design and implementation of ultra-high frequency …design and implementation of ultra-high...

Design and Implementation of Ultra-High Frequency (UHF) Radio Frequency Identification

(RFID) Transceiver for Passive RFID Tags

Session 2003(F)

PROJECT ADVISOR Prof. Zubair Ahmed Khan

Project Members

Haani Masood 2003(F)-E-201 Nauman Anwar 2003(F)-E-142 Syed Waqar Azeem 2003(F)-E-154 Shakeel Ahmad 2003(F)-E-139

Department of Electrical Engineering

University of Engineering and Technology, Lahore

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Table of Contents 1 Introduction................................................................................................................. 9 2 RFID System Overview............................................................................................ 12

2.2 The background to RFID technology ..................................................................... 12 2.2.1 RFID Technology comparison with Barcodes................................................. 13

2.3 RFID System Components ..................................................................................... 15 2.2.1 Tags (Transponders) ........................................................................................ 16

2.2.1.1 Classification of Tags According To Power............................................. 16 2.2.1.2 Classification of Tags According to EPC ................................................. 18

2.2.2 Readers............................................................................................................. 18 2.2.3 Antenna ............................................................................................................ 19 2.2.4 Host Computer ................................................................................................. 20

2.4 RFID APPLICATIONS.......................................................................................... 20 2.4.1 Manufacturing.................................................................................................. 20 2.4.2 Distribution and Inventory............................................................................... 20 2.4.3 Retail ................................................................................................................ 21 2.4.4 Security ............................................................................................................ 21 2.4.5 Food Supplies................................................................................................... 22 2.4.6 Healthcare ........................................................................................................ 22 2.4.7 Animal Tracking .............................................................................................. 22

3 RFID System Design ................................................................................................ 23 3.1 Frequency Band Selection ...................................................................................... 23

3.1.1 Comparison of different Frequency Bands ............................................... 24 3.1.2 Frequency Band selection of RFID system .............................................. 27

3.2 Tag Selection .......................................................................................................... 29 3.2.1 Specification Of Tags ...................................................................................... 29

3.2.1.1 Data Capacity........................................................................................ 31 3.2.1.2 Data read rate ........................................................................................ 32 3.2.1.3 Data Programming Options .................................................................. 32 3.2.1.4 Physical Form ....................................................................................... 32 3.2.1.5 Costs...................................................................................................... 32

3.2.2 Comparison of Tag market ....................................................................... 33 3.3 Antenna Selection ................................................................................................... 34

3.3.1 Frequency......................................................................................................... 35 3.3.2 Return Loss ...................................................................................................... 35 3.3.3 Gain.................................................................................................................. 35 3.3.4 Polarization ...................................................................................................... 35 3.3.5 Power Handling ............................................................................................... 36 3.3.6 Input Impedance............................................................................................... 36

3.4 Modulation Schemes............................................................................................... 36 3.4.1 Load Modulation.............................................................................................. 37 3.4.2 Backscatter Modulation ................................................................................... 37

3.4.2.1 Explanation of Backscatter Modulation................................................ 38 3.5 Reader Design......................................................................................................... 39

3.5.1 RF (Carrier) part ....................................................................................... 40 3.5.1.1 UHF read range estimation ................................................................... 41

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3.5.1.2 Reader sensitivity limit ......................................................................... 41 3.5.1.3 Conclusion ............................................................................................ 42

3.5.2 Microcontroller Part.................................................................................. 42 3.5.3 Specifications of Reader ........................................................................... 42

4 RFID System Implementation (Part I) ...................................................................... 45 4.1 Implementation OF RFID System .......................................................................... 45

4.1.1 RF (Carrier) Part .............................................................................................. 45 4.1.1.1 Integrated PLL and VCO Frequency Synthesizer ............................... 46 4.1.1.2 Driver for Power Amplifier .................................................................. 50 4.1.1.3 Power Amplifier.................................................................................... 52 4.1.1.4 Transistor Switch for OOK................................................................... 54

4.1.2 Receiver ........................................................................................................... 57 4.1.2.1 Operational Theory ............................................................................... 58 4.1.2.2 Pin selection of TH7122 for desired results.......................................... 62 4.1.2.3 Operation of RFID Reader.................................................................... 66 4.1.2.4 Manufacturing of Evaluation Board of TH7122................................... 67

4.1.3 Antenna Design................................................................................................ 70 4.1.3.1 Patch Antenna Design Formulas and Calculations............................... 71 4.1.3.2 HFSS software Simulation.................................................................... 75

4.1.4 Tags Used for RFID system implementation................................................... 80 5 RFID System Implementation (Part II)..................................................................... 82

5.1 Overview................................................................................................................. 82 5.1.1 Physical Layer.................................................................................................. 82 5.1.2 Tag – Identification Layer ............................................................................... 83 5.1.3 Reader Tag Communication Process.......................................................... 83 5.1.4 Tag Selection and Detection ............................................................................ 85

5.1.4.1 Sessions and Inventoried flags.............................................................. 85 5.1.4.2 Tag States and Slot Counter.................................................................. 85

5.1.5 Reader Commands and Tag Replies ................................................................ 87 5.1.6 Link Timing ..................................................................................................... 88 5.1.7 Flowcharts and Line Diagrams ........................................................................ 89

5.2 Transmitted/Received Waveforms and Data Models ............................................. 91 5.2.1 Transmitted Waveform and Data Model ......................................................... 91

5.2.1.1 Modulation............................................................................................ 91 5.2.1.2 Data Encoding....................................................................................... 91 5.2.1.3 Data Rates ............................................................................................. 92 5.2.1.4 Transmission Order............................................................................... 92 5.2.1.5 Preamble and Frame-sync..................................................................... 92 5.2.1.6 SELECT Command .............................................................................. 93 5.2.1.7 Invertory Command.............................................................................. 95

5.2.2 Receiver Waveform and Data Model ....................................................... 97 5.2.2.1 Modulation............................................................................................ 97 5.2.2.2 Data Encoding....................................................................................... 97 5.2.2.3 Data Rates ........................................................................................... 100 5.2.2.4 Transmission Order............................................................................. 101 5.2.2.5 Tag Memory........................................................................................ 101

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5.2.2.6 EPC Memory ...................................................................................... 102 5.3 Implementation and Design of Data Model.......................................................... 103

5.3.1 Microcontroller Unit – ATMEL ATMega32................................................. 103 5.3.2 Implementation of the Transmitted Data Model............................................ 104 5.3.3 Implementation of Receiver Data Model....................................................... 106 5.3.4 Clock Options and Clock Frequency Used.................................................... 107 5.3.5 Calculations and Results ................................................................................ 108

5.3.5.1 Reader to Tag Communication ........................................................... 108 5.3.5.2 Tag to Reader Communication ........................................................... 111 5.3.5.3 Link Timings....................................................................................... 112

6 Testing and Results ................................................................................................. 115 6.1 RF Portion Results ................................................................................................ 115

6.1.1 Component-Level Testing ............................................................................. 115 6.1.1.1 Integrated PLL and VCO Frequency Synthesizer .............................. 115 6.1.1.2 Driver .................................................................................................. 117 6.1.1.3 Power Amplifier.................................................................................. 120 6.1.1.4 Switch for ON-OFF Keying ............................................................... 122 6.1.1.5 Antenna ............................................................................................... 122

6.1.2 System Level Testing.............................................................................. 124 6.2 Signal Generation and Processing on ATMega32 Results ................................... 127

6.2.1 FrameSync Command.................................................................................... 128 6.2.2 Preamble Command....................................................................................... 129 6.2.3 Select Command ............................................................................................ 131 6.2.4 Query Command............................................................................................ 132 6.2.5 QueryRep Command ..................................................................................... 133 6.2.6 T4, Minimum Time Wait between Reader Commands ................................. 133

7 Conclusions and Outlook........................................................................................ 135 7.1 Conclusions........................................................................................................... 135 7.2 Outlook ................................................................................................................. 136

References....................................................................................................................... 137 Appendix......................................................................................................................... 138

List of Tables Table 3.1 RFID frequency bands ...................................................................................... 23 Table 3.2 Comparison of the characteristics of RFID frequency bands ........................... 25 Table 4.1 Frequency resolution and operating frequency................................................. 60 Table 4.2 Selection of frequency. ..................................................................................... 63 Table 4.3 Operation Mode ................................................................................................ 63 Table 4.4 Modulation Type............................................................................................... 63 Table 4.5 LNA gain mode ................................................................................................ 63 Table 5.1 – Tag’s response to Reader commands............................................................. 87 Table 5.2 – Transition of Tag due to Select command ..................................................... 87 Table 5.3 – Link Timing Parameters ................................................................................ 88 Table 5.4 – Tari Values..................................................................................................... 92

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Table 5.5 – Select Command ............................................................................................ 93 Table 5.6 – Matching and Non-Matching Conditions for Tags........................................ 94 Table 5.7 – Query Command............................................................................................ 95 Table 5.8 – QueryRep Command ..................................................................................... 96 Table 5.9 – Tag’s response to QueryRep command......................................................... 97 Table 5.10 – ACK Command ........................................................................................... 97 Table 5.11 – Link Frequencies........................................................................................ 100 Table 5.12 – Data Rates .................................................................................................. 101 Table 5.13 - Precursor..................................................................................................... 103 *Table 5.14 – Clocking Options in ATMega32.............................................................. 107 Table 5.15 – Parameter Values ....................................................................................... 108

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List of Figures Figure 2.1 A typical barcode............................................................................................. 12 Figure 2.2 RFID Systems.................................................................................................. 16 Figure 2.3 A typical Tag. .................................................................................................. 17 Figure 3.1 Comparison of read range of different types of coupling................................ 24 Figure 3.2 Frequency bands in different countries ........................................................... 26 Figure 3.3 Tags at different frequencies. .......................................................................... 31 Figure 3.4 tags manufactured by different vendors .......................................................... 33 Figure 3.5 An Alien Class 1 Gen 2 tag for library............................................................ 34 Figure 3.6 circularly polarized 900MHz Panel antenna by POYNTING Co. .................. 36 Figure 3.7 Inductive coupling between reader and tag. .................................................... 37 Figure 3.8 Backscatter modulation. ................................................................................. 39 Figure 3.9 A typical reader anatomy................................................................................. 40 Figure 3.10 An Alien Class 1 Gen 2 reader. ..................................................................... 44 Figure 4.6 A block diagram of RFID system reader........................................................ 46 Figure 4.7 Functional Block Diagram of Synthesizer ...................................................... 47 Figure 4.8 ADL4360-7 Evaluation board. ........................................................................ 48 Figure 4.9 Snap shot of the ADL4360-7 programming software. .................................... 50 Figure 4.10 Functional block diagram of ADL5530 ........................................................ 51 Figure 4.11 Schematic of the Evaluation board of ADL5530. ......................................... 51 Figure 4.12 Evaluation Board of ADL5530 ..................................................................... 52 Figure 4.13* Functional block diagram of ADL5322. ..................................................... 52 Figure 4.14 Schematic of the evaluation board of ADL5322. .......................................... 53 Figure 4.15 Evaluation Board of ADL5322. .................................................................... 53 Figure 4.16 Waveform of transistor having distorted high frequency response............... 54 Figure 4.17 A driving circuit to drive the power MOSFET ............................................. 56 Figure 4.18 Circuit diagram of On Off Keying (OOK) switch......................................... 56 Figure 4.19 pin description of TH7122............................................................................. 58 Figure 4.20 TH7122 block diagram.................................................................................. 59 Figure 4.21 Main window of TH7122 software ............................................................... 65 Figure 4.22 Register view of TH7122 software................................................................ 65 Figure 4.23 Extended parameters window........................................................................ 66 Figure 4.24 TH7122 evaluation board. ............................................................................. 67 Figure 4.25 LPKF CNC machine for PCB designing....................................................... 68 Figure 4.26 Layout of evaluation board of TH7122, board size is 3.95cm*5.65cm. ....... 69 Figure 4.27 Circuit diagram of peak detector ASK demodulator option.......................... 70 Figure 4.28 Physical PCB layout of the evaluation board of TH7122. ............................ 70 Figure 4.29 Schematic of patch rectangular patch antenna. ............................................. 72 Figure 4.30 HFSS main window snap shot....................................................................... 75 Figure 4.31 Result of sweep of x_feed point versus return loss. ...................................... 77 Figure 4.32 The patch antenna with its ground and air dimensions after new values. ..... 78 Figure 4.33 Frequency verses return loss of patch antenna. ............................................. 78 Figure 4.34 Frequency versus VSWR of the patch antenna. ............................................ 79 Figure 4.35 radiation pattern of patch antenna. ................................................................ 79 Figure 4.36 Our fabricated patch antenna......................................................................... 80

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Figure 4.37 RX-UHF-00C01-03 tag. ................................................................................ 80 Figure 5.1 – Reader Tag Communication Process ...................................................... 83 Figure 5.2 – Link Timing.................................................................................................. 88 Figure 5.3 – Line Diagram of Communication Process ................................................... 89 Figure 5.4 – Flowchart of Communication Process between Reader and Tag................. 90 Figure 5.5 – PIE encoded Data-0 and Data-1 Waveforms................................................ 91 Figure 5.6 – Preamble and Frame Sync ............................................................................ 93 Figure 5.7 – FM0 Basis Functions and State Transition Diagram.................................... 98 Figure 5.8 – FM0 Symbols and Sequences....................................................................... 99 Figure 5.9 – FM0 Preamble .............................................................................................. 99 Figure 5.10 - Memory Banks .......................................................................................... 102 Figure 5.11 – CTC Mode, Timing Diagram ................................................................... 105 Figure 5.12 – Crystal Resonator Configuration.............................................................. 107 Figure 5.13 – Data-0 Waveform ..................................................................................... 109 Figure 5.14 – Data-1 Waveform ..................................................................................... 109 Figure 5.15 – RTcal Waveform ...................................................................................... 110 Figure 5.16 – TRcal Waveform ...................................................................................... 110 Figure 5.17 – Delimiter................................................................................................... 111 Figure 5.18 – Data-0 symbols......................................................................................... 112 Figure 5.19 – Data-1 symbols......................................................................................... 112 Figure 6.1.Testing setup for frequency synthesizer. ....................................................... 116 Figure 6.2 .A snapshot of Spectrum Analyzer showing Synthesizer output. ................. 116 Figure 6.3.Time domain waveform of synthesizer output. ............................................. 117 Figure 6.4 Testing of driver by Network Analyzer......................................................... 117 Figure 6.5 S 11 parameter of driver. ............................................................................... 118 Figure 6.6 S21 parameter of driver. ................................................................................ 118 Figure 6.7 S12 parameter of driver ................................................................................. 119 Figure 6.8 S22 parameter of driver ................................................................................. 119 Figure 6.9.Testing of power amplifier by Network Analyzer ........................................ 120 Figure 6.10 S11 parameter of power amplifier ............................................................... 120 Figure 6.11 S22 parameter of power amplifier ............................................................... 121 Figure 6.12 S21 parameter of power amplifier ............................................................... 121 Figure 6.13 Output of the switch .................................................................................... 122 Figure 6.14 Testing setup for patch antenna................................................................... 123 Figure 6.15 Return loss of patch antenna having minimum value at 915MHz .............. 123 Figure 6.16 Synthesizer connected to driver................................................................... 124 Figure 6.17 Output after driver module. ......................................................................... 125 Figure 6.18 Power amplifier connected .......................................................................... 125 Figure 6.19 Output after power amplifier ....................................................................... 126 Figure 6.20.Complete transmitter part ............................................................................ 126 Figure 6.21 On Off Keying of carrier signal................................................................... 127 Figure 6.22 – Simulation FrameSync ............................................................................. 128 Figure 6.23 – Simulation FrameSync highlighting RTcal .............................................. 129 Figure 6.24 – Simulation Preamble ................................................................................ 130 Figure 6.25 – Simulation Preamble highlighting RTcal ................................................. 130 Figure 6.26 – Simulation Select Command .................................................................... 131

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Figure 6.27 Simulation Query Command....................................................................... 132 Figure 6.28 Simulation QueryRep Command ................................................................ 133 Figure 6.29 – Simulation T4 ........................................................................................... 134

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1 Introduction

The trend in the automated industry is moving towards fast and real time

identification. The main objective is to further improve the high level of accuracy needed

to enable continuous identification and monitoring. Such a level of real-time knowledge

is often called ambient intelligence. One of the technologies that made this concept viable

is known as Radio Frequency Identification or more simply RFID. The invention of

RFID technology is playing a key role in becoming a ubiquitous enabler for doing things

better than we used to do them.

RFID is an automatic identification method which relies on storing and remotely

retrieving data using devices called RFID tags or transponders. An RFID tag is an object

that can be attached to or incorporated into a product, animal, or person for the purpose of

identification using radio waves. Chip-based RFID tags contain silicon chips and

antennas. Passive tags require no internal power source, whereas active tags require a

power source.

RFID is important because it enables machines to perceive. Machine perception is

common in science fiction where sentient robots walk and talk as a matter of course, but

it is rare and primitive in every day life. Airport faucets struggle to sense people

impatiently waiting to wash their hands, barcode scanners frequently fail to beep, and

home burglar alarms have trouble distinguishing between pets and intruders. During the

next few decades RFID will help change all that. It will usher in a new wave of

computing in which devices can effectively sense and interpret the world around them.

Warehouses will sense whether they are low on stock or over stock, airports will find and

route luggage automatically, cars will know whether their tires are about to blow, homes

will know if lights are left on, doors are unlocked, or windows are open. Because of

RFID, we are entering what Paul Saffo has called “The Sensor Age”. In the 19th century,

machines could do, in the 20th century they could think, in the 21st century they will

perceive.

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The British pioneered RFID during World War II to identify their own air craft

returning from sorties over occupied Europe. Early radar system could spot an incoming

aircraft but not determine its type. But with the transponders in planes, RFID could

differentiate Allied from enemy aircraft. In the late 1960s, the U.S. Government began

using RFID to tag and monitor nuclear and other hazardous materials.

Along with garage door openers, an early major use of RFID was animal

identification. Livestock could be tagged or collared, for example, to control access to

feeding stations at feedlots. Meanwhile, companion animals could be implanted with

RFID tags for identification purposes. Interestingly, RFID animal tagging has become

important enough for the International Standards Organization (ISO) to develop the

animal ID standards ISO11784 and ISO 11785.

Today, RFID is used for automatic toll collection, access control, security,

equipment tracking, payment at gas pumps, fast food establishments and other retail

outlets, and a wide range of other applications. Additionally, an entire committee inside

the ISO (ISO/IEC JHC1/SC31/WG4) is responsible for international RFID standards.

