AASCIT Journal of Physics 2017; 3(4): 18-27 http://www.aascit.org/journal/physics ISSN: 2381-1358 (Print); ISSN: 2381-1366 (Online)

Keywords Balanced Full-Wave Detector,

Micro Controller,

Demodulation,

Phase Locked Loop

Received: April 29, 2017

Accepted: July 30, 2017

Published: September 20, 2017

Development of Am Field Strength Meter

Babalola Micheal Toluwase1, *

, Akeredolu Bunmi Jacob2, *

,

Ewetumo Theophilus3

1Department of Physics, Afe Babalola University, Ado Ekiti, Nigeria 2Department of Pure and Applied Physics, Federal University, Wukari, Nigeria 3Department of Physics Federal University of Technology, Akure, Nigeria

Email address bunmi_dolu2002@yahoo.co.uk (A. B. Jacob) *Corresponding author

Citation Babalola Micheal Toluwase, Akeredolu Bunmi Jacob, Ewetumo Theophilus. Development of Am

Field Strength Meter. AASCIT Journal of Physics. Vol. 3, No. 4, 2017, pp. 18-27.

Abstract The Amplitude Modulated field strength meter circuit consists of an antenna, AM

demodulation unit with signal indicator output, tuning circuit comprising of a phase lock

loop (PLL), integrator, differential amplifier, frequency divider, microcontroller output

with serial communication interface and a varactor. The AM demodulator IC uses

balanced full-wave detector method for demodulation. Internally, it has a full wave

rectifier circuit that gives signal strength indication output, and frequency oscillation that

can give audio output as well. The output signal is fed into an analogue to digital

converter (ADC) of a microcontroller (PIC 16F877) to display the measured value. This

system will display the frequency received by dividing the frequency from the oscillator,

and this will be fed into PIC16F877 to give station frequencies that will be locked into.

To be able to confirm the stations, an audio output is incorporated so that the station

programmes can be heard. Tuning to different stations is done using PLL circuit which

generates a local oscillation via a varactor for balanced full-wave demodulation. The

system is equipped with facility to interface with computer via comport using visual

studio2008.net program to store the data at every two seconds time interval.

1. Introduction

Communicating over long distances has been a challenge throughout history. In

ancient times, runners were used to carry important messages between rulers or other

important people.

Modern telecommunications began in the 1800s with the discovery that electricity can

be used to transmit a signal. For the first time, a signal could be sent faster than any other

mode of communication e.g. transportation. Yet there are many challenges which face

the so called modern communication system, such as a barrier that reflects the signal

away from reaching the targeted audience. More importantly the strength of signal

transmission which conveys information to people varies with the time of the day

because of topography and weather [1]. Addressing this problem leads to development of

monitoring equipment to express the quality or value of a particular signal strength at a

certain distance from its transmission station [29]. The paper will assist in the

development of field strength meter.

In Telecommunications, a field strength meter is a measuring device which measures

the intensity of the electric field caused by a transmitter. A field strength meter in its

simplest form is a simple radio receiver. The tuner circuit is at the front end; then the

19 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter

signal is detected and fed to a micro-ammeter, which is

scaled in dBųv [29]. The frequency range of the tuner is

usually within the terrestrial broadcasting bands, though

some field strength meters can also receive signals in the

gigahertz range. In an ideal free space, the electric field

strength produced by a transmitter with isotropic radiation is

readily calculated as [27]

E= √

(1)

where E is the electric field strength in volts per meter, P is

the transmitter power output in watts and d is the distance

from the radiator in metre.

Field intensity measurements are made in support of a

reference. Section 12.4.2.1 of the Nigeria broadcasting code

NBC [22] specifies that a broadcaster shall maintain

specified minimum values of field strength within its

assigned coverage area depending on its mode of

transmission and its location’. Accordingly, the acceptable

minimum values shall be as indicated in table 1:

Table 1. The acceptable minimum values of field strength within assigned coverage area in Nigeria.