There are different standard frequency bands for RFID applications like Low

Frequency (LF), High Frequency (HF), Ultra High Frequency (UHF) and 2.45 GHz

Industrial Scientific Medical (ISM) band. At present the available standards are the

Electronic Product Code (EPC) Global initiative and the ISO 18000 standard for UHF

RFID applications. The ISO only deals with air interface whereas EPC also includes the

data structure of the ID. EPC Global describes the whole protocol of reader to tag

communication.

In this project we have designed and implemented an RFID system. This includes

design and development of hardware portion(RF carrier) and implementation of EPC

Global protocol for standardized communication between RFID tags and reader. Among

the various applications of RFID, it is being widely deployed in libraries. Unlike Electro-

Mechanical (EM) and Radio Frequency (RF) systems, which have been used in libraries

for decades, RFID-based systems move beyond security to become tracking systems that

combine security with more efficient tracking of books throughout the library, including

easier and faster charging and discharging, inventorying, and material handling. The use

of RFID also reduces the amount of time required to perform book circulation operations.

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The most significant time savings are attributable to the facts that information can be read

from RFID tags much faster than from barcodes and that several items in a stack can be

read at the same time. While initially unreliable, the anti-collision algorithm that allows

an entire stack to be charged or discharged now appears to be working well.

We have worked at UHF band (868MHz) which is a European standard for RFID

applications. The reasons for choosing this frequency are comprehensively explained in

chapter 3. We have used tags from Texas Instruments which are in accordance with EPC

Global Class 1 Gen2 standard. Selection criteria of tags are also explained in chapter 3.

On the receiving side from tag to reader we have used a receiver chip TH7122 after the

receiving antenna. The details of this portion are mentioned in chapter 4. EPC Global

Class 1 Gen2 protocol is implemented using Atmel ATmega32 microcontroller. The

detailed explanation of the whole protocol and its implementation can be found in chapter

5. The carrier (RF) part is operating at 868MHz with output power level of 30dBm.

Technology overview and components of RFID system have been explained in

chapter 2 and 3. Next, the design and implementation of RFID system has been divided

into two parts. Part I is contained in chapter 4 which describes the implementation of the

hardware part. Part II is contained in chapter 5 which describes the software part i-e the

protocol implementation for RFID system on microcontroller. The testing and results of

our project are outlined in chapter 6. The project report concludes with a brief overview

and recommendations for future prospects in chapter 7.

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2 RFID System Overview

2.2 The background to RFID technology

The ubiquitous barcode, such as diagrammatically portrayed in Figure 2.1, has

been used since the late 1960’s as a printed means of identifying product categories. The

barcode system was effectively standardized with the widespread adoption in 1973, by

users and equipment makers, of the Universal Product Code (UPC) 8 or 12 digit

zymology. Since then, there have developed different versions of this barcode technology

(EAN, JAN) encompassing 8 to 14 digit systems. Barcode technology continues to

perform an essential role in inventory control and distribution, particularly in the

consumer product market sector.It can be found in use from car manufacture to library

stock control.

Figure 2.1 A typical barcode.

Barcodes require close scanning with an optical reader, much like the laser scanning readers found at most supermarket.

Barcodes are limited to the data printed on them and cannot be updated,

other than by replacement or sticking a label over them (which may be

labor-Intensive).

Need to be substantially flat for reliable reading.

Are typically (but not always) paper labels, or printed on paper based

packaging, and therefore prone to damage.

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Typically provide inventory data to the level of product category. [For

Example it might indicate that the product is a 250g packet of Danish

‘Product Name’ unsalted butter, but is unlikely to indicate the sell by

neither date (shelf life), nor the “best before date".]

Are very unlikely to show through which distribution depots and transport

means the product arrived at the point of sale.

2.2.1 RFID Technology comparison with Barcodes Compared to barcode inventory control systems RFID technology has both

advantages and disadvantages, many of which are outside of product manufacture and

distribution chain applications.

Advantages versus Barcodes

Do not require line of sight access to be read.

The tag can trigger security alarm systems if removed from its correct

location.

Scanner/reader and RFID tag are not (so) orientation sensitive.

Automatic scanning and data logging is possible without Operator

intervention.

Each tag can hold more data than just a unique product code.

Each item can be individually ‘labeled’.

Tag data can be comprehensive, unique in parts/common in parts, and is

compatible with data processing.

With the right technology a plurality of tags can be concurrently read

It can be read only or read-write.

There is a very high level of data integrity (character check sum

encoding).

Provides a high degree of security and product authentication – a tag is

more difficult to counterfeit than a barcode.

The supporting data infrastructure can allow data retrieval and product

Tracking anywhere provided the scanner/reader is close enough to the tag.

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Combined with its authentication tag has the ability to monitor product

shelf life – a societal advantage in the pharmaceutical and food industry.

Since each tag can be unique they can act as a security feature if lost or

stolen e.g. a stolen SMART travel card can be cancelled.

The technology is rugged and can be used in hostile environments such as

down oil wells (heat and pressure) to carry data to remote equipment.

The technology tends to be tolerant updated, for example, as a car goes

through its life its service record can be electronically logged with the car.

The technology could be inserted within a suit so that when it is sent to the

cleaners it automatically gets the right cleaning procedure applied to it.

The technology can be used to increase security so that, for example, it

may be construed that a child is at school as their tag in their school bag

was logged when they came through the school gates. [Clearly, this does

not necessarily mean that the child is at school, but only that their bag with

the contained RFID tag has been taken into the school, which in most

circumstances will mean that the bag was carried by the child at that time.]

Disadvantages versus Barcodes

Even in six figure production quantities, the simplest of these tags is more

expensive (say tens of pence) than a printed barcode – this extra cost, plus

the potential greater infrastructure capital cost, has to be bettered by other

benefits in the distribution chain or represent an application for which the

barcode is not suitable e.g. SMART Cards.

There is a high cost (long pay-back) for integrating RFID technology into

existing inventory control systems.

External influences such as metalwork, material dielectric properties and

radio interference can constrain RFID remote reading.

If a significant number of RFID’s greater system capabilities are

implemented then the host system and infrastructure have a higher capital

cost and complexity than for barcode systems.

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There is currently no international RFID numbering system equivalent to

UPC, and thus uptake is constrained due to uncertainty. The International

Standards Organization (ISO) and Electronic Product Code [EPC] Global

consortium, amongst others, are working to address this issue.

Currently there are not internationally agreed frequencies for RFID

operation (other than the ‘SMART card’ 13.56 MHz) and permitted

scanner/reader powers differ between countries. This limits product take-

up. [For example, there are significant differences between the USA and

European UHF frequencies.]

2.3 RFID System Components

An RFID system may consist of several components: tags, tag readers, edge

servers, middleware, and application software.

The purpose of an RFID system is to be able to transmit data by a mobile device,

called a tag, which is read by an RFID reader and processed according to the needs of a

particular application. The data transmitted by the tag may provide identification or

location information, or other specifications of the product tagged, such as price, color,

date of purchase, etc. The use of RFID in tracking and access applications first appeared

in 1932, to identify aircraft as friendly or unfriendly ("identify friend or foe" (IFF)).

RFID quickly gained attention because of its ability to track moving objects. As the

technology is refined, more pervasive and possibly invasive uses for RFID tags are in the

works.

In a typical RFID system, individual objects are equipped with a small,

inexpensive tag. The tag contains a transponder with a digital memory chip that is given a

unique electronic product code. An RFID reader is infact, an antenna packaged with a

transceiver and decoder which emits a signal activating the RFID tag so it can read and

write data to it. When an RFID tag passes through the electromagnetic zone of reader, it

detects the reader's activation signal. The reader decodes the data encoded in the tag's

integrated circuit (silicon chip) and the data is passed to the host computer. The

application software on the host processes the data, and may perform various filtering

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operations to reduce the numerous often redundant reads of the same tag to a smaller and

more useful data set.

For example,in a library, security gates can detect whether or not an RFID tagged

book has been properly checked out of the library. When users return items, the security

bit is re-set and the item record in the integrated library system is automatically

updated. In some RFID solutions, a return receipt can be generated. At this point,

materials can be roughly sorted into bins by the return equipment..

Figure 2.2* RFID Systems.

2.2.1 Tags (Transponders) An RFID tag acts as a transceiver which generates a reply signal upon proper

electronic interrogation. A typical RFID tag is shown in Figure 2.3.

2.2.1.1 Classification of Tags According To Power

There are two types of tags according to their power requirement.

Passive

Active

Passive

These type of tags use the energy in readers close-coupled reactive field as a

power source for any on-chip computation and also for communication back to the

* Courtesy: www.google.com/images

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reader.As the communication back to the reader is by means of the same signal that

provides power to it,so these transponders can only be read from a short-range distance of

only few feet.These transponders are cheaper and hence can be applied in high quantities

to individual items.

Active

These type of tags use on-board battery power for computation and for

communication back to the reader.Due to availability of sufficient energy these can be

read from a long distance of more than 100 feet.They are ideal for tracking items over

long ranges,such as tracking shipping containers in transit.They have high power and

battery requirements,so they are heavier and expensive.

Substrate

Die attach

Tag IC

Antenna

Figure 2.3* A typical Tag.

* Courtesy: www.RFID.com

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2.2.1.2 Classification of Tags According to EPC

The standards body EPC Global™ has categorized RFID tags into four

different classes based on their functionality.

Class 0 and 1

Passive identity tags (usable range of 3 meters)

Backscatter (interrogator speaks first)

Lowest cost

Class 2

Passive identity and memory tags (usable range of 3 meters)


Security

Low cost

Class 3

Battery-assisted passive tags

More functionality on chip – memory, sensors, etc


100 meter range

Moderate cost

Class 4

Active battery tags (tags transmit carrier)

Active transmission (permits tag-talks-first operating modes)

100 meter range

High cost

2.2.2 Readers The RFID reader sends a pulse of radio energy to the tag and listens for the tag’s

response. The tag detects this energy and sends back a response that contains the tag’s

serial number and possibly other information as well.

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In simple RFID systems, the reader’s pulse of energy functioned as an on-off

switch; in more sophisticated systems, the reader’s RF signal can contain commands to

the tag, instructions to read or write memory that the tag contains, and even passwords.

Historically, RFID readers were designed to read only a particular kind of tag, but

so-called multimode readers that can read many different kinds of tags are becoming

increasingly popular.

RFID readers are usually active, meaning continually transmitting radio energy

and awaiting any tags that enter their field of operation. However, for some applications,

this is unnecessary and could be undesirable in battery-powered devices that need to

conserve energy. Thus, it is possible to configure an RFID reader so that it sends the

radio pulse only in response to an external event. For example, most electronic toll

collection systems have the reader constantly powered up so that every passing car will

be recorded.

Like the tags themselves, RFID readers come in many sizes. The largest readers

might consist of a desktop personal computer with a special card and multiple antennas

connected to the card through shielded cable. Such a reader would typically have a

network connection as well so that it could report tags that it reads to other computers.

The smallest readers are the size of a postage stamp and are designed to be embedded in

mobile telephones.

2.2.3 Antenna The antennas are the conduits for data communication between the tag and the

reader. Antenna design and placement plays a significant role in determining the

coverage zone, range and accuracy of communication. When analyzing the energy that is

radiated from an antenna, its field is divided into two parts: the near field, which is the

part of radiation that is within a small number of wavelengths of the antenna, and the far

field, which is the energy that is radiated beyond the near field. Because the wave-length

of LF and HF devices tends to be much larger than the ranges at which RFID systems

typically operate, these systems operate in the near field, while UHF and ISM systems

operate in the far field. Antennae in RFID systems perform dual functions, they transmit

power as well as data.

19

The packaging characteristics for the antenna on a reader vary depending on

application requirements. In certain cases such as handheld readers, the antenna is

mounted directly on the reader. In other cases, several antennae can be mounted away

from a reader unit and positioned strategically to enhance the quality and range of radio

signals.

2.2.4 Host Computer The final stop for data in the RFID system is the host computer. Usually the

interface of a reader with a host computer is a serialized shared bus link. Once the data

reaches to this point the host computer makes sense of this data and saves it in

appropriate database table. This data can be later retrieved based on some query or

research result.

The host computer can also determine what mode in which the reader is

operating. For example, in an environment where tags are passing near antenna, the

reader can be instructed to continuously look for tags. If the reader antenna were to

become portable, the host computer may instruct the reader to energize only at the users

command.

2.4 RFID APPLICATIONS

The following section describes some key RFID applications.

2.4.1 Manufacturing Manufacturing is a complex task. It requires that the right materials arrive at the

right station and receive the right process and that the manufacturing itself is done

correctly. RFID is being used for ensuring that the right label goes on a product or that a

box contains everything it should. It is being used in tracking an item through every

workstation and recording every tool that performed an operation on it. Tool checkouts

that uses RFID employee ID badges has saved companies tens of thousands of dollars

annually in “lost” tools and has helped boost productivity.

2.4.2 Distribution and Inventory Maintaining an accurate inventory is critical to any operation. Cycle counting

everything in a warehouse has always been a necessary but loathsome task. A properly

20

designed and implemented RFID program, however, can eliminate the need for physical

cycle counting, saving companies hundreds of employee hours and days of down time.

During “Operation Iraqi Freedom” the U.S.Department of Defence (DoD) used

passive RFID tags to identify shipping units stored inside metal cargo containers that

were themselves equipped with active RFID tags. The DoD’s experience in 1991’s

“Operation Desert Storm” demonstrated that shipping massive quantities of materials to

an operational theatre was of no use if it could not be quickly and easily identified and

located. Thus, DoD issued a mandate to its 30,000 suppliers that all shipments to the DoD

must be RFID-tagged.

2.4.3 Retail RFID has proven to be extremely useful in maintaining adequate stock levels in

distribution of goods such as in food or fashion industry. The ability to automatically

record sales, check inventories, and replenish or order additional stock reduces the

amount of inventory and waste in the system while helping to assure shoppers fresh

products on the shelves.

RFID tags used as both” smart labels” and EAS (Electronic Article Surveillance)

systems, have been proven to reduce theft at the store level as well as product diversions

in the supply chain.

2.4.4 Security Access to secure areas is already being monitored through the use of a variety of

automatic identification technologies. Given recent lapses in security at U.S.

governmental facilities, one can expect greater use of RFID for internal security and asset

tracking, not just access control. With the mounting concerns about national security, the

U.S. government is beginning to require biometrically enabled passports and visas to

identify the bearers and verify the documents’authenticity.The use of RFID in such

documents helps prevent counterfeits, RFID unlike holograms and other security features

is extremely difficult to duplicate.

Overseas shipping facilities and cargo containers are increasingly being secured

by the use of RFID to prevent the importation of chemical or biological weapons.

21

2.4.5 Food Supplies Recent outbreaks of” mad cow” disease, hepatitis, and other food-borne illnesses

have focused international attention on the need to maintain better records on the world’s

food supply.

Agencies are using RFID to maintain a data history of animals and food products

in various parts of the world. This use will undoubtedly increase as the public and

governments become more sensitized to food safety concerns.

In the United Kingdom, for example, companies are already required to be able to

trace an individual package of meat in the supermarket back to the original animal and to

document that animal’s heritage and feed history.

2.4.6 Healthcare The healthcare sector is just now taking up RFID technology, with significant

opportunities for increased efficiency and patient safety. Today, many healthcare

providers use RFID in routine treatment, especially for patient identification and

automatic recording or updating of records on procedures, medications and outcomes.

RFID can be used throughout the medical supply chain to maintain adequate

inventories at a facility, ensure the authenticity of medications and medical devices, and

give healthcare professionals more time to focus on patients rather than paperwork.

2.4.7 Animal Tracking RFID is used in animal identification and tracking at large farm houses. RFID

tags are tied to the cattle and each cattle can be uniquely identified and tracked. This

helps in studying animal behavior.

22

3 RFID System Design

3.1 Frequency Band Selection

The speed with which the reader can interrogate the tag and write to it depends

upon the RFID technology used, in particular the radio frequency used. Importantly, the

necessary proximity between the reader antenna and the RFID tag for successful

operation is dependent upon the radio frequency and whether the tag is active or passive.

There are four main frequency bands, used for RFID systems. They are depicted

in Table 3.1.

Table 3.1 RFID frequency bands

3.1.1 Generic Band Name 3.1.2 Frequency Range

3.1.3 Comment (National Frequency Allocations Vary)

Low Frequency (LF) 120 - 135 kHz Short range inductive applications.

High Frequency (HF) 13.56 MHz Worldwide common frequency, smart cards and labels.

433 MHz Active low power tags. Ultra High Frequency (UHF) 860 - 960 MHz Band with major supply chain

development activity. Microwave 2450 MHz Active tag technology gives

range and fast data rates. Hz (Hertz) unit of frequency measurement kHz thousands of Hertz, MHz millions of Hertz.

RFID systems that use frequencies between approximately 100 kHz and 30 MHz

operate using inductive coupling. By contrast microwave systems in the frequency range

2.45-5.8GHz are coupled using electromagnetic fields.

The specific absorption rate (damping) for water or non conductive substances is

lower by a factor of 100 000 at 100 kHz than it is at 1GHz. Therefore virtually no

damping or absorption takes place. Lower frequency up to 13.56MHz systems are

primarily used due to the better penetration of the objects.

Microwave systems significantly have a higher range than inductive coupled

systems typically 2-15m. Another important factor is sensitivity to electromagnetic

interference fields, such as generated by strong electric motors. The performance of

23

inductive transponders is lower in such situation. Microwave systems have therefore

established themselves in the production lines and the painting systems of the automotive

industry.

3.1.1 Comparison of different Frequency Bands The choice of frequency band specifies the range and speed in most cases of the

RFID systems. A small comparison of relative interrogation zones of different systems is

shown below in Fig 3.1.

Figure 3.1 Comparison of read range of different types of coupling

The frequency selection is so much critical and to choose a perfect frequency for

an RFID application there are many factors which influence the choice of frequency.

Table 3.2 gives detailed comparison of different RFID frequency bands according to their

speed, applications and read/write ranges.

1m 2m 3m

Inductive coupling

Electromagnetic coupling (backscatter) directional

Electromagnetic coupling (back scatter) non directional

24

Table 3.2* Comparison of the characteristics of RFID frequency bands Typical Operating Range

Typical Tag Size

Band MHz

Read Only

Read Write

Active (vol.)