Urban Areas microvolt/m Rural Areas

i. AM Sound Broadcasting 72 dB i. AM Sound Broadcasting 66 dB

ii. VHF-FM Sound Broadcasting 60dB ii. VHF-FM Sound Broadcasting 48dB

iii. VHF-Television (Band III) 60dB iii. VHF-Television (Band III) 49dB

iv. UHF-Television (Band IV) 65dB iv. UHF-Television (Band IV) 60dB

v. UHF- Television (Band V) 70dB v. UHF- Television (Band V) 60dB

The AM field strength meter measures the strength level of a

signal that is being received from any AM transmitter. It detects

the electric field of AM radio frequency (RF) signals. The meter

reads the signal strength directly on the display [3, 17, 29]. To

design sensible and sensitive AM field strength meter, it is

useful to understand the amplitude modulation process.

Amplitude modulation (AM) occurs when the amplitude of

a carrier wave is modulated, i.e the process of impressing a

low frequency intelligent signal into a high frequency carrier

signal [19], and it has an equation of the form:

= [ + (t)]cos 2 t+ ∅ ) (2)

where the modulating signal component (t) is added to the

carrier amplitude , then the modulated carrier wave is

, the term [ + (t)] describes the envelope of the

modulated wave. The frequency is the frequency of the

carrier while ∅ is its phase [12]. A device that performs modulation is known as a modulator and a device that performs the inverse operation of a modulator is known as a demodulator (sometimes detector or demodulator). A device that can do both operations is a modem. [2]. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones [28]. A perfect AM transmitter, modulated with a single sine wave tone would radiate a constant average power regardless of modulation level, with spectral power shared among the assigned frequency carrier and two sideband frequencies above and below the assigned centre frequency by the frequency of the modulating tone. A field strength meter, or calibrated power bolometer, would read a constant value, with or without a modulating tone [17]

2. Method

2.1. Basic Description of the Block Diagram

The block diagram of an AM field strength meter is shown

in figure 1. It works as follows. An antenna A1 intercepts an

AM radio wave and supplies the signal to a radio frequency

amplifier S1 of a receiver chip U0. The radio wave is

amplified by the RF amplifier S1 and is fed to a mixer S2

together with oscillator signal from controlled oscillator S3.

The controlled oscillator signals is generated from frequency

synthesiser and fed into the mixer; multiplicative mixing

occurs between the AM rf input and oscillator signals. The

mixer output provides the intermediate frequency (IF) at pin

1 of the receiver U0. The output signal from IF filter is fed to

IF amplifier via pin 3. The output signals from IF amplifier

S4 is internally fed to the balance full-wave detector S5. The

balanced full-wave detector demodulates an audio signal

which corresponds to a modulated broadcast signal. The

detected audio frequency output is passed through an AF pre-

amplifier S6 to audio amplifier S7 through pin 6.

The Local oscillator signal from the local oscillator S3 via

pin 10 of AM receiver U0 is fed to comparator and inverter

which convert the sine wave to square wave before input to

programmable divider U4. The divider divides the frequency

fx of the channel signal with a predetermined ratio N. The

new signal is fed to a phase detector of PLL U19 which is

supplied further with the reference signal from reference

oscillator U21. The detector compares the frequency divided

signal with the reference signal and produces a signal which

represents the frequency difference and/or phase difference.

The signal has its ripple component removed by a low pass

filter (LPF), and is therefore amplified into a control signal.

The DC voltage level of the control signal is supplied to the

local oscillator S3. The oscillation frequency of the local

oscillator is changed according to the voltage level of the

control signal. The local oscillator S3 is therefore called

"voltage controlled oscillator (VCO)". The signal from a

field strength indicator is amplified and fed through the

microcontroller to the display. The control key input the

frequency of the channel station, and both the field strength

readings and the frequency of channel station are displayed

AASCIT Journal of Physics 2017; 3(4): 18-27 20

on the LCD. The audio signal is taken from pin 6 of U1 and re-amplifies before it is fed to the speaker.