Passive (area)**

Typical Application

Relative Data Transfer Rate

Comment

0.125 – 0.134 (Low Frequency LF)

Up to 2m

Few cm

5 – 10cc

2-5cm2

• Animal identification • Car immobilizer • Controlled access • Work in progress pallets

• Slow*** • Non - concurrent multiple access

• Inductive applications • Expensive tag • Susceptible to electrical noise

13.56 (High Frequency HF)

Up to 1m

Up to 0.5m

3 – 5cc

10cm2

• Smart Cards • Smart labels • Domestic electrical goods • Access and security systems

• Medium*** • Multiple concurrent read <50 items

• Worldwide RFID frequency • Max. reader power North America 3W, Europe 4W • Medium cost tag

433 MHz

Tens m

Few m

• Specialist animal tracking

• Fast***

• Active tags

860 – 960 (Ultra High Frequency UHF)

Up to 5m

Up to 0.5m

1 – 2cc

4cm2

• Item level tracking in factory, warehouse and distribution chain • Mass produced consumer durables

• Fast • Multiple concurrent read >100 items

• High ability to concurrently read multiple tags • Highly integrated manufacture • Inexpensive tag • Tag modulates read to send

* courtesy of TheIET.org ** The passive devices tend to be very thin (<1mm) and for this reason the comparative area is given. *** Approximate data rates: Slow up to 2 kilobits/s Medium up to 10kilobits/s fast up to 100 kilobits/s

25

data – back scatter • Different frequencies North America, Europe, Japan

2450 (Microwave)

Up to 10m

Up to 1m

- 1cm2

• Moving car electronic toll collection • Prepaid travel cards

• Fast

• Bluetooth and WiFi systems • Highly integrated manufacture • Inexpensive tag

The regulatory authorities for frequency allocation have allocated different

frequency bands in different countries. The above factor also limits the frequency use for

RFID system in different countries of the world irrespective of the applications. For UHF

RFID systems in some countries most of the UHF band has already been allocated for

GSM (Global System for Mobile communication). The below figure 3.2 specifies some

countries and the allocated bands for RFID.

Figure 3.2 Frequency bands in different countries

US, Canada EU Countries Japan 125 KHz 125 KHz 125 KHz 13.56MHz 13.56MHz 13.56MHz 902-928MHz 868-870MHz 950-956MHz

26

3.1.2 Frequency Band selection of RFID system

13.56 MHz was the most common band in use but now the RFID system

developers are moving towards the UHF band to get the full advantage of the RFID

systems. The UHF Technology was chosen above the other frequencies such as 13.56

MHz due to much longer read ranges and reduced cost. Read distances of UHF

technology in 860-960 MHz frequency range are greater than 13.56 MHz due to

maximum legal power restraints on the readers by regulators. The frequency is almost 70

times faster than 13.56 MHz, which makes many more reads per second possible. With

the new EPC Global Gen 2 standard running at top speed, over 1000 tags can be read per

second in an application, where they are insulated from RF noise. UHF also has the

option to slow down to read 100 tags per second with high reliability in noisy

applications. This, compared to a maximum of around 60 tags per second with the 13.56

MHz reader frequency tagging systems, makes the UHF Technology the ideal choice for

many applications including Library management.

The cost of the UHF tags is cheaper than other tags and is utilized in other

industry verticals by some of the world’s largest companies. EG Retail giant Wal-Mart

has mandated that their top 400 suppliers use UHF RFID in their supply chain at case and

pallet level. It is anticipated that over time that individual retail items may well be tagged.

The United States, Department of Defense is also using UHF RFID in its supply chain

[2].

Advantages of UHF (902-927 MHz) compared to HF (13.56 MHz)

Longer Read ranges.

Up to 4 meters and further.

Long read range in hand held devices.

1 meter is achieved reading more than 12 books at a time allowing quick effective

stock takes. Greater read ranges are also achieved dependant on application such

as searching for an individual item.

Speed of the frequency

Giving more book read per second.

27

Lower cost

UHF is targeted at a much lower cost than 13.56 MHz

Software adjustable readers

Gives fine tuning for distance and speed of reads. The new Gen 2 readers are also

able to run applications directly on the reader allowing remote programs running

in isolated locations.

UHF RFID benefits in Libraries.

Self Check out

Allowing faster check out of books as more can be read at the same time.

Auto sorting in returns

Provides sorting of books as they are returned.

Stock takes

Due to the read ranges of UHF readers, stock takes can achieved by walking

down the aisle, the RFID reader reads the tags on the books without being taken off a

shelf, the system can warn if a book looks to be out of place.

Book Search

If a book is not found in its correct location, a hand held PDA reader can be used

to search for the book. Read ranges of up to 1.5- 2 meters can be achieved with the PDA

readers which makes searching quick and easy.

Security

RFID brings an added feature of security. Gateways of 2.5 meters can be achieved

reducing the incidence of stolen books. UHF readers can read 100’s of tags a second,

many items can be checked as they are going through the security gates rather than only a

few.

Conclusion

This comparison of 13.56MHz and UHF for RFID systems show that to get

advantage of technology and implement it at cheaper level, UHF is the better choice.

28

3.2 Tag Selection

Following are the criteria for the selection of tags.

Frequency Of Operation

Power Source

Memory type and Size

Air Interface Protocol

Methods employed for Anti-Collision management

Operating Environment Support

All the above criteria are critical in deciding a tag for a specific application. The

data rates, speed, range and the size are dependent on the frequency. While the power is

critical in deciding the range and the advance features of tags.

Memory decides the purpose of tags and what services tags are going to give to

user. So if a tag is simply for detection then it’ll have so much small memory but for tags

of higher classes as explained in Chapter 2, large memory is required.

Air interface protocol is also application dependent. Anti-collision is usually used

where reader detects a number of tags at a time. The tags are also specified with respect

to their operating environment. It highly depends that on what material the tags are going

to be implanted.

3.2.1 Specification Of Tags Following specifications of tags are usually given in any tag datasheet. The

specification of Alien’s ALL Gen 2 squiggle family tags are specified below.

PRODUCT SPECIFICATIONS*

OPERATING FREQUENCY:

860-960MHz – optimized for 915MHz ISM Band

OPERATING MODE:

Passive

MEMORY: Gen 1 tags 128 bits total

User Programmable 96 bits * Taken from www.alien.com.

29

CRC 16 bits

Lock Code 8 bits

Kill Code 8 bits

MEMORY: Gen 2 tags 240 bits NVM

EPC size 96 bits

Protocol Control bits 16 bits

Lock Bits 10 bits

Kill Bit 1 bit

Access Code 32 bits

Kill Code 32 bits

Reserved 53 bits

MINIMUM PROGRAMMING CYCLES:

Gen 2 tags 10,000 write/erase cycles

Gen 1 tag > 25

OPERATIONAL TEMPERATURE RANGE:

Gen 2 tags -25º C to =65º C

Gen 1 tags -25ºC to +70º C

RECOMMENDED BENDING RADIUS:

70+ mm

THICKNESS:

Gen 2 tags: Over IC 0.42mm

Gen 1 tag: Over strap 0.41mm

Over antenna 0.20mm

Figure 3.3 shows some tags at different frequencies.

30

Figure 3.3* Tags at different frequencies.

3.2.1.1 Data Capacity

In terms of data capacity tags can be obtained that satisfy needs from single bit to

kilobits. The single bit devices are essentially for surveillance purposes. Retail electronic

article surveillance (EAS) is the typical application for such devices, being used to

activate an alarm when detected in the interrogating field. They may also be used in

counting applications.

Devices characterized by data storage capacities up to 128 bits are sufficient to

hold a serial or identification number together, possibly, with parity check bits. Such

devices may be manufacturer or user programmable. Tags with data storage capacities up

to 512 bits, are invariably user programmable, and suitable for accommodating

identification and other specific data such as serial numbers, package content, key

process instructions or possibly results of earlier interrogation/response transactions.

Tags characterized by data storage capacities of around 64 kilobits may be

regarded as carriers for portable data files. With increased capacity the facility can also

* Courtesy: www.google.com/images.

31

be provided for organizing data into fields or pages that may be selectively interrogated

during the reading process.

3.2.1.2 Data read rate

It has been already mentioned that data transfer rate is essentially linked to carrier

frequency. The higher the frequency, the higher would be the data transfer rates. It should

also be appreciated that reading or transferring the data requires a finite period of time,

even if rated in milliseconds, and can be an important consideration in applications where

a tag is passing swiftly through an interrogation or read zone.

3.2.1.3 Data Programming Options

Depending upon the type of memory a tag contains the data carried may be read-

only, write once read many (WORM) or read/write. Read-only tags are invariably low

capacity devices programmed at source, usually with an identification number. WORM

devices are user programmable devices. Read/write devices are also user-programmable

but allowing the user to change data stored in a tag. Portable programmers may be

recognized that also allow in-field programming of the tag while attached to the item

being identified or accompanied.

3.2.1.4 Physical Form

RFID tags come in a wide variety of physical forms, shapes sizes and protective

housings. Animal tracking tags, inserted beneath the skin, can be as small as a pencil lead

in diameter and ten millimeters in length. Tags can be screw-shaped to identify trees or

wooden items, or credit-card shaped for use in access applications. The anti-theft hard

plastic tags attached to merchandise in stores are also RFID tags, as are heavy-duty 120

by 100 by 50 millimeter rectangular transponders used to track inter-modal containers, or

heavy machinery, trucks, and railroad cars for maintenance and tracking applications.

3.2.1.5 Costs

The cost of tags obviously depends upon the type and quantities that are

purchased. For large quantities (tens of thousands) the price can range from less than a

32

few tens of pence for extremely simple tags to tens of pounds for the larger and more

sophisticated devices.

The manner in which the tag is packaged to form a unit will also have a bearing

on cost. Some applications where harsh environments may be expected, such as steel

mills, mines, and car body paint shops, will require mechanically robust, chemical and

temperature tolerant packaging. Such packaging will undoubtedly represent a significant

proportion of the total transponder cost.

3.2.2 Comparison of Tag market RFID tags at different frequencies manufactured by different vendors.

125 KHz TI Philips Others

13.56 MHz Tagsys Philips TI Microchip Others

915 MHz Intermec SCS Matrics Alien Philips TI

2.4 GHz Intermec SCS Hitachi

Figure 3.4* tags manufactured by different vendors

3.2.3 Tags For Library Application For library Application two types of tags are now in use now a day. One type of

tags is used for circulation desk and for handheld reader while the other type of tags is

detected only at security exits.

Tags detected at circulation desk

* Courtesy: www.RFID.com

33

The tags which are used to detect the books at circulation desk should their EPC

code which shows the uniqueness of the tag and the book. And this tag gives it code after

the complete handshake with the reader for security reasons. So class 1 or class 2 tags are

used to be detected at circulation desk.

Tags detected at exit

At the library exits the tags only have to show their presence by just giving

response to reader power signals and some handshaking protocol. No need to send the

code, so class 0 tags can be used for such purposes.

A better solution propose by us

Instead of using two types of tags, simply a single tag can be used for both

purposes i.e. circulation desk and at exit. Only to modify the software that it will compare

the folder of issue books with the books at the exit. If the tag number is present in the

issued book folder than the reader at exit will not invoke the alarm but if it is not then

beep is blown out. So using UHF band class 1 Gen2 tags are the better option. Figure 3.5

shows a class 1 Gen 2 tag from Alien Technologies. This tag can be used for library

application.

Figure 3.5* An Alien Class 1 Gen 2 tag for library.

3.3 Antenna Selection

As the frequency of choice for RFID devices rises into the microwave region, the

problem of designing antennas to match the devices on the protected object becomes

more acute. The objective of any such antenna must be to maximize the transfer of power

into and out of the device on the protected object. This requires careful design to match

the antenna to free space. The major considerations in choosing an antenna are: the type

of antenna; its impedance; RF performance when applied to the object; and RF

performance when the object has other structures around it. Antennas which are omni

* Courtesy: Alien technologies website.

34

directional should be avoided and, wherever possible, directional antennas should be used

because they have the advantage of fewer disturbances to the radiation pattern and to the

return loss.

Antenna Specifications

Following are the critical factors in deciding an appropriate antenna.

Frequency

Return Loss

Gain

Polarization

Power Handling

Input Impedance

3.3.1 Frequency The antenna Frequency should be in the required range. It should work over a

bandwidth of frequencies instead of just a single point. For low frequency applications

inductive coils are used as coupling elements but for HF and higher frequencies, proper

antennas are used.

3.3.2 Return Loss The return loss of the antenna at the specified band is normally -20 to -30 dB for

good operation. For such return loss the VSWR is 1.4 to 1.1. The lower the VSWR and

the return loss better the performance of the antenna and the system.

3.3.3 Gain The gain for UHF band is typically 6dBi or 8 dBi. dBi units show the gain of

directional antenna with reference to isotropic antenna. Gain is also specified in the units

of dBc and dBd , gain with reference to circular polarization and dipole antenna.

3.3.4 Polarization Polarization of the antenna is dependent on the polarization of the tag antenna. If

the tag antenna is linear polarized then to get maximum power efficiency the polarization

of the reader antenna should be linear. If tag is circularly polarized then the reader

35

antenna should also be circularly polarized. Usually tags are circularly polarized for

better performance.

3.3.5 Power Handling The antenna should be capable to sustain the maximum power transmitted without

heating or any other performance degradation.

3.3.6 Input Impedance Usually the RF system operates with standard input and output 50 ohm

impedance. So the antenna input impedance should be matched to the feeding system

output impedance which is usually 50 ohm to minimize the return loss.

Figure 3.6 shows a typical antenna for UHF RFID.

Figure 3.6 circularly polarized 900MHz Panel antenna by POYNTING Co.

3.4 Modulation Schemes

The communication techniques for the RFID systems are

Load Modulation

36

Backscatter Modulation

3.4.1 Load Modulation In low frequency RFID systems the inductive coupling is used which is based

upon a tag type coupling between the primary coil in the reader and the secondary coil in

the tag. This only happens when the tag is in the near field of the reader antenna. When a

transponder or tag is placed within the magnetic field of the reader’s antenna the tag

draws energy from the magnetic field. The resulting feedback of the tag on the reader’s

antenna can be represented as transformed impedance in the antenna coil of the reader.

Switching a load resistor on and off at the tag’s antenna therefore brings about a change

in the impedance, and thus voltage changes at the reader’s antenna. This has the effect of

an amplitude modulation of the voltage at the reader’s antenna coil by the remote tag. If

the timing with which the load resistor is switched on and off is controlled by data, this

data can be transferred from the tag to the reader. This type of data transfer is called load

modulation [2].

Figure 3.7* Inductive coupling between reader and tag.

3.4.2 Backscatter Modulation For distance between tag and the reader greater than 1m backscatter modulation is

used. For frequencies higher than 13.56MHz, for RFID, backscatter modulation is used.

* Courtesy: RFID Handbook, 2nd Edition by Klaus Finkenzeller.

37

The backscattering modulation technique is based on the variation of the

reflection coefficient at the input of tag. Reflection coefficient can vary either in

amplitude or in phase. As a result, two modulation types are possible: ASK (Amplitude

Shift Keying) and PSK (Phase Shift Keying).

ASK

To implement ASK, a reflected power is switched between two or more values at

a given rate. In the case of passive tags and considering the amount of date to transmit, a

Binary Amplitude Shift Keying is sufficient.

PSK

PSK backscattering modulation relies on the relative phase variation between the

in coming and the reflected waves at the antenna. The tag switches its input impedance

between two values, ensuring the widest modulation angle between the power waves

being reflected in the two modulation states.

Comparison Of ASK and PSK

PSK modulation is superior to ASK by a factor of .74dB which is rather small

difference at the cost of a more complex implementation. The impedances necessary to

implement PSK induce a smaller available voltage at the rectifier input compared to

ASK. This results in a lower sensitivity for PSK. This effects the maximal operating

range and in that sense, ASK can be advantageous. Furthermore, the average power

available to the tag is the same in both cases when considering optimal parameters. This

shows that there is no clear advantage in implementing PSK rather than ASK in the frame

work of passive tags [3].

It is well known that BPSK is superior to BASK in terms of BER, but in remotely

powered applications where backscattering modulation is used, there exists a strict trade

off between the power part given to the application and the power dedicated to the

communication [3].

3.4.2.1 Explanation of Backscatter Modulation

Power P1 is emitted from the reader’s antenna, a small proportion of which (free

space attenuation) reaches the tag’s antenna. Backscatter modulation is depicted in

38

figure3.9. The power P1

is supplied to the antenna connections as HF voltage and after

rectification by the diodes D1 and D2 this can be used as turn-on voltage for the

deactivation or activation of the power saving ‘power down’ mode. The diodes used here

are low barrier Schottky diodes, which have a particularly low threshold voltage. The

voltage obtained may also be sufficient to serve as a power supply for short ranges.

A proportion of the incoming power P1

is reflected by the antenna and returned as

power P2.The reflection characteristics(=reflection cross-section) of the antenna can be

influenced by altering the load connected to the antenna. In order to transmit data from

the tag to the reader, a load resistor RL connected in parallel with the antenna is switched

on and off in time with the data stream to be transmitted. The amplitude of the power P2

reflected from the tag can thus be modulated (modulated backscatter).

The power P2 reflected from the tag is radiated into free space. A small proportion

of this (free space attenuation) is picked up by the reader’s antenna. The reflected signal

therefore travels into the antenna connection of the reader in the back-wards direction and

can be decoupled using a directional coupler and transferred to the receiver input of a

reader. The forward signal of the transmitter, which is stronger by powers of ten, is to a

large degree suppressed by the directional coupler.

Figure 3.8* Backscatter modulation.

3.5 Reader Design

Functions performed by the reader may include quite sophisticated signal

conditioning, parity error checking and correction. Once the signal from a tag has been

correctly received and decoded, algorithms may be applied to decide whether the signal is

a repeat transmission, and may then instruct the tag to cease transmitting. This is known

* Courtesy: RFID Handbook, 2nd Edition by Klaus Finkenzeller.

39

as the "Command Response Protocol" and is used to circumvent the problem of reading

multiple tags in a short space of time. Using readers in this way is sometimes referred to

as "Hands Down Polling". An alternative, more secure, but slower tag polling technique

is called "Hands Up Polling" which involves the reader looking for tags with specific

identities, and interrogating them in turn. This is contention management, and a variety of

techniques have been developed to improve the process of batch reading. A further

approach may use multiple readers, multiplexed into one reader, but with attendant

increases in costs.

Mainly there are two parts of RFID readers:

1. Hardware Portion: RF (Carrier) part

2. Software Portion: Microcontroller part

Figure 3.9 shows a typical RFID reader anatomy.

Digital Signal

Figure 3.9* A typical reader anatomy.

3.5.1 RF (Carrier) part The main purpose of the RF part is to make power at the input of the antenna of

reader available to it communicate with the tags. Modulation of the data is also done in

the RF part of the reader. It has two parts:

Transmitting part * Courtesy: www.google.com/images.

915MHz Radio

Network Processor

Processor (DSP)

Power Supply

13.56MHz Radio

40

http://www.google.com/

Receiving part.

On transmitting part frequency synthesizers, drivers, filters, modulators and the

power amplifiers are used. The design of these parts depends upon the frequency ranges,

type of modulation and the power transmitted. The components should be well matched

for the maximum power transfer and minimum losses.

On the receiving side LNA (low noise amplifier, filter, down converter and

demodulators are used. The design of these components is also dependent on the same

factors on which the transmitter components depend. Receiver side is usually more

critical as the noise level is so much high and the Signal to noise ratio (SNR) is very

much low. So on receiver side the components should have low losses.

The power level of UHF RFID is given in the following lines.

Reader Transmit Power rP = 30dBm (1 Watt)

Reader Receiver Sensitivity rS = -80dBm

Reader Antenna Gain rG = 6dBi

Tag Power Requirement tP = -10dBm (100 microwatts)

Tag Antenna Gain tG = 1dBi

Tag Backscatter Efficiency tE = -20dB

System operating wavelength = 33cm (915MHz)

3.5.1.1 UHF read range estimation

Well designed systems are tag power limited.

ttrr

trrt

PGGPd

dGGPP

2)4()(

2)4(

2max

22

dmax = 5.8 meters, theoretical maximum

3.5.1.2 Reader sensitivity limit

Let’s assume we can build a tag IC requiring 1 microwatt (100 times better than

current practice)

dmax = 194 meters tag power limit for this hypothetical IC.

41

dBmrP

dEGGPrP

t

ttrrt

99

2)4( 22

3.5.1.3 Conclusion

Since Pt 1/d2 , doubling read range requires 4X the transmitter power.

Larger antennas can help, but at the expense of larger physical size because

Gt,r Area.

More advanced CMOS process technology will help by reducing Pt.

At large distances, reader sensitivity limitations dominate.

3.5.2 Microcontroller Part The purpose of microcontroller part is to do all type of processing. The main

handshaking protocols and the detection protocols are implemented in microcontroller.

On transmitting side the modulation is done from the microcontroller data while on the

receiving side it detects the data from tag and processes it. After processing the data it

sends this data to host computer where application software further process it. For UHF

applications EPC Global protocol is implemented. So in choosing the microcontroller, the

clock frequency, memory and the out put ports should be considered.

3.5.3 Specifications of Reader Typical specifications of Alien class 1 Gen 2 upgrade reader are given below.

PRODUCT SPECIFICATIONS*

FREQUENCY:

865.6–867.6 MHz

POWER:

2 Watts ERP

REGULATORY COMPLIANCE :

Carries the CE mark. EM Compatibility: EN 302- 208, EN 301 489; Safety: EN

60950, EN 50364


42

RFID PROTOCOL:

EPC Class 1,

EPC Class 1 Gen 2 upgrade

ANTENNAS:

4-ports, circular or linear polarization, 6 dBi, 6 meter cables, reverse polarity TNC

POWER SUPPLY:

12VDC, 5A (unregulated).

90-264 VAC input at 47-63 Hz.

COMMUNICATION INTERFACE:

Serial RS232; 9-pin, Sub D (female)

LAN INTERFACE:

10baseT Ethernet

GENERAL PURPOSE INPUTS/OUTPUTS:

4 programmable logic I/O

INDICATORS:

Reader (On), Serial (Tx, Rx), LAN (Link, Active), Tag (Sniff, Lock), RF (On)

OPERATING TEMPERATURE:

0º C to +50º C

STORAGE TEMPERATURE:

-20º C to +70º C

DIMENSIONS:

30.4 cm x 22.9 cm x 4.4 cm

43

WEIGHT:

1.81 KG

Figure 3.10 shows physical structure of Alien class 1 Gen 2 reader.

Figure 3.10* An Alien Class 1 Gen 2 reader.


44

4 RFID System Implementation

(Part I)

4.1 Implementation OF RFID System

In our project we have designed and implemented the RFID reader. Our reader

has two parts.

1. Hardware portion: RF (Carrier) part

2. Software portion: Microcontroller part

The block diagram of figure 4.6 shows reader for RFID system.

4.1.1 RF (Carrier) Part This portion consists of two sub portions.

Transmitter

Receiver

45

M

Figure 4.6 A block diagram of RFID system reader

The frequency for this system is 868 MHz. The power level at the antenna input is

30 dBm which is according to the standards of EPC Global. So the designing of the RF

portion is all according to standards of EPC Global. The transmitter has following parts.

I. Integrrated PLL and VCO frequency synthesizer

II. Driver for power amplifier

III. Power amplifier

IV. Transistor switch for OOK

4.1.1.1 Integrated PLL and VCO Frequency Synthesizer

We have used the integrated PLL and VCO synthesizer made by Analog Devices

for generating the carrier signal. The ADF4360-7 is an integrated integer-N synthesizer

and voltage controlled oscillator (VCO). The ADF4360-7 center frequency is set by

external inductors. This allows a frequency range of between 350 MHz to 1800 MHz. In

addition, a divide-by-2 option is available, whereby the user receives an RF output of

between 175 MHz and 900 MHz.

Control of all the on-chip registers is through a simple 3-wire interface. The

device operates with a power supply ranging from 3.0 V to 3.6 V and can be powered

POWER

AMPLIFIER

ANTENNA

Driver

SWITCH FOR ASK

Receiver Chip TH7122.2

I C

VCO

R O C O N T A

NTENNA

R O L L E R

46

down when not in use. Detailed description of synthesizer can be found in its datasheet in

appendix.

Figure 4.7 Functional Block Diagram of Synthesizer

Evaluation Board

We have used the evaluation board of ADF 4360-7 in our project. It is a self

contained board for generating RF frequencies. It contains the ADF4360-7BCP, a PC

connector, plus SMA* connectors for the RF outputs. Unpopulated SMA footprints are

available for the power supplies, Chip enable (CE) and external reference input. It also

contains the loop filter to complete the PLL. The evaluation board has the flexibility for

changing external inductor to allow different VCO output frequency ranges.

Courtesy: Analog Devices * A type of high frequency connector.

47

Figure 4.8 ADL4360-7 Evaluation board.

Operation of Frequency Synthesizer for RFID Reader

The evaluation board can be used according to the requirements by configuring it

through software. The ADL4360-7 is connected to the computer with a serial to parallel

connector. Snapshot of software window is shown in figure 4.9. When the software

window was opened, first of all we specified the evaluation board which was ADL 4360-

7. This evaluation board would be automatically displayed in device in use and

Evaluation board tab. We have used the frequency synthesizer at out put frequency of

868 MHz, the frequency of our reader. Frequency was set by clicking RF VCO Output

Frequency button. The power level was -3dBm which is the maximum possible of

ADL4360-7.Core Power Level was set at 5 mA which is the minimum level of current.

48

We could use the high value but 5mA was enough for our application. RF Prescaler value

was selected to be 16/17 according to datasheet*.

The 14-bit R counter allows the input reference frequency to be divided down to

produce the reference clock to PFD (Phase Frequency Detector). Division ratios from 1 to

16,383 were allowed. A and B counters combinee with prescaler to allow a wide range of

division ratio in PLL feedback counter.Prescaler along with counters A and B enabled the

large division ratio,N, to be realized .The PFD takes inputs from the R counter and N

counter and produces an output proportional to the phase and frequency difference

between them. The PFD includes a programmable delay element that controls the width

of antibacklash pulse. This pulse ensured that there was no dead zone in PFD transfer

function and minimized phase noise and reference spurs. VCO core used eight

overlapping bands to allow a wide frequency range to be covered without a large VCO

sensitivity. The correct band was chosen automatically by the band select logic at power-

up or whenever N counter latch was updated. The registers were loaded with the bit

patterns according to datasheet to get the required results. The correct programming

sequence for ADF 4360-7 is R counter latch, control latch and N counter latch

respectively. On initial power up, an interval was required between programming control

latch and N counter latch. This interval was necessary to allow the transient behavior

during initial power-up to settle. After that, synthesizer could be removed from cable and

used stand alone. Further details about synthesizer working can be found in appendix at

the end of the chapter and the datasheet of ADL4630-7. The rest of buttons (CP Gain,

three state output, power down settings etc.)were not changed by us. We used the default

values of those fields for 868 MHz frequency.

* Data sheet can be downloaded from www.analog.com.

49

Figure 4.9 Snap shot of the ADL4360-7 programming software.

4.1.1.2 Driver for Power Amplifier

After getting signal from VCO we have used a gain block which is in fact an

amplifier which increases the power level of our carrier signal from 1 dBm to 17 dBm.

The reason of using driver before power amplifier is that power amplifier is usually not

capable of amplifying very low level of power. So a driver in fact amplifies the very low

power level to the power level sufficient for the input of power amplifier. We are

modulating the RF carrier signal, based on the signal coming from microcontroller, by

switching the supply of driver. We used evaluation board ADL 5530 manufactured by

Analog Devices, for this purpose. The ADL5530 is a broadband, fixed-gain, linear

amplifier that operates at frequencies up to 1000 MHz. The ADL5530 provides a gain of

16.5 dB. It achieves an third order intercept point (OIP3) of 37 dBm with an output

compression point of 21.8 dB and a noise figure of 3 dB.

50

This amplifier is single-ended and internally matched to 50 Ω with an input return

loss of 11 dB. Only input/output ac-coupling capacitors, a power supply decoupling

capacitor, and an external inductor are required for operation .The ADL5530 operates

with supply voltages of 3 V or 5 V with a supply current of 110 mA.Its functional block

diagram is shown in Figure 4.10. The complete data sheet of the ADL5530 is given in

appendix.

Figure 4.10 Functional block diagram of ADL5530

Evaluation Board

We have used the evaluation board of ADL5530 in our application. Figure 4.11

shows the schematic for the ADL5530 evaluation board. The board is powered by a

single supply (between 3 V and 5 V).

Figure 4.11 Schematic of the Evaluation board of ADL5530.

Courtesy: Analog Devices

51

Power can be applied to the board through clip-on leads (J5, J6), through an edge

connector (P1), or through Jumper W1. IN2, OUT2, T1, T2, C6, C7 and C10 have no

function. Because Pin 1, Pin 3 and Pin 6 of ADL5530 are No Connects, these pins are

grounded on this PCB (this has no effect on electrical performance). Evaluation board

layout is given in Figure 4.12.

Figure 4.12 Evaluation Board of ADL5530

4.1.1.3 Power Amplifier

We have used a power amplifier after the driver .It increases the power level upto

30 dBm which is then coupled to antenna for transmission. We have used the power

amplifier ADL5322 manufactured by Analog Devices. The ADL5322 is a high linearity

GaAs driver amplifier that is internally matched to 50 Ω for operation in the 700 MHz to

1000 MHz frequency range. A functional block diagram of power amplifier is shown in

Figure 4.13.

Figure 4.13* Functional block diagram of ADL5322.

Courtesy: Analog Devices

52

Evaluation Board

We have used the evaluation board of ADL5322 for our application. Figure 4.14

shows the schematic of the ADL5322 evaluation board.

Figure 4.14 Schematic of the evaluation board of ADL5322.

The board is powered by a single supply in the 4.75 V to 5.25 V range. The power

supply is decoupled on each of the three power supply pins by 10 μF, 10 nF, and 100 pF

capacitors. All three VCC pins (Pin 1, Pin 2, and Pin 5) should be independently

bypassed for proper operation. Figure 4.15 is a snapshot of evaluation board.

Figure 4.15 Evaluation Board of ADL5322.

Courtesy :Analog Devices

53

4.1.1.4 Transistor Switch for OOK

We have used a transistor switch for modulating the RF carrier signal form VCO

by the signal coming from microcontroller. This switch receives microcontroller signal at

its input, which is in the form of pulses, and it switches the power supply of driver

accordingly. Hence in this way OOK (On Off Keying) is achieved.

To design this switch we have considered the following parameters which should

be fulfilled by the transistor switch in order to properly switch the driver which is driving

the power amplifier.

Switching Speed

The data for transmission comes from the microcontroller at a rate of 128

KHz(maximum value). The data appears to be in the form of square wave showing zeros

and ones of a particular coding scheme. So the high frequency response of the transistor

should not be distorted. Usually MOSFETs have good frequency response than the BJTs.

A poor frequency response of a transistor causes charging and discharging of the junction

capacitances to become prominent in the output switching waveform. The collector of

low frequency transistor does not switch according to the switching speed of its base.

Figure 4.16 shows frequency response of a low frequency transistor.

Time

Amplitude

Figure 4.16 Waveform of transistor having distorted high frequency response

54

So we selected the MOSFET IRFZ44N for switching the power supply. It has

very low on-resistance up to 0.5 ohm. It can operate well over our frequency which is 128

KHz.

Driving Circuit

The output (5V) signal from the microcontroller is of very low power with low

current. If the microcontroller is directly attached to the IRFZ44N it will be loaded and

the this voltage level is not sufficient enough to drive the MOSFET IRFZ44N (although it

can turn it on). A driving circuit is necessary to drive the transistor specially the

MOSFET. At least 10 volts should be applied at MOSFET gate to properly drive it and

let the required current of 325mA pass through it which is the rated current for power

amplifier driver.

The frequency response of the driving circuit should also be distortion free at out

operational frequencies (about 130 KHz). At first we were using the simple BJT BC547

to drive the MOSFET. Its frequency response at 128 KHz was acceptable but when the

output is attached to the gate of MOSFET, the whole voltage was sourced (constant at

5V).The reason for this was found to be the high capacitance between the gate and the

source of the MOSFET. A simple BJT transistor could not give the sinking path for the

MOSFET. A simple transistor could not drive a MOSFET properly because to drive it

properly, proper source and sink is required.

We tried CMOS circuits to drive the MOSFET. To make our circuit simple and

economical, we used two transistors in push pull topology. This push pull worked and

gave the output voltage which was not sourced as was with the case of using single BJT.

But it could not give the output voltage greater than its input voltages.Hence using a

driving circuit in the form of push pull topology of BJTs was also an unacceptable

approach for our problem. We ended up with two appropriate solutions.

In first approach, we used the push pull topology accompanied another transistor

before each PNP and NPN transistor as inverter. This scheme made sure that the emitter

of the two output driving transistors (push-pull), which were C945 (NPN) and A1015

(PNP) in our case had more than 10 volts at their emitters if the Vcc is 12 and -12 volts.

Figure 4.17 shows the above scheme to drive the MOSFET IRFZ44N.

55

Figure 4.17 A driving circuit to drive the power MOSFET

The second option was to use the operational amplifier based circuit which

switches the power supply between +12 and -12. We have used this technique to drive the

IRFZ44N. This technique worked well and we successfully switched the supply of driver

for power amplifier. Figure 4.18 shows the whole circuit diagram of our switch.

+12

+5V

Figure 4.18 Circuit diagram of On Off Keying (OOK) switch

56

Current Rating

Another important factor in designing and implementing the switch was the

current capability of the switching transistor. The MOSFET IRFZ44N have current rating

of its drain current up to 49 amperes. While we required only 340mA maximum. So this

MOSFET worked well and switched supply of driver with out sinking the output voltage.

Common Source Configuration

We used the MOSFET in the common source configuration. This configuration

has low output impedance and delivers the high currents to the load without sinking the

voltage. This configuration is also used in impedance matching and for buffer purposes as

well.

The output wave forms of the switch are given in 6th chapter.

4.1.2 Receiver In our reader we have used a transceiver chip. We are using only its receiver part.

It is TH7122.2 by MELEXIS. The TH7122 is a single chip FSK/ASK transceiver IC.

Figure 4.7 shows its pin description. It is designed to operate in low power multi channel

programmable or single channel stand-alone, half duplex data transmission systems. It

can be used for applications in automotive, industrial-scientific-medical (ISM), short

range devices (SRD) or similar applications operating in the frequency range of 300MHz

to 900MHz. In programmable used mode, the transceiver can operate down to 27MHz by

employing an external VCO varactor diode.

It has following features:

Single chip solution with only few external components.

Stand alone fixed frequency user mode.

Programmable multi channel user mode.

Low current consumption in active mode and very low standby current.

PLL stabilized RF VCO (LO) with internal varactor diode.

Lock detect output in programmable user mode.

On chip AFC for extended input frequency acceptance range.

3wire bus serial control interface.

FSK/ASK mode selection.

FSK for digital data or FM for analog signal reception.

57

RSSI output for signal strength indication and ASK reception

Peak detector for ASK detection.

Switch able LNA gain for improved dynamic range.

32-pin low profile Quad Flat Package (LQFP).

Figure 4.19* pin description of TH7122

4.1.2.1 Operational Theory

The main building block of the transceiver is a programmable PLL frequency

synthesizer that is based on an integer-N topology. The PLL is used for generating the

carrier frequency during transmission and for generating the LO signal during reception.

The carrier frequency can be FSK-modulated either by pulling the crystal or by

modulating the VCO directly. ASK modulation is done by on/off keying of the power

amplifier. The receiver is based on the principle of a single conversion superhet. In

receive mode, the default LO injection type is low-side injection. The figure 4.20 shows

the block diagram of the TH7122.

The TH7122 transceiver IC consists of the following building blocks:

Low-noise amplifier (LNA) for high-sensitivity

RF signal reception with switchable gain

Mixer (MIX) for RF-to-IF down-conversion

* Courtesy: Analog devices

58

IF amplifier (IFA) to amplify and limit the IF signal and for RSSI

generation

Phase-coincidence FSK demodulator with external ceramic discriminator

or LC tank

Operational amplifier (OA1), connected todemodulator output

Operational amplifier (OA2), for general use

Peak detector (PKDET) for ASK detection

Control logic with 3wire bus serial control interface (SCI)

Reference oscillator (RO) with external crystal

Reference divider (R counter)

Programmable divider (N/A counter)

Phase-frequency detector (PFD)

Charge pump (CP)

Voltage controlled oscillator (VCO) with internal varactor

Power amplifier (PA) with adjustable output power

Figure 4.20* TH7122 block diagram.

User Modes

The transceiver can operate in two different user modes. It can be used either as a

3wire-bus-controlled programmable or as a stand-alone fixed-frequency device. After

power up, the transceiver is set to Standalone User Mode (SUM). In this mode, pins

* Taken from TH7122 datasheet.