Figure 1. Block diagram of AM Field Strength Meter.

2.2. Circuit Description

The circuit in figure 3 can conveniently be divided into

three parts; the AM receiver circuit, frequency synthesizer

circuit and controller circuit.

2.2.1. Receiver Circuit

The sensing unit (antenna) used, was a monopole antenna

of 50 ohms, which was capable to intercept or capture a radio

wave travelling through space and able to deliver them to a

receivers. Other features of the antenna include its length,

power received and antenna factor, made it an ideal choice

for this project. The main component of the receiver is a

TDA1572 AM receiver chip U, around which everything else

was based, TDA1572T is a 16-lead mini pack plastic package,

normally measuring and yet it contains 10 major circuit

blocks, these blocks are integrated on a chip. TDA 1572T

performs all the main function of an AM superhet radio

receiver including RF Amplifier, Local Oscillator, Mixer,

Detector, AF Amplifier, AGC system and Field strength

indicator. The captured signal from an antenna is fed to pin

14 of RF amplifier via capacitor C23. The differential

amplifier in the RF stage of TDA 1572T employs an AGC

negative feedback network to provide a wide dynamic range.

Very good cross-modulation behaviour is also achieved by

AGC delays at the various signal stages. Low noise working

is achieved in the differential amplifier by using transistors

with a low base resistance. A double balanced mixer provides

the IF output at pin 1 of TDA1072. Local oscillator operates

at 455kHz above the wanted incoming signal frequency i.e if

the wanted signal RF is 1000kHz, then the local oscillator

frequency will be 1455kHZ. The oscillation frequency is

generated by a frequency synthesizer with varactor D1, and a

parallel tank (made up of coil L1, and capacitor C20 and C2)

connected between pins 11 and 12 of IC TDA1072. The local

oscillator needs to be tuned to a range 990 to 2100 kHz, such

that, it’s frequency plus the IF frequency (455 kHz) equals

the received frequency, that is a range of 990 to 2100kHz. i.e

from (535 + 455 to 1645 + 455kHz). A varactor diode D1

which gives about 90 to 200pF along with the capacitor C21

set the tuning span and the tank inductor L1 of 95, formed

by a copper wire wound round a ferrite core. Extra oscillator

output from pin 10 of IC TDA1072 is fed to programmable

divider through transistors Q1 and Q2 and a comparator U32

which are used to amplify and convert the analogue signals

(sine wave) to digital (Square wave) to form input signal to

programmable divide The intermediate frequency signal from

21 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter

the mixer output via pin1 of TDA 1072 is filtered through IF

filter. Designing of IF filter was done using the formula,

= 12√ (3)

Where is the centre frequency, L is the inductor and C is the capacitor.

Since 455kHz was selected as centre frequency, the IF

filter, FL1, consists of two pole ladder crystal filter X3 and

X4 of 455kHz, a matching transformer TR1, capacitors C28

and C29. The matching transformer has 13 turns on the

primary coil and 9 on the secondary coil of TR2. A trimmer

capacitor C50 is used to resonate the inductance of 31H of

the primary coil of TR1.

C29 can be calculated as follows: = 12√ , the

value of C29 is given as; = 3.9$%

The TR1 and C29 are used to tune to 455 kHz, giving a

good match for IF filter: a 2 pole ladder filter is from toko.

Inside the TDA 1572T the full- wave balanced envelop

detector is used for demodulation of the intermediate

frequency to audio frequency signal, internally it has a low

pass filter to block residual IF carrier from signal path. The

audio frequency signal is amplified by AF pre-amplifier,

which uses an emitter follower with a series resistor (inside

the TDA1072), together with an external capacitors, provides

the required low-pass filtering for AF signals. The audio

filter at pin 6 of TDA1072 consisting of the capacitor C27,

C26 and resistor R33. The AF output voltage