59

FS0/SDEN and FS1/LD must be connected to VEE or VCC in order to set the desired

frequency of operation. There are 4 pre-defined frequency settings: 315MHz,

433.92MHz, 868.3MHz and 915MHz. The logic level at pin FS0/SDEN must not be

changed after power up in order to remain in fixed-frequency mode.

A mode control logic allows several operating modes. In addition to standby,

transmit and receive mode, two idle modes can be selected to run either the reference

oscillator only or the whole PLL synthesizer. The PLL settings for the PLL idle mode are

taken over from the last operating mode which can be either receive or transmit

mode.The different operating modes can be set in SUM and PUM (Programmable User

Mode) as well. In SUM the user can program the transceiver via control pins RE/SCLK

and TE/SDTA. In PUM the register bits OPMODE are used to select the modes of

operation while pins RE/SCLK and TE/SDTA are part of the SCI.

Frequency Resolution and Operating Frequency

At a given frequency resolution fR, the maximum operating frequency of the VCO

is limited by the maximum N-counter setting. The table 4.1 below provides some

illustrative numbers.

Table 4.1* Frequency resolution and operating frequency.

Crystal

frequency fro

Frequency

resolution fr

R counter

N counter

Operating

frequency fvco

3.0000MHz 2.93kHz 1023 13107 38.437MHz 3.0000MHz 2.93kHz 1023 131071 384.372MHz 8.0000MHz 12.5kHz 640 35812 447.65MHz 8.0000MHz 25kHz 320 34746 868.65MHz 8.0000MHz 250kHz 32 3660 915.0MHz

Receiving Side of the Chip TH7122

The RF front-end of the receiver part is a super-heterodyne configuration that

converts the input radiofrequency (RF) signal into an intermediate frequency (IF) signal.

The most commonly used IF is 10.7 MHz, but IFs in the range of 0.4 to 22 MHz can also * Taken from TH7122 datasheet.

60

be used. According to the block diagram, the front-end consists of a LNA, a Mixer and an

IF limiting amplifier with received signal strength indicator (RSSI). The local oscillator

(LO) signal for the mixer is generated by the PLL frequency synthesizer.

As the receiver constitutes superhet architecture, there is no inherent suppression

of the image frequency. It depends on the particular application and the system’s

environmental conditions whether an RF front-end filter should be added or not. If image

rejection and/or good blocking immunity are relevant system parameters, a band-pass

filter must be placed either in front or after the LNA. This filter can be a SAW (surface

acoustic wave) or LC-based filter (e.g. helix type). This filter is implemented in the

evaluation board of TH7122 manufactured by us.

LNA

The LNA is based on a cascode topology for low-noise, high gain and good

reverse isolation. The open collector output has to be connected to an external resonance

circuit which is tuned to the receive frequency. The gain of the LNA can be changed in

order to achieve a high dynamic range. There are two possibilities for the gain setting

which can be selected by the register bit LNACTRL. External control can be done via the

pin GAIN_LNA, internal control is given by the register bit LNAGAIN. In case of

external gain control, a hysteresis of about 340 mV can be chosen via the register bit

LNAHYST. This configuration is useful if an automatic gain control loop via the RSSI

signal is established. In transmit mode the LNA-input is shorted to protect the amplifier

from saturation and damaging.

Mixer

The mixer is a double-balanced mixer which down converts the receive frequency

to the IF. The default LO injection type is low side (fVCO = fRX . fIF). But also high side

injection is possible (fVCO = fRX + fIF). In this case, the data signal’s polarity is inverted due

to the mixing process. To avoid this, the transmitted data stream can be inverted too by

setting DTAPOL to .1.

The output impedance of the mixer is about 330Ω in order to match to an external

IF filter.

61

IF Amplifier

After passing the channel select filter which sets the IF bandwidth the signal is

limited by means of an high gain limiting amplifier. The small signal gain is about 80 dB.

The RSSI signal is generated within the IF amplifier. The output of the RSSI signal is

available at pin RSSI. The voltage at this pin is proportional to the input power of the

receiver in dBm. Using this RSSI output signal the signal strength of different

transmitters can be distinguished.

ASK Demodulator

The receive part of the TH7122 allows for two ASK demodulation configurations:

standard ASK demodulation

ASK demodulation with peak detector.

The default setting is standard ASK demodulation. In this mode SW1 and SW2

are closed and the RSSI output signal directly feeds the data slicer setup by means of

OA1*. The data slicer time constant equals to

T = 200kΩ·C3,

with C3 external capacitor to pin INT1. This time constant should be larger than

the longest possible bit duration of the data stream. This is required to properly extract

the ASK data’s DC level. The purpose of the DC (or mean) level at the negative input of

OA1 is to set an adaptive comparator threshold to perform the ASK detection.

Alternatively a peak detector can be used to define the ASK detection threshold.

In this configuration the peak detector PKDET is enabled, SW1 is closed and SW2 is

open, and the peak detector output is multiplexed to pin INT2/PDO. This way the peak

detector can feed the data slicer, again constituted by OA1 and a few external R and C

components. The peak detection mode is selectable in programmable user mode.

4.1.2.2 Pin selection of TH7122 for desired results

As described above the chip can be used either in stand alone mode or user

programmable mode. In stand alone mode the specific pins as described in the next

section should be given proper biasing. We have used this device in user programmable

mode as we are using it for RFID system where there is always a host computer attached * See datasheet of TH7122 for pin description.

62

to the reader. In this way there is no problem of managing a separate computer for chip

operation. Following sections show the details of both modes of operation.

Specification of pins for Stand Alone user mode

After power up the transceiver is set to stand-alone user mode. In this mode, pins

FS0/SDEN and FS1/LD must be connected to VEE or VCC to set the desired frequency of

operation. The logic level at pin FS0/SDEN must not be changed after power up in order

to remain in stand-alone user mode. The default settings of the control word bits in stand-

alone user mode are described in Table 4.2.

Table 4.2 Selection of frequency.

Channel

Frequency 433.92MHz 868.3MHz 315MHz 915MHz

FSO/SDEN 1 0 1 0

FS1/LD 0 0 1 1

In stand-alone user mode, the transceiver can be set to Standby, Receive,

Transmit or Idle mode (only PLL synthesizer active) via control pins RE/SCLK and

TE/SDTA which is depicted in Table 4.3. The modulation scheme and the LNA gain are

set by pins ASK/FSK and GAIN_LNA, which are depicted in Table 4.4 and Table 4.5

respectively.

Table 4.3 Operation Mode

Operation

Mode Stand by Receiver Transmit Idle

RE/SCLK 0 1 0 1

TE/SDATA 0 0 1 1

Table 4.4 Modulation Type

Modulation Type ASK FSK

ASK/FSK 0 1

Table 4.5 LNA gain mode LNA gain High Low

63

Gain_LNA 0 1

Programmable User Mode Operation

The transceiver can also be used in programmable user mode. After the first logic

level change at pin FS0/SDEN, the transceiver enters into Programmable User Mode

(PUM). In this mode, the user can set any PLL frequency or mode of operation by the

SCI (Serial Control Interface). In SUM pins FS0/SDEN and FS1/LD are used to set the

desired frequency, while in PUM pin FS0/SDEN is part of the 3-wire serial control

interface (SCI) and pin FS1/LD is the look detector output signal of the PLL synthesizer.

When the power is switched on the default mode is stand alone user mode but by

changing the logic as described above, the chip enters into the user programmable mode.

In this mode we have set the frequency 868.3 MHz by software. We made

following settings in the software.

The modulation type was ASK

Frequency was 868.3 MHz.

IF was selected to 10.7 MHz which is by default set by the software.

The register values changed to the default settings of the 868.3 MHz.

The mode of operation was receiving mode.

Data polarity was set to positive.

Peak detector option was chosen.

Our frequency of operation was greater than 500MHz. So we selected the

IRF>500MHz option. The detail of IRF can be found in datasheet of

TH7122.

The main window of the software TH7122 made all the necessary settings

for specific frequencies.

The other windows of the software are for advanced settings. If somebody wants

some frequency other than the default, then that person have to calculate the register

values and fix it in the register columns. The details of the software settings are given in

the datasheet. The register description is also given in the Appendix. We have used the

standard frequency, so with some modifications, which are explained above, we have

64

used the default register values. The figure 4.21 to figure 4.23 shows different snapshots

of the software of TH7122.

Figure 4.21 Main window of TH7122 software

Figure 4.22 Register view of TH7122 software

65

Figure 4.23 Extended parameters window

Register description

In Stand-alone User Mode, the intrinsic default values with respect to the applied

levels at pins FS0 and FS1 lay down the configuration of the transceiver. In

Programmable User Mode, the register settings can be changed via 3-wire interface SCI.

The default settings which vary with the desired operating frequency depend on the

voltage levels at the frequency selection pins FS0 and FS1 before entering the PUM. It

should be noted that the channel frequency which is listed in the Table 4.4 will be

achieved with a crystal frequency of 7.1505 MHz. The table in the appendix depicts an

overview of the register configuration of the TH7122.

4.1.2.3 Operation of RFID Reader

We have used the chip in the user programmable mode as it gives more options to

get good results. As described above, no new computer is required for reader application.

We can also use this chip in standalone mode because we are working at 868 MHz which

can be achieved by stand alone mode. Stand alone mode is preferable where the system

does not require any update after installation But for the applications like RFID reader, as

the device should be programmable, so we have preferred User Programmable mode and

used the transceiver chip in receiver mode.

66

The tags chosen for our project are ASK modulated so we required an ASK

demodulator. This functionality was achieved by TH7122 chip. There are two modes of

operation for the ASK demodulation. One is the simple data slicer while the other one is

peak detector option. The data slicer can also work well for return to zero but for NRZ

(non return to zero) data the ratio of ones to zero’s should not be greater than 5:1.

The peak detector option is much robust than the data slicer option. Peak detector

option also works well where the dc component in the data is not constant which is the

case with NRZ. This option can be used for RZ and NRZ with good results and no hard

rules. Hence for these reasons we have used the chip in the peak detector option because

the power in the passive RFID system is the limited factor. Usually the signal from the

tag is so much noisy. So with ASK and 868 MHz we have used this chip TH7122.

Figure 4.24 shows the chip evaluation board manufactured by us. The

manufacturing details of the evaluation board are given in the next sections.

Figure 4.24* TH7122 evaluation board.

4.1.2.4 Manufacturing of Evaluation Board of TH7122

The layout of the chip in Gerber files was available on the Melexis website. To

edit this layout we worked on protel DXP and Gerb view software. As the PCB for

TH7122 evaluation board was double sided and pin through holes. So the first process

was to drill the holes in double sided FR4 copper board. The job position was saved in

the board master the software of CNC machine in the PCB lab of electrical engineering

* Taken from TH7122 Evaluation board datasheet.

67

UET Lahore. After Pin Through Holes (PTH) from Micropak Islamabad. This PCB for

evaluation board was manufactured from the PCB lab of EE department UET Lahore.

The figure 4.25 shows the CNC machine for PCB manufacturing placed at EE

department UET Lahore.

Figure 4.25 LPKF CNC machine for PCB designing.

Soldering Of Components

Following components* are soldered on the evaluation board of the PCB. Some

soldering was done in SUPARCO while the rest was done in the Research Center at UET

Lahore.

Co1.8 pF ±5% VCO tank capacitor C1 0603 1 pF ±5% LNA output tank capacitor C2 0603 1.5 pF ±5% MIX input matching capacitor C5 0603 1.5 nF ±10% RSSI output low pass capacitor C6 0603 100 nF ±10% PKDET capacitor CB0 1210 10 μF ±20% de-coupling capacitor CB1 0603 10 nF ±10% de-coupling capacitor CB2 0603 330 pF ±10% de-coupling capacitor CB5 0603 100 nF ±10% de-coupling capacitor CB6 0603 100 pF ±10% de-coupling capacitor CB7 0603 100 nF ±10% de-coupling capacitor CF1 0603 100 pF ±10% loop filter capacitor CF2 0603 39 pF ±5% loop filter capacitor CPS 0603 1 nF ±10% power-select capacitor CX1 0805 18 pF ±5% RO capacitor CRX0 0603 10 pF ±5% RX coupling capacitor CTX0 0603 10 pF ±5% TX coupling capacitor

* Courtesy: TH7122 evaluation board datasheet.

68

CTX1 0603 4.7 pF ±5% TX impedance matching capacitor CTX2 0603 3.9 pF ±5% TX impedance matching capacitor CTX4 0603 1.8 pF ±5% TX impedance matching capacitor R1 0603 100 kΩ ±5% PKDET resistor R2 0603 680 kΩ ±5% PKDET resistor RB1 0603 100 Ω ±5% protection resistor RF 0603 33 kΩ ±5% loop filter resistor RP 0603 3.3 KΩ ±5% CERDIS loading resistor RL0 0603 390 Ω ±5% CERFIL loading, optionally RPS 0603 43 kΩ ±5% power-select resistor RS1...RS3 0603 10 kΩ ±5% protection resistor L0 0603 3.9 nH ±5% VCO tank inductor from Würth-Elektronik (WE-KI series) or equivalent part L1 0603 4.7 nH ±5% LNA output tank inductor from Würth-Elektronik (WE-KI series) or equivalent part LRX2 0603 15 nH ±5% LTX0 0603 3.9 nH ±5% LTX1 0603 10 nH ±5% impedance matching inductor from Würth-Elektronik (WE-KI series) or equivalent part XTAL HC49 SMD 7x5 8.0000 MHz ±20ppm cal., ±20ppm temp fundamental-mode crystal from:Telcona/Hong Kong X.tals C5L8000000D10F3EHK01 CERFIL SMD 3.45x3.1 SFECF10M7HA00 B3dB = 180 kHz ceramic filter from Murata, or equivalent part

The exact location of components with respect to their number e.g. LTX1, are

given on TH7122 datasheet.

The layout of the TH7122 evaluation board is given in the figure 4.26.

Figure 4.26* Layout of evaluation board of TH7122, board size is 3.95cm*5.65cm.

Figure 4.27 shows the circuit diagram of peak detector ASK demodulation circuit.

*Courtesy: TH7122 datasheet.

69

Figure 4.27 Circuit diagram of peak detector ASK demodulator option

Figure 4.28 shows the top view of our PCB.

Figure 4.28 Physical PCB layout of the evaluation board of TH7122.

4.1.3 Antenna Design Selection of Antenna

The advantages of microstrip patch antennas make them a perfect candidate for

use in RFID applications. Microstrip patch antennas are being promoted for new high-

speed RFID Reader Systems. This antenna style provides a low profile, aesthetically

pleasing and low cost as well as highly effective and efficient antenna design. Microstrip

70

patch antennas are well suited for RFID Reader systems due to their versatility,

conformability, low cost and low sensitivity to manufacturing tolerances.

Advantages of Microstrip Patch Antennas

Some of the principal advantages of microstrip patch antennas compared to

conventional microwave antennas are:

Light weight, low volume, and thin profile configuration, which can be

made conformal

Low fabrication cost; readily amenable to mass production

Linear and circular polarizations are possible with simple feed

Dual-frequency and dual-polarization antennas can be easily made

No cavity backing is required

Can be easily integrated with microwave integrated circuits

Feed lines and matching networks can be fabricated simultaneously with

the antenna structure.

All these advantages, made us select microstrip patch antenna for our RFID system.

4.1.3.1 Patch Antenna Design Formulas and Calculations

Design Specifications

The three essential parameters for the design of a rectangular Microstrip Patch

Antenna are:

Frequency of operation ( fo ): The resonant frequency of the antenna must

be selected appropriately. In our application, we have selected 868MHz as

center frequency for patch antenna.

Dielectric constant of the substrate ( εr ): The dielectric material selected

for design is FR4 which has a dielectric constant of 4.4.

Height of dielectric substrate ( h ): For the microstrip patch antenna to be

used in RFID applications, it is essential that the antenna is not bulky.

Hence, the height of the dielectric substrate is selected as 1.6 mm.

71

Figure 4.29 Schematic of patch rectangular patch antenna.

Design Procedure

Step 1: Calculation of the Width (W ):

The width of the Microstrip patch antenna is given by

2

12

rof

cW

Using

c = 3×108 m/s

f o = 868 MHz

r = 4.4

We got W = 10.5 cm.

Step 2: Calculation of Effective dielectric constant (ε reff ):

The effective dielectric constant is given as:

W

hrrreff 121

2

1

2

1

Using

r = 4.4

h = 1.6×10-3 m

72

W = 10.5 cm

We got reff = 4.7

Step 3: Calculation of the Effective length (L eff ):

The effective length is given as:

reffo

efff

cL

2

Using

c = 3×108 m/s

f o = 868 MHz

reff = 4.7

We got Leff = 8 cm.

Step 4: Calculation of the length extension (ΔL):

The length extension can be calculated as:

8.0258.0

264.03.0412.0

hW

hW

hL

reff

reff

Using

reff = 4.7

h = 1.6×10-3 m

W = 10.5 cm

We got L = 0.073 cm

Step 5: Calculation of actual length of patch (L):

The actual length is calculated as:

LLL eff 2

Using

Leff = 8 cm

73

L = 0.073 cm

We got L = 7.85 cm.

Step 6: Calculation of the ground plane dimensions (Lg and Wg):

The transmission line model is applicable to infinite ground planes only.

However, for practical considerations, it is essential to have a finite ground plane. Similar

results for finite and infinite ground plane can be obtained if the size of the ground plane

is greater than the patch dimensions by approximately six times the substrate thickness all

around the periphery. Hence, for our design, the ground plane dimensions were selected

as:

LhLg 6

Using

h = 1.6×10-3 m

L = 7.85 cm

We got Lg = 8.81 cm.

WhWg 6

Using

h = 1.6×10-3 m

W = 10.5 cm

We got Wg = 11.46 cm.

Step 7: Determination of feed point location (Xf ,Yf ):

We used a coaxial probe type feed in our design. The center of the patch was

taken as the origin and the feed point location was given by the co-ordinates (Xf ,Yf ) from

the origin. The feed point must be located at that point on the patch, where the input

impedance is 50 ohms for the resonant frequency. Hence, a trial and error method was

used to locate the feed point. For different locations of the feed point, the return loss

(R.L) is compared and that feed point is selected where the R.L. is most negative. There

exists a point along the length of the patch where the R.L. is minimum.

74

4.1.3.2 HFSS software Simulation

After designing the length, width of the patch and ground plane, we simulated our

results in HFSS. The patch antenna we have used is probe fed. The feeding point was first

calculated by calculations of formulas in the previous section. Then the whole design

with the calculated lengths and widths is drawn in the HFSS software. The substrate is

FR4 with height 1.6mm and relative permittivity 4.4. Proper design of probe is also done

on the calculated point. Figure 4.30 shows a snap shot of HFSS software.

Figure 4.30 HFSS main window snap shot.

Then proper excitations and boundaries of E-field according to the HFSS help, so

that the boundaries did not overlap were given to design and the air. After validating the

design we simulated the design. This design did not show the exact frequency response

and return loss, the reason was the formulas for the patch antenna design are

approximating not exact. Then parametric sweeps were done in the optimetrics options in

75

HFSS. In parametric sweeps, one variable is swept and its value, which is required by the

user, is calculated at every given interval. The frequency is a single frequency or a range

or frequencies depending upon the design.

Design of Antenna with parametric sweeps

With the calculated lengths and widths of the patch and the ground, the first

parametric sweep has done on the feed point. Feed point was given in XY coordinate

system. One point was considered Y/2 while the sweep was done on the whole length of

X, with return loss at the specified frequency 870 MHz, to get a bandwidth in which 868

MHz was operatable. The result of this sweep showed that with changing the feed point,

return loss also changed. We selected the point with minimum return loss, modified the

design and gain simulated the design with new value of feed point. We repeated

parametric sweeps run until we got required frequency response. We did parametric

sweep on the length of the patch and selected the value of length of patch with minimum

return loss. Similarly we performed the parametric sweeps on the width of the patch,

length of ground and width of ground and selected the values with minimum return loss

and modified the design step by step. Note, on each parametric sweep, only one variable

was swept while the others were kept constant.

In the end we performed parametric sweep of feed point across length of the patch

and selected the point with minimum return loss. The result of last parametric sweep is

shown in the Figure 4.31. This parametric sweep was done with the following values of

patch and ground dimensions.

Length of patch = 7.5cm

Width of patch = 7.5cm

Length of ground = 18.5cm

Width of ground = 16.5cm

Patch coordinates in XY= -3.25,-2

Ground coordinates in XY= -8.25,-9.25

Y axis of feed point= 0cm

76

Figure 4.31 Result of sweep of x_feed point versus return loss.

The value of x for the feed point selected in the result of this parametric sweep

result is 1.25 cm.

Simulation Results

The values specified in the previous section were given with the new value of

x_feed point then the whole design was simulated in the HFSS. We got the following

results from the software simulation shown in figure 4.32 to figure 4.35.

77

Figure 4.32 The patch antenna with its ground and air dimensions after new values.

Figure 4.33 shows the frequency response of the antenna with return loss. This

shows a dip at 870 MHz with return loss of almost -25 dB. At return loss below -15 dB,

very low power is lost due to reflections. So this antenna design was fine and

approaching the ideal situations. The dip was not so much sharp so it can also operate on

868 MHz with good performance in the sense of gain and losses.

Figure 4.33 Frequency verses return loss of patch antenna.

78

The Figure 3.34 shows the VSWR for the patch antenna. The operatable range of

VSWR is 1 to 1.5. So the frequency 868MHz lies in this range.

Figure 4.34 Frequency versus VSWR of the patch antenna.

Another plot shows the radiation pattern with phi 0deg and 90 deg. This radiation

pattern is shown in fig 4.35.

Figure 4.35 radiation pattern of patch antenna.

79

Physical Antenna Implementation

After getting all the desired specifications of the patch antenna, we made layout of

patch antenna on PCB Wizard software. We generated the Gerber files from PCB files.

We implemented the above antenna physically by the CNC machine in the PCB Lab of

EE department UET Lahore.

Figure 4.36 shows the photograph of our patch antenna.

Figure 4.36 Our fabricated patch antenna.

4.1.4 Tags Used for RFID system implementation We have used Class 1, Gen II tags. Their number is RX-UHF-00C01-03

Physical structure of tag is shown in figure 4.37 from Texas Instruments TI.

Figure 4.37 RX-UHF-00C01-03 tag.

80

Following are the specifications of the tags. This section only shows the

specification of the tags.

The memory contents of the tags are given in the appendix.

Supported SKU types UHF friendly SKU’s* IC Supported Standard

EPC UHF Gen II

Operating frequency

860- 960 MHz

EPC Memory 96 bits EPC user programmable

TID Memory 32 bits factory pre-programmed

Data retention

2 years at + 25°C

Write/erase cycle

1000 at + 25°C

Operating temperature

-40°C to + 65°C

Storage temperature (single)

-40°C to + 85°C

Storage temperature (on reel)

-40°C to + 45°C

Bending radius

15 mm (0.59”)

Antenna Size

3.5” X 1” [ 88.90mm X 25.40mm ]

Inlay pitch

1.5” [38.1mm (± 0.5mm)]

Width of inlay

3.75” [95.25mm (± 0.5mm)]

Material/ thickness

75 micron (~2.95 mils) PET substrate

Antenna Material

Printed silver ink

Reel diameter

ID: 3” core (76.2mm); OD: Max 15” (381mm)

Delivery Single row inlay wound on cardboard reel

Quantity 10K per reel

81

5 RFID System Implementation

(Part II)

This chapter explains the software implementation related to the signal generation

and processing part of an RFID reader. In this part the detail of EPC Global class 1 Gen 2

UHF RFID protocol is explained. It starts with the explanation of the EPC Global Class 1

Generation 2 UHF RFID Protocol for communication between an RFID Reader and Tag.

5.1 Overview

5.1.1 Physical Layer A Reader sends information to one or more Tags by modulating an RF carrier at

868MHz. The modulation schemes supported are:

double-sideband amplitude shift keying (DSB-ASK)

single-sideband amplitude shift keying (SSB-ASK)

phase-reversal amplitude shift keying (PR-ASK)

Any one of the above mentioned modulation schemes can be chosen for the

Reader-to-Tag communication link complemented with a pulse-interval encoding (PIE)

format for detecting RFID Tags. Tags receive their operating energy from this same

modulated RF carrier.

A Reader receives information from a Tag by transmitting an unmodulated RF

carrier which enables the passive Tag to generate a backscattered reply. Tag

communicates to the Reader by backscatter-modulating the amplitude and/or phase of the

received RF carrier. The encoding format, selected in response to Reader commands, is

either FM0 or Miller-modulated subcarrier. The communication link between Reader and

Tag is half-duplex, meaning that at one time only one-way communication is possible,

either from Tag to Reader or vice versa.

82

5.1.2 Tag – Identification Layer A Reader manages Tag populations using three basic operations:

a) Select: It performs the operation of choosing a Tag population for Inventory

and Access. A Select command may be applied successively to select a particular Tag

population based on user-specified criteria.

b) Inventory: It performs the operation of identifying the Tag. A Reader begins

an inventory round by transmitting a Query command. One or more Tags may reply. The

Reader detects a single Tag reply and requests the Protocol Control (PC) bits, Electronic

Product Code (EPC) bits, and Cyclic Redundancy Check (CRC-16) bits from the Tag.

Inventory session comprises of multiple commands.

c) Access: It performs the operation of communicating with (reading from and/or

writing to) a Tag. An individual Tag must be uniquely identified prior to access.

(Access commands are useful, however, are not essential and have not

been implemented as a part of our project)

5.1.3 Reader Tag Communication Process The reader to tag communication process is explained as under.

COMMAND STATES

READY SELECT

ARBITRATE REPLY

INVENTORY ACKNOWLEDGED

OPEN SECURED ACCESS KILLED

Figure 5.1 – Reader Tag Communication Process

As soon as the Tag enters the RF field of Reader, it changes to the Ready

state and will accept Select commands. Select commands are sent to all Tags to inform

each one if they are to take part in the Inventory process that is to follow.

83

Multiple Select commands can be used to precisely define which Tags are

to respond. All exchanges between Reader and Tag start with one or more Select

commands. Tags do not respond to the Select command.

The Inventory group of commands, that follow the Select command, are

used to initiate the singulation process, where each individual Tag is identified and

processed. Each Inventory round starts with a Query command being broadcasted – this

command passes a Q-value (0 to 15) from which each Tag generates a slot counter

number in the range (0, 2Q - 1). Most Readers can dynamically adjust the Q-value

depending on the number of Tags in the field, increasing the potential reading rate this

way.

The Tag which generates a slot counter value of zero is allowed to reply

by sending a 16-bit random number and at the same time transitioning to the Reply state.

The other Tags change state to Arbitrate and wait for further commands. If the Tag’s

response is successfully received, the Reader replies by sending an ACK command,

together with the same 16-bit random number. This response now allows the IC to send

back its EPC data and change state to Acknowledged.

It is at this point that the Reader is able to transition the Tag to the Open

(or Secured) state allowing operations such as Read, Write, Lock and KILL, but normally

this exchange would terminate when the reader sends a QueryAdjust command and the

Tag switches state back to the Ready state and changes its inventoried flag to show it has

been singulated.

The QueryAdjust command also affects the other Tags causing them to

decrement their slot counters and any Tag whose counter is now zero is allowed to reply

– so in this way, with successive QueryAdjust or QueryRep commands, all Tags will be

found. If two Tags reply at the same time, unless the Reader is able to identify each one

and send an ACK as well as the correct 16-bit random number, each one will timeout, re-

generate a slot counter value and return to Arbitrate State.

If further actions are needed to be performed on a Tag, once the Tag has

returned its EPC number and is in the Acknowledged state, the Reader sends a Req_RN

(Request Random Number) command. The Tag replies with a new 16-bit random number

that is called the Handle and changes its state to the Open State.

84

The Open State, however, is not mandatory and has not been implemented as a

part of our project.

5.1.4 Tag Selection and Detection

5.1.4.1 Sessions and Inventoried flags

The Reader must support all of the 4 sessions provided by the Tags

(denoted S0, S1, S2, and S3). Tags participate in one and only one session during an

inventory round and maintain an independent inventoried flag for each session.

Each of the four inventoried flags has two values, denoted by A and B.

At the beginning of each and every inventory round a Reader chooses to inventory either

A or B Tags in one of the four sessions. Tags participating in an inventory round in one

session neither use nor modify the inventoried flag for a different session. The

inventoried flags are the only resource a Tag provides separately and independently to a

given session. All other Tag resources are shared among sessions. The Reader can also

issue a command that causes the Tag to invert its inventoried flag for that session (i.e.

A→B or B→A).

A Tag powers-up with its inventoried flags set as follows:

The S0 inventoried flag set to A.

The S1 inventoried flag shall be set to either A or B, depending on its stored

value, unless the flag was set longer in the past than its persistence time, in which

case the Tag powers-up with its S1 inventoried flag set to A.


value, unless the Tag has lost power for a time greater than its persistence time, in

which case the Tag powers-up with the S2 inventoried flag set to A.


value, unless the Tag has lost power for a time greater than its persistence time, in

which case the Tag powers-up with its S3 inventoried flag set to A.

5.1.4.2 Tag States and Slot Counter

EPC Global Class1 Gen2 Tags support the following states and slot counter values:

85

Ready state

The Ready state can be viewed as a “holding state” for energized Tags

that are neither killed nor currently participating in an inventory round. Upon entering an

energizing RF field the first state that a Tag enters is the Ready state. The Tags remain in

Ready state until they receive a Query command, whose inventoried parameter (for the

session specified in the Query) and SEL parameter match its current flag values.

Matching Tags draw a Q-bit number from their Random Number Generator, load this

number into their slot counter, and transit to the Arbitrate state if the number is nonzero,

or to the Reply state if the number is zero.

Arbitrate state

Arbitrate state can be viewed as a “holding state” for Tags that are

participating in the current inventory round but whose slot counters hold nonzero values.

Tags in Arbitrate state decrement their slot counter values each time they receive a

QueryAdjust command whose session parameter matches the session for the inventory

round currently in progress. They, however, transit to the Reply state once their slot

counter value reaches 0000h. Tags that return to Arbitrate (for example, from the Reply

state) with a slot value of 0000h

decrement their slot counter from 0000h

to 7FFFh

at the

next QueryAdjust command (with matching session) and, because their slot value is now

nonzero, remain in Arbitrate state.

Reply state Upon entering the Reply state, a Tag backscatters an RN16 (Random

Number – 16) number. If the Tag receives a valid acknowledgement (ACK) for the RN16

from the Reader, it transits to the Acknowledged state, backscattering its PC, EPC and

CRC-16. If the Tag fails to receive an ACK, or receives an invalid ACK, it returns to

Arbitrate state.

Acknowledged state

A Tag in the acknowledged state may transition to any state except killed,

depending on the received command.

86

Slot counter

Tags implement a 15-bit slot counter. Upon receiving a Query or

QueryAdjust command a Tag preloads a value between 0 and 2Q–1, drawn from the Tag’s

RNG (Random Number Generator), into its slot counter. Q is an integer in the range

(0,15). A Query specifies Q while a QueryAdjust may modify Q from the prior Query.

Upon receiving a QueryAdjust command a Tag shall decrement its slot counter. The slot

counter shall be capable of continuously counting, meaning that, after the slot counter

decrements to 0000h it rolls over and begins counting down from 7FFF

h.

5.1.5 Reader Commands and Tag Replies Table 5.1 shows the commands sent from the Reader and their corresponding

replies from the Tag.

Table 5.1 – Tag’s response to Reader commands

Reader-to TagCommad Tag Response

Select No reply (only causes the Tag’s state to change as shown in table 5.2)

Query RN16 (Random Number – 16)

QueryAdjust RN16

QueryRep RN16

ACK PC+EPC+CRC16

Table 5.2 – Transition of Tag due to Select command

Action Matching Non-Matching

000 Assert SL or inventoriedA Deassert SL or inventoriedB 001 Assert SL or inventoriedA Do nothing 010 Do nothing Deassert SL or inventoriedB 011 Negate SL or (AB, BA) Do nothing 100 Deassert SL or inventoriedB Assert SL or inventoriedA 101 Deassert SL or inventoriedB Do nothing 110 Do nothing Assert SL or inventoriedA 111 Do nothing Negate SL or (AB, BA)

87

5.1.6 Link Timing Figure 5.2 illustrates Reader-to-Tag and Tag-to-Reader link timing. Table

5.3 shows the timing requirements for Figure 5.2. Tags and Readers must meet all timing

requirements shown in the Table. RTcal is defined in Article 5.2.1.5; Tpri

is the Tag-to-

Reader link period (Tpri

= 1 / LF, where LF is the Link Frequency).

Reader SELECT QUERY ACK QUERYREP Commands

T4 PC + EPC + CRC16

TAG RN16Responses

T1 T2 T1 T2

Figure 5.2 – Link Timing

Table 5.3* – Link Timing Parameters

* Courtesy: EPC Global Class 1 Gen 2 UHF RFID Protocol

88

5.1.7 Flowcharts and Line Diagrams The Figure 5.3 shows the Line Diagram explaining the communication process

between Reader and Tags.

Reader Tag Communication Line Diagram

Tag Replies Reader Commands

QueryRep

ACK

Query

Select

No Reply

Tag with slot counter = 0 replies with RN16 all others change their state to Arbitrate

The Acknowledged Tag responds with PC+EPC+CRC16 Bits

Tag decrement their slot counter values and the Tag whose value becomes zero

Figure 5.3 – Line Diagram of Communication Process

89

Reader to Tag Commands Flowchart The flowchart in Figure 5.4 describes the order of commands sent to the Tag and

their replies.

QueryRep commands =16

Q-Value=4

Tag replies with RN16

Tag replies with PC+EPC+CRC16

No Reply

SELECT

Time wait T4=187.5 µsec

Query

Time wait T1=236 µsec


Time wait T1=236 sec


QueryRep

Tags do not reply to SELECT command

“ACK” Sent

QueryRep commands <16

Figure 5.4 – Flowchart of Communication Process between Reader and Tag

90

5.2 Transmitted/Received Waveforms and Data Models

5.2.1 Transmitted Waveform and Data Model UHF RFID Tags are capable of receiving power from and communicating

with Readers within the frequency range from 860 MHz to 960 MHz, inclusive. A

Reader’s choice of operational frequency is determined by local radio regulations and by

the local radio-frequency environment. We have chosen 868 MHz as our carrier

frequency.

5.2.1.1 Modulation

Our Reader communicates with one or more Tags by modulating an RF

carrier using DSB-ASK Modulation with PIE encoding.

5.2.1.2 Data Encoding

The Reader-to-Tag link shall use PIE encoding as shown in Figure 5.5.

Tari is the reference time interval for Reader-to-Tag signaling, and is the duration of a

data-0. High values represent transmitted CW, low values represent attenuated CW. The

tolerance on all parameters is +/–1%.

Tari 0.5Tari<=x<=Tari

PW PW

Figure 5.5 – PIE encoded Data-0 and Data-1 Waveforms

91

5.2.1.3 Data Rates

Readers can communicate using Tari values between 6.25μs and 25μs,

inclusive. Reader compliance is evaluated using the preferred Tari values specified in

Table 5.4 and the encoding shown in Figure 5.5. A Reader uses fixed data-0 and data-1

symbol lengths for the duration of an Inventory round. Since our Reader modulates data

using DSB-ASK Modulation, the Tari value is set at 25μs.

*Table 5.4 – Tari Values

Tari Value Tari-Value Tolerance 6.25 s +/- 1% 12.5 s +/- 1% 25 s +/- 1%

5.2.1.4 Transmission Order

The following conventions were followed for all Reader-to-Tag

Communications.

Within each message, the most-significant word shall be transmitted first,

and

Within each word, the most-significant bit (MSB) shall be transmitted

first.

5.2.1.5 Preamble and Frame-sync

All Reader-to-Tag signaling begins with either a preamble or a

frameSync, both of which are shown in Figure 5.6. A Preamble always precedes a

Query command and denotes the start of an inventory round. All other signaling begins

with a frameSync. The tolerance on all parameters specified in units of Tari are +/–1%.

PW denotes the pulse width in the figure. The Preamble and frameSync are used to define

certain parameters to the Tag. The length of data-0 and Pulse width (PW) can be obtained

from the Preamble and frameSync. Other than this a Tag may compare the length of the

data-0 with the length of RTcal to validate the Preamble.

* Courtesy: EPC Global Class 1 Generation 2 UHF RFID Protocol

92

Preamble

RT Calibration (RTcal) TR Calibration (TRcal)

12.5 µs

Delimiter

PW PW

Data-0 RT Calibration (RTcal)

1 Tari 2.5 Tari<=RTcal<=3.0 Tari

Frame-SyncDelimiter

12.5 µs PW PW PW

Data-0

1 Tari 2.5 Tari<=RTcal<=3.0 Tari 1.1RTcal<=TRcal<=3 RTcal

Figure 5.6 – Preamble and Frame Sync

5.2.1.6 SELECT Command

The Select command is the first message that is sent to the Tag. It contains

User-defined criteria to allow the precise selection of a sub-population of Tags. It can

also set or re-set the Tag’s SL flag or change the Inventoried flag. The parameters of the

Select command are shown in Table 5.5 and Table 5.6.

Table 5.5 – Select Command

Command Target Action MemBank Pointer Length Mask Truncate CRC-

16

# of bits 4 3 3 2 EBV 8 Variable 1 16

Description 1010 000:Inventoried(S0) 001:Inventoried(S1) 010:Inventoried(S2) 011:Inventoried(S3)

100: SL 101: RFU 110: RFU 111: RFU

See Table below

00:RFU 01:EPC 10:TID 11:User

Starting Mask

Address

Mask Length (bits)

Mask Value

0:Disable Truncation 1:Enable

Truncation

93

Table 5.6 – Matching and Non-Matching Conditions for Tags

Action Matching Non-Matching 000 Assert SL or inventoried A Deassert SL or inventoried B 001 Assert SL of inventoried A Do nothing 010 Do nothing Deassert AL or inventoried B 011 Negate SL or (A B, B A) Do nothing 100 Deassert SL or inventoried B Assert SL or inventoried A 101 Deassert SL or inventoried B Do nothing 110 Do nothing Assert SL or inventoried A 111 Do nothing Negate SL or (A B, B A)

These parameters are explained as under.

Target indicates whether the Select command modifies a Tag’s SL flag or

its inventoried flag, and in the case of inventoried it further specifies one

of four sessions. A Select command that modifies SL does not modify

inventoried, and vice versa. Class-1 Tags ignore Select commands whose

Target is 1012, 110

2, or 111

2.

Action indicates whether matching Tags assert or deassert SL flag, or set

their inventoried flag to A or to B. Tags conforming to the contents of the

MemBank, Pointer, Length, and Mask fields are considered matching.

Tags not conforming to the contents of these fields are considered non-

matching

MemBank specifies whether Mask applies to EPC, TID, or User memory.

Select commands shall apply to a single memory bank. Successive Selects

may apply to different banks. MemBank does not specify Re-served

memory; if a Tag receives a Select specifying MemBank = 002

it will

ignore the Select.

Pointer, Length, and Mask Pointer and Length describe a memory range.

Pointer references a memory bit address (Pointer is not restricted to word

boundaries). Length is 8 bits, allowing Masks from 0 to 255 bits in length.

Mask, which is Length bits long, contains a bit string that a Tag compares

against the memory location that begins at Pointer and ends Length bits

94

Truncate If a Reader asserts Truncate, and if a subsequent Query

specifies Sel=10 or Sel=11, then matching Tags truncate their reply to an

ACK to that portion of the EPC immediately following Mask, followed by

the CRC-16 stored in EPC memory 00h to 0F

h.

CRC-16 Checksum calculated on command string

5.2.1.7 Invertory Command

The inventory command set comprises Query, QueryRep, ACK/NAK.

Query Command

This is a mandatory command and initiates the Inventory round where

individual Tags are identified. The parameters of the Query command are specified in

Table 5.7.

Table 5.7 – Query Command Command DR M TRext Sel Session Target Q CRC-

5 # of bits 4 1 2 1 2 2 1 4 5

Description 1000 0:DR=8 1:DR=64/3

00:M=1 01:M=2 10:M=4 11:M=8

0:No Pilot tone 1:use Pilot tone

00:All 01:All 10:~SL 11:SL

00:S0 01:S1 10:S2 11:S3

0:A 1:B

0-15

These parameters are explained as under.

DR (TRcal divide ratio) sets the Tag-to-Reader link frequency as

described in Article 5.2.2.3.

M (cycles per symbol) sets the Tag-to-Reader data rate and modulation

format as shown in Article 5.2.2.3.

TRext chooses whether the Tag-to-Reader preamble is preceded with a

pilot tone.

95

Sel chooses which Tags respond to the Query command

Session chooses a session for the Inventory round

Target selects whether Tags whose inventoried flag is A or B participate

in the Inventory round. Tags may change their inventoried flag from A to

B (or vice versa) as a result of being singulated

Q sets the number of slots in the round (see Article 5.1.3)

CRC-5 is calculated over the first command-code bit to the last Q bit. If a

Tag receives a Query with a CRC-5 error, it ignores the command. For

detail on generation of CRC-5 refer to Appendix.

Preamble needs to be preceded with every Query Command.

QueryRep Command

QueryRep instructs Tags to decrement their slot counters and, if slot = 0 after

decrementing, to backscatter an RN16 to the Reader. The parameters of the QueryRep

are shown in Table 5.8.

Table 5.8 – QueryRep Command

Command Session# of bits 2 2

Description 00 00:S0 01:S1 10:S2 11:S3

This parameter are explained as under.

Session corroborates the session number for the inventory round. If a Tag

receives a QueryRep whose session number is different from the session

number in the Query that initiated the round it shall ignore the command.

If a Tag, in response to the QueryRep, decrements its slot counter and the

decremented slot value is zero, then its reply to a QueryRep shall be as shown in Table

5.9. Otherwise the Tag shall remain silent. Tags shall respond to a QueryRep only if they

received a prior Query.

96

Table 5.9 – Tag’s response to QueryRep command

Response

# of bits 16

Description RN16

A QueryRep is always preceded with a frameSync

ACK Command

The Reader implements the ACK command shown. Reader sends an ACK to

acknowledge the Tag’s RN16. ACK echoes the Tag’s backscattered RN16.

An ACK is preceded with a frame-sync.

The parameters of the ACK command are shown in Table 5.10.

Table 5.10 – ACK Command

Command RN # of bits 2 16

Description 01 Echoed RN16

5.2.2 Receiver Waveform and Data Model A Tag communicates with a Reader using backscatter modulation, in

which the Tag switches the reflection coefficient of its antenna between two states in

accordance with the data being sent. A Tag shall backscatter using a fixed modulation

format, data encoding, and data rate for the duration of an inventory round. The Tag

selects the modulation format, the Reader selects the encoding and data rate by means of

the Query command that initiates the inventory round.

5.2.2.1 Modulation

Tag backscatter shall use ASK and/or PSK modulation. The Tag vendor

selects the modulation format. We have used Texas Instruments, Part number ‘RI-UHF-

00C02-04’; ASK modulation supporting Tags for our project.

5.2.2.2 Data Encoding

Tags can encode the backscattered data as either FM0 baseband or Miller

modulation of a subcarrier at the data rate. The Reader commands the encoding choice by

choosing the value of M in the Query command. We have chosen FM0 baseband as the

encoding employed by the Tags

97

FM0 Baseband

FM0 inverts the baseband phase at every symbol boundary. A data-0 has

an additional mid-symbol phase inversion. Figure 5.7 shows basis functions and a state

diagram for generating FM0 (bi-phase space) encoding. The state diagram in Figure 5.7

maps a logical data sequence to the FM0 basis functions that are transmitted. Labels, S1–

S4, indicate four possible FM0-encoded symbols, represented by the two phases of each

of the FM0 basis functions. The state labels also represent the FM0 waveform that is

transmitted upon entering the state. The labels on the state transitions indicate the logical

values of the data sequence to be encoded. For example, a transition from state S2 to S

3 is

disallowed because the resulting transmission would not have a phase inversion on a

symbol boundary. The state diagram in Figure 5.7 does not imply any specific

implementation.

FM0 Basis Functions FM0 Generator State Diagram

Data - 0

Am

pli

tud

e

-1

0

+1

S2(t)

S3(t)= - S2(t)

Time(t)

Time(t)

S1(t)

S4(t)= - S1(t)

+1

0

Data - 1

Figure 5.7 – FM0 Basis Functions and State Transition Diagram

0

S1

S2

S3

S4 1

0

1

0

1

0

Figure 5.8 shows generated baseband FM0 symbols and sequences. The duty cycle of a 00 or 11 sequence, measured at the modulator output, shall be a minimum of 45% and a maximum of 55%, with a nominal value of 50%. FM0 encoding has memory; consequently, the choice of FM0 sequences in Figure 5.8 depends on prior transmissions.

98

Figure 5.8 – FM0 Symbols and Sequences

FM0 Preamble

Tag-to-Reader FM0 signaling begins with one of the two preambles

shown in Figure 5.9. The choice depends on the value of the TRext bit specified in the

Query command that initiated the inventory round. The “v” shown in Figure 5.9 indicates

an FM0 violation (i.e. a phase inversion should have occurred but did not).

1 0 1 0 v 1

FM0 Preamble(TRext=0)

FM0 Preamble(RText=1)

0 0 0 0 1 0 1 0 v 1

12 leading zeros

0 0

1 1

00

01

10

11 11

10

00

01

FM0 Sequences

FM0 Symbols

Figure 5.9 – FM0 Preamble

99

5.2.2.3 Data Rates

A Reader shall specify a Tag’s backscatter link frequency (its FM0 datarate or the

frequency of its Miller subcarrier) using the TRcal and divide ratio (DR) in the preamble

and payload, respectively, of a Query command that initiates an inventory round. The

following equation specifies the relationship between the backscatter link frequency (LF),

TRcal, and DR. A Tag shall measure the length of TRcal, compute LF, and adjust its

Tag-to-Reader link rate to be equal to LF. The TRcal and RTcal that an Reader uses in

any inventory round shall meet the constraints in the following equation.

LF = DR/TRcal 1.1×RTcal≤TRcal≤3×RTcal

The Query command that initiates an inventory round specifies DR in

Table 5.7 and M in Table 5.12 ; the preamble that precedes the Query specifies TRcal. LF

is computed using the above equation. These four parameters together define the

backscatter frequency, modulation type (FM0 or Miller), and Tag-to-Reader data rate for

the round. Table 5.11 shows LF values and tolerances.

*Table 5.11 – Link Frequencies

* Courtesy: EPC Global Class 1 Generation 2 UHF RFID Protocol

100

*Table 5.12 – Data Rates

5.2.2.4 Transmission Order

The transmission order for all Reader-to-Tag communications shall

respect the following conventions:

Within each message, the most-significant word shall be transmitted first,

and

Within each word, the most-significant bit (MSB) shall be transmitted

first.

5.2.2.5 Tag Memory

The Tag memory is logically separated into four distinct banks, each of

which may comprise zero or more memory words. A logical memory map is shown in

Figure 5.10. The memory banks are:

Reserved memory contains the kill and access passwords. This section of the memory is not used in our project. If a Tag does not implement the kill and/or access passwords, the Tag can act as though it had zero-valued passwords that are permanently read/write locked, and the corresponding memory locations in Reserved memory need not exist.

Electronic Product Code(EPC) memory contains a CRC-16 at memory addresses 00h to 0Fh, Protocol-Control (PC) bits at memory addresses 10h to 1Fh, and a code that identifies the object to which the tag is or will be attached beginning at address 20h which is called EPC. The EPC, which is 96 bits long, ends at address 7Fh. The CRC-16, PC, and EPC are stored MSB first (the EPC’s MSB is stored in location 20h). The contents of the EPC memory are explained in detail in the next section.

TID memory contains an 8-bit ISO/IEC 15963 allocation class identifier (111000102 for EPCglobal) at memory locations 00h to 07h. TID memory


101

contains sufficient identifying information above 07h for a Reader to uniquely identify the custom commands and/or optional features that a Tag supports.

User memory allows user-specific data storage. The memory organization is user-defined.

Bank Memory

Bank 11 User Bank 10 TID Bank 01 EPC Bank 00 Reserved

Figure 5.10 - Memory Banks

5.2.2.6 EPC Memory

Protocol Control (PC) Bits

The PC bits contain the physical-layer information that a Tag backscatters

with its EPC during an inventory operation. There are 16 PC bits, stored in EPC memory

at addresses 10h to 1F

h, with bit values defined as follows:

Bits 10h

– 14h: The length of the (PC + EPC) that a Tag backscatters, in

words:

000002: One word (addresses 10

h to 1F

h in EPC memory).

000012: Two words (addresses 10

h to 2F

h in EPC memory).

000102: Three words (addresses 10

h to 3F

h in EPC memory).

111112: 32 words (addresses 10

h to 20F

h in EPC memory).

Bits 15h

– 16h: RFU (shall be set to 00

2 for Class-1 Tags).

Bits 17h

– 1Fh: A numbering system identifier (NSI) whose default value

is 0000000002. The MSB of the NSI is stored in memory location 17

h. If

bit 17h contains a logical 0, then PC bits 18

h – 1F

h contain an EPCglobal™

Header as defined in the EPC™ Tag Data Standards. If bit 17h

contains a

logical 1, then PC bits 18h

– 1Fh

contains the entire AFI defined in

ISO/IEC 15961.

The default (unprogrammed) PC value shall be 0000h.

102

Electronic Product Code (EPC) Bits

An EPC is an electronic product code that identifies the object to which a

Tag is affixed. The EPC is stored in EPC memory beginning at address 20h, MSB first.

Readers issue a Select command that includes all or part of the EPC in the mask. Readers

also issue an ACK command to cause a Tag to backscatter its PC, EPC, and CRC-16.

Cyclic Redundancy Check (CRC-16) Bits

A CRC-16 is a cyclic-redundancy check that a Reader uses when

protecting certain Reader-to-Tag commands and that a Tag uses when protecting certain

backscattered Tag-to-Reader sequences. To generate a CRC-16 a Reader or Tag must

first generate the CRC-16 precursor shown in Table 5.13, then take the ones-complement

of the generated precursor to form the CRC-16.

One sequence protected by a CRC-16 is the PC bits and EPC that a Tag

backscatters during an inventory operation. For detail on CRC-16 refer to Appendix.

*Table 5.13 - Precursor

5.3 Implementation and Design of Data Model

5.3.1 Microcontroller Unit – ATMEL ATMega32 The implementation of the Transmitted and Received Data Model required

the use of a microcontroller supporting PWM Generation for the exact implementation of

our PIE encoded DSB-ASK waveform and the Input Capture Unit. The Atmel

ATMega32 Microcontroller was used for this purpose and the software for the generation

of the encoded data waveform was developed in C Programming Language.

The features provided by an ATmega32 microcontroller include:

maximum frequency clock of 16MHz using external resonators

32K bytes of In-System Programmable


103

Flash Program memory with Read-While-Write capabilities

1024 bytes EEPROM

2Kbyte SRAM

32 general purpose I/O lines

32 general purpose working registers

On-chip Debugging support and programming

Three flexible Timer/Counters with compare modes

Internal and External Interrupts

SPI serial port

Among other several features, it is suitable for our application.

5.3.2 Implementation of the Transmitted Data Model For the implementation of Transmitted Data Model we used the PWM

Generator of ATMega32. This feature is explained as under.

PWM Generation using Timer/Counter Unit

The Transmitted waveform by the Reader is a DSB-ASK Modulated and

PIE encoded waveform which can be easily generated using Pulse Width Modulation.

The PWM was implemented using the in-built Timer/Counter unit in the Atmel

ATMega32 Microcontroller.

We used the 16-bit Timer 1 for generating the desired waveform. The Timer can

be clocked internally, via the prescaler, or by an external clock Source. It then uses the

selected clock source to increment (or decrement) its value on every clock edge.

The 16-bit Timer Unit also has two buffered Output Compare Registers

(OCR1A/B) that are compared with the Timer value at all time. The result of the compare

can be used by the Waveform Generator to generate a PWM. The compare match event

also triggers the Compare Match Flag (OCF1A/B) which is used to generate an output

compare interrupt request. The Waveform Generator uses the match signal to generate an

output according to operating mode set. The following modes of the 16-bit timer were

used in our application.

104

Clear Timer on Compare(CTC) Mode

We used the CTC for the generation of our Reader to Tag transmitted

waveform. In CTC mode, the Output Compare Registers are used to manipulate the timer

resolution. In this mode the timer is cleared to zero when the timer value matches Output

Compare Register Value. Hence, this mode allows greater control of the compare match

output frequency. It also simplifies the operation of counting external events. The timing

diagram for the CTC mode is shown in Figure 5.11.

*Figure 5.11 – CTC Mode, Timing Diagram

The counter value (TCNT1) increases until a compare match occurs with

OCR1A, and then counter (TCNT1) is cleared. An interrupt is generated at each time the

counter value reaches the TOP value by using the OCF1A Flag. The interrupt handler

routine is then used for updating the TOP value.

In this mode there is also an option of toggling the OCnA pin on each

compare match, but we have not used this feature. The formula for generating a square

wave on any pin, using the CTC mode is given as under.

fOC1A = f clk_I/O / [ 2 x N x (1 + OCR1A)]

where OCR1A is the value of the Output Compare Register. The waveform

generated can have a maximum frequency of fOC1A = fclk_I/O/2 when Output Compare

Register is set to zero (0x0000). The N variable represents the prescaler factor (1, 8, 64, 256, or 1024). Article 5.3.5 shows the calculations for setting the value of OCR1A using the above formula.


105

5.3.3 Implementation of Receiver Data Model The Reader waveform detection required the use of a capture unit that

could trigger an interrupt on the occurrence of data on its input. Processing of data to

individually recognize each Tags data was also required. For this we used the in-built

Input Capture Unit available in the Atmel ATMega32 microcontroller.

Using the Input Capture Unit

In order to capture the data coming from the tag side we used the Timer Input

Capture Unit. This Input Capture unit can capture external events and give them a time-

stamp indicating time of occurrence. The external signal coming from the tag side can be

applied via the ICP1 pin. The time-stamps can be used to determine whether a data-0 or a

data-1 is received from the tag.

The Input Capture Unit works in the following way. When a change of the logic

level (an event) occurs on the Input Capture pin (ICP1), and this change confirms to the

setting of the edge detector, a capture will be triggered. The edge detector bit (ICES1)

selects the edge on the Input Capture Pin (ICP1) that is used to trigger a capture event.

When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and

when the ICES1 bit is written to one, a rising (positive) edge will trigger the capture.

When a capture is triggered, the 16-bit value of the counter (TCNT1) is written to

the Input Capture Register (ICR1). The Input Capture Flag (ICF1) is set at the same

system clock as the TCNT1 value is copied into ICR1 Register. If enabled (TICIE1 = 1),

the Input Capture Flag generates an Input Capture interrupt. The ICF1 Flag is

automatically cleared when our Interrupt Service Routine (ISR) is executed.

Processing of Captured Data

The main challenge of successfully capturing incoming data was using the Input

Capture unit is to assign enough processor capacity for handling the incoming events.

The time between two events is critical. If the processor has not read the captured value

in the Incoming Capture Register before the next event occurs, the Input Capture Register

is overwritten with a new value. In this case the result of the capture will be incorrect.

When using the Input Capture interrupt, the Input Capture Register was designed

such that it is read as early in the interrupt handler routine as possible because even

106

though the Input Capture interrupt has relatively high priority, the maximum interrupt

response time is dependent on the maximum number of clock cycles it takes to handle

any of the other interrupt requests.

5.3.4 Clock Options and Clock Frequency Used The microcontroller has the following clock source options, selectable by

Flash Fuse bits as shown below. The clock from the selected source is input to the AVR

clock generator, nd routed to the timer/counter unit and other modules for operation. The

clocking options available with ATMega32 are listed in Table 5.14.

*Table 5.14 – Clocking Options in ATMega32

In order to achieve the highest clock rate, and hence the fastest processing,

we used the External Crystal/Ceramic Resonator. The External Crystal can provide a

maximum system clock frequency of 16MHz when a crystal of 16MHz is connected

between XTAL1 and XTAL2 pins in the configuration shown below.

Figure 5.12* – Crystal Resonator Configuration


107

The value of capacitors can be from 12pF to 22pF. In order to use this mode we

programmed the CKSEL3..0 bits to 1111 with CKOPT programmed.

5.3.5 Calculations and Results This section demonstrates the calculations of different values of parameters for

the Timer CTC Unit and Input Capture Unit in order to generate the desired waveform.

5.3.5.1 Reader to Tag Communication

We preferred the following values of parameters in the Reader to Tag

communication.

Table 5.15 – Parameter Values

Parameter Value Tari Value 25 μs Pulse Width(PW) 12.5 μs Delimiter 12.5 μs Data-0 Length=1 Tari 25 μs Data-1 Length=2 Tari 50 μs RTcal Length=3 Tari 75 μs TRcal Length=2.4 RTcal 180 μs

These values set the Reader to Tag and Tag to Reader data rates, and also

the length of the Reader to Tag preamble and frame Sync. The above given values were

preferred in order to achieve the minimum possible data rate from the Tag to Reader

communication. This minimum possible data rate was required in order to achieve

maximum time between any two bits received on the reader side, so that the ISR for that

corresponding bit is completed before the next bit arrives.

Based on the above values of parameters, the values of OCR1A register

for the 16-bit timer 1 are calculated as follows. The formula used in these calculations,

which was stated in the CTC mode section, is restated as under.

fOC1A = f clk_I/O / [ 2 x N x (1 + OCR1A)]

We have the prescaler value of ‘1’ throughout our calculation so N=1.

System Clock Time Period

System Clock Frequency= f clk_I/O = 16MHz

So, System Time Period=1/16M = 62.5ns

108

Pulse Width (PW)

PW=12.5 μs

So, fOCnA = 1/12.5μs = 80 kHz

Using the formula: (1+OCR1A) = f clk_I/O/ fOCnA

(Removing the factor of ‘2’ because PW is only half the square wave)

We get OCR1A=199 = C7hex

Data-0

The waveform for Data-0 for PIE encoding is given in Figure 5.13.

PW=12.5μs

12.5μs

1 Tari=25 μs

Figure 5.13 – Data-0 Waveform PW had already been calculated, so data-0 high time is calculated as:

High Time = 12.5 μs

So, fOCnA = 1/12.5 μs = 80 kHz



Data-1

The waveform for Data-1 for PIE encoding is given in Figure 5.14

37.5 μs

2 Tari=50 μs

PW=12.5μs

Figure 5.14 – Data-1 Waveform

PW had already been calculated, so data-1 high time is calculated as:


109

So, fOCnA = 1/37.5 μs = 26.67 kHz


We get OCR1A=599 = 257hex

RTcal Waveform

The waveform for RTcal is shown if Figure 5.15. 62.5 μs

3 Tari=75 μs

PW=12.5μs

Figure 5.15 – RTcal Waveform

PW had already been calculated, so RTcal high time is calculated as:


So, fOCnA = 1/62.5 μs = 16 kHz


We get OCR1A=999 = 3E7hex

TRcal Waveform

The waveform for TRcal is shown in Figure 5.16.

167.5 μs

2.4 RTcal=180 μs

PW=12.5μs

Figure 5.16 – TRcal Waveform

PW had already been calculated, so TRcal high time is calculated as:


So, fOCnA = 1/167.5 μs = 5.970 kHz

110


We get OCR1A=2679 = A77hex

Delimiter

All the preamble and frameSync waveforms start with a delimiter of the type,

shown in Figure 5.17

12.5 μs

Figure 5.17 – Delimiter

Its OCR value is calculated as:

Delimiter = 12.5 μs

So, fOCnA = 1/12.5μs = 80 kHz



5.3.5.2 Tag to Reader Communication

The following values need to be calculated for implementing the tag to reader

communication protocol. The Tag to Reader communication protocol implements FM0

encoding.

Link Frequency

As specified previously, under the Article 5.2.2.3, the link frequency for the Tag

to Reader communication is calculated by the formula.

LF = DR/TRcal

We have chosen the value of DR=8 and the value of TRcal, as stated previously,

is 180 μs. We got LF=44.44 kHz. Hence, the Tag to Reader data rate for FM0 baseband

modulation is 44.44kbps.

Data-0

Time Period of one bit = 1/LF = 22.5 μs

With FM0 encoding, data-0 is represented in Figure 5.18.

111

Figure 5.18 – Data-0 symbols

Data-1

Time Period of one bit = 1/LF = 22.5 μs

With FM0 encoding, data-1 is represented in Figure 5.19

22.5 μs

11.25μs

11.25μs

22.5 μs

11.25μs 11.25μs OR

s 22.5 μ

22.5 μs OR

Figure 5.19 – Data-1 symbols

5.3.5.3 Link Timings

The Tag to Reader and the Reader to Tag timings were explained previously

under the Article 5.1.6. These timings are dependent on the value of Tprj and FT. The

value of FT for a link frequency of 44.44 kHz is known from Table 5.9 as:

FT= +/- 4%

The value of Tprj is the inverse of link frequency which comes out to be:

Tprj = 1/LF = 22.5 μs

The values of the link timings and their corresponding values of OCR1A register

for 16-bit Timer are calculated as follows.

T1, Time from Reader’s transmission to Tag’s response

T1=MAX (RTcal, 10Tprj) x (1+FT) + 2 μs

Hence, T1=236 μs


We get: OCR1A = 3775 = EBFhex

112

T2, Time from Tag’s response to Reader’s transmission

Taking T2 = 15Tprj = 337.5 μs


We get: OCR1A = 5399 = 1517hex

T4, Minimum Time between Reader Commands

Taking T4 = 2.5 RTcal = 187.5 μs


We get: OCR1A = 2999 = BB7hex

113

6 Testing and Results

6.1 RF Portion Results

Before implementing our complete design, we tested each component of our

system individually. Testing was done according to the optimal parameters as given in

data sheets. We checked the output responses, waveforms and power levels of each

component. After which, complete system was implemented with the assurance that

individual components are working properly. Following are the results of our testing and

implementation.

6.1.1 Component-Level Testing In RF portion, we tested our frequency synthesizer, driver, power amplifier and

then finally patch antenna.

6.1.1.1 Integrated PLL and VCO Frequency Synthesizer

For testing the synthesizer, first of all we configured it according to our

requirements by using software of evaluation board. We selected 868 MHz as our carrier

frequency. Output power level of -3dBm (maximum possible for VCO) was selected.

Then registers were loaded with the bit patterns as described in datasheet. In the end, we

connected RFout port of VCO to Spectrum Analyzer via a 50 ohm male to male wire

connector. Our testing setup is shown in Figure 6.1.

115

Figure 6.1.Testing setup for frequency synthesizer.

After initial adjustment, Spectrum Analyzer gave the output. It contained a peak

at 868 MHz with a power level of 0.5dBm. This is shown in Figure 6.2.

Figure 6.2 .A snapshot of Spectrum Analyzer showing Synthesizer output.

The time domain waveform of Synthesizer output was observed on high frequency oscilloscope. It is shown in Figure 6.3.

116

Figure 6.3.Time domain waveform of synthesizer output.

6.1.1.2 Driver

First of all, we measured S parameters of the driver. It was connected to the

network analyzer and one by one all the four S parameters were checked. Following is

the diagram of Our testing setup for driver is shown in Figure 6.4.

Figure 6.4 Testing of driver by Network Analyzer Following are the results showing S parameters of the driver.

117

S11 The value of S11 parameter at 866 MHz is -9.9280 dB which is depicted in Figure

6.5.

Figure 6.5 S 11 parameter of driver. S22

The value of S22 parameter of driver 866 MHz is 14.266 dB which is shown in

figure 6.6.

Figure 6.6 S21 parameter of driver.

118

S12 The value of S12 parameter at 866MHz is -22dB, depicted in figure 6.7.

Figure 6.7 S12 parameter of driver S22

The S22 parameter of driver at 866MHz is -11.24 dB which is shown in figure in

6.8.

Figure 6.8 S22 parameter of driver

119

6.1.1.3 Power Amplifier

First of all, we measured S parameters of the power amplifier. It was connected to

the network analyzer and one by one all the four S parameters were checked. Figure 6.9

shows the testing setup.

Figure 6.9.Testing of power amplifier by Network Analyzer Following are the results showing S parameters of the driver. S11

The value of S11 parameter at 866 MHz is -19.83 dB, depicted in figure 6.10.

Figure 6.10 S11 parameter of power amplifier

120

S22

The value of S22 parameter at 866 MHz is -24.596 dB which is depicted in figure

6.11.

Figure 6.11 S22 parameter of power amplifier S21

The value of S21 parameter at 866 MHz is -19.710 dB which is depicted in figure

6.12.

Figure 6.12 S21 parameter of power amplifier

121

6.1.1.4 Switch for ON-OFF Keying

We tested the switch and observed its output waveform on high frequency

oscilloscope. For testing purposes, we used a dummy input signal from function

generator. It was switching properly without loading the input signal. Figure 6.13 shows

its output with modulated signal.

Figure 6.13 Output of the switch

6.1.1.5 Antenna

For testing antenna, we connected it to a port of Network Analyzer and S11

parameter was measured. Then, using the Marker search option we located the minimum

point on the curve. The dip was on 915 MHZ meaning that it was giving minimum return

loss at this frequency. Figure 6.14 shows the testing setup.

122

Figure 6.14 Testing setup for patch antenna

Figure 6.15 shows the curve obtained on Network Analyzer showing a dip of -

19.165dB at 915MHz.

Figure 6.15 Return loss of patch antenna having minimum value at 915MHz

Remarks: Due to the substandard substrate, we got minimum return loss at 915 MHz instead of 868 MHz. But our tag and reader could operate from 860-960MHz. Therefore we could work with this antenna with out any losses.

123

6.1.2 System Level Testing After testing each component individually, we integrated the system. First of all,

synthesizer was loaded. Its output was connected to the driver which raised its power

level from 0.5 dBm to 17 dBm. After that, power amplifier was used to further enhance

the power level to 29 dBm. This signal was then coupled to antenna for transmission. The

power supply of driver was switched on and off by using the switch which has been

discussed above. At the output of power amplifier, we connected the high frequency

oscilloscope to check the time domain waveform of our transmitted signal. Following

figures showing step by step testing of our complete RFID system.

Figure 6.16 Synthesizer connected to driver.

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Figure 6.17 Output after driver module.

Note the peak at 868 MHz with power level of 17 dBm.

After that power amplifier was connected via connector cable. Figure 6.18 shows

this setup.

Figure 6.18 Power amplifier connected

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The result shown by Spectrum Analyzer is given below. It is giving a peak at

868MHz with a power level of 29dBm.

Figure 6.19 Output after power amplifier

After that antenna was connected and complete setup is shown in Figure 6.20.

Figure 6.20.Complete transmitter part

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After that we analyzed the output signal on high frequency oscilloscope. It is

shown in figure 6.21.

Figure 6.21 On Off Keying of carrier signal.

6.2 Signal Generation and Processing on ATMega32 Results

In implementing the signal generation and processing part on microcontroller we

experienced a variety of practical challenges.

Firstly, since all of the code was written in Interrupt Service Routine (ISR), timely

execution of ISR before next event can occur, was critical to our project. This required

the use of fast processing microcontroller having high clock speed. Most of the

microcontrollers have the limitations on the maximum achievable clock frequency.

Controllers with maximum clock frequency of 8 MHz were commonly available. The

Atmel ATMega32 provided us with maximum 16 MHz clock and a throughput of

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16MIPS at this frequency. This clock frequency of 16 MHz worked fine for the

transmitter side but this was still less than our requirements for the receiver side. So, we

approached this problem in two ways. Firstly, we optimized our code in order to achieve

timely execution of ISR before the next data bit can arrive. Secondly, we decreased our

data rate from the Tag to Reader (as was allowed by EPC protocol). This gave us

maximum time between two ISR events and hence the ATMega32 worked well for our

RFID system. Using a higher clock frequency controller was also another option, but this

would have caused delays because such controllers are not commonly available and their

cost is too high to be affordable at student level projects.

It is noteworthy that testing of such a code on a simulator was not possible. Our

requirement was the generation of real time data which gave erroneous results using the

simulator. So throughout the course of our implementation, we had to emulate our code

in order to validate it. Also, we had to test our generated commands separately so that we

could get a better view of their timings. The following simulation results show different

commands generated individually. These simulation results are the output of a storage

oscilloscope. The timings of different commands are according to our design, as

explained previously in Article 5.3.5.

6.2.1 FrameSync Command As observed on the storage oscilloscope, the FrameSync command looked like as

shown in Figue 6.22. In this figure the FrameSync command is shown in a loop, with

each FrameSync command separated from the next by a short high time interval.

Total Time for a Fram and eSync comm

Figure 6.22 – Simulation FrameSync

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Figure 6.23 shows the FrameSync command highlighting RTcal time interval..

∆ showing the high time duration for RTcal

Figure 6.23 – Simulation FrameSync highlighting RTcal

6.2.2 Preamble Command The following simulations results show the preamble command in a loop. Figure

6.24 highlights different parameters of a preamble command. Figure 6.25 shows the time

interval of RTcal. The interval between a preamble command and the next is separated by

a short high time interval.

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Figure 6.24 – Simulation Preamble

Figure 6.25 – Simulation Preamble highlighting RTcal

Delimiter

RTcal TRcal

∆ showing RTcal time

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6.2.3 Select Command The following simulation results show the Select command. Figure 6.26 shows

the total time for Select command. For this run, the Pointer field in Select command was

taken to be 8 bits and the Mask field was also taken 8 bits. With these values, the total no.

of bits came out to be 53.

∆ showing the total duration for Select command

Figure 6.26 – Simulation Select Command

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6.2.4 Query Command The following simulations show the Query command as seen on storage

oscilloscope. Figure 6.26 shows the total duration of Query command. The Query

command has 22 bits.

Figure 6.27 Simulation Query Command

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6.2.5 QueryRep Command The following simulations show the QueryRep command as seen on storage oscilloscope.

Figure 6.28 Simulation QueryRep Command

6.2.6 T4, Minimum Time Wait between Reader Commands The following simulation shows time wait T4 after sending a Select command.

This is the minimum time between Reader commands. Its length was calculated in Article

5.3.5.3.

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∆ showing the T4 duration

Figure 6.29 – Simulation T4

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7 Conclusions and Outlook

7.1 Conclusions

RFID system implementation has provided us a deep insight into the principles

and working of RF circuits, antenna design, EPC Global protocol and its implementation.

In the beginning, we were using a local oscillator and a mixer for up conversion and

down conversion of our Tx. and Rx. signal. But then we devised a simpler and a more

efficient approach. We generated our carrier signal directly from frequency synthesizer

and modulated the carrier by simply switching the power supply of power amplifier

driver before transmitting over the air interface. Initially, we planned to use circulator

before antenna to avoid using separate antennas for Tx. and Rx. But then, to make our

system economical, we ended up using two separate antennas for transmitting and

receiving data. Another practical issue was the availability of components. All the

components were purchased from abroad and then shipped to Pakistan. This caused a

long delay in the actual implementation of our project. Other than that, we faced some

problems in PCB manufacturing and antenna implementation due to lack of resources and

adequate facilities.

As a result of above mentioned problems, we were only able to develop our

application to a primitive level. Had the components come earlier, we would have taken

our project to a much advanced level.

During the course of this project, we used the ADS, HFSS, Ansoft designer

softwares which greatly enhanced our design skills. The hardware testing was done on

the spectrum analyzer, network analyzer and high frequency oscilloscope.

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7.2 Outlook

There are still improvements and a lot of future work which can be done to fully

utilize the advantages of RFID in libraries. From circulation desk to tracking, and annual

record keeping of books to security checks at exit, and a lot of other prospects are there.

In our project, we were able to read a tag and get the information stored on it. If smart

cards are used in conjunction with RFID, it can provide contact less check-in and check-

out as well.

In future RFID can be operated with wireless sensor networks. A lot of research

work is being carried out in this field. In this way, using RFID with sensors would bring

forth a whole new look to our world.

The main future development of our RFID system can be in library management,

to search for a book. This thing can be achieved by networking many readers in the

library. These all readers can have their own specific code for detection of its location. So

each time, when a book has to be searched, the master reader checks its vicinity first of

all and in similar manner contacts the other readers (slave readers). The other readers

now become masters and this process carries on. If in the end no reader finds the book it

means the book is not present other wise some reader sends the code. In this case the

other readers are commanded to stop searching and keep their search results in a cache

memory.

In one line, we can summarise the future prospects of RFID systems by narrating

that with the ongoing work on RFID systems, near future will see the whole world

Tagged.

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References [1] Simson Garfinkel, Beth Rosenberg, “RFID Applications, Security, and Privacy”

PEARSON Education 2006 ISBN 81-317-0166-2

[2] Klaus Finkenzeller, “RFID Handbook Fundamentals and Applications in

Contactless Smart Cards and Identification: 2nd Edition” John Wiley & Sons 2003

ISBN 0-470-84402-7

[3] Jari-Pascal Curty, Mitchel Dectercq, Catherine Dehollain, Nobert Joehl, “Design and

Optimization of Passive UHF RFID Systems” Springer Science 2007

ISBN 0-387-35274-0

[4] LibBest Library RFID Management System.http://www.rfid-library.com/

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Appendix

Data sheet of RF components

Data sheet of VCO ADL4360-7

138

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

140

For performance characteristics visit www.analog.com

141

http://www.analog.com/

Data sheet of Power Amplifier ADL5322

142

Data sheet of Driver5530

144

ABSOLUTE MAXIMUM RATINGS

TYPICAL SCATTERING PARAMETERS

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

145

Data sheet of TH7122

Pin Description

146

Register Description

151

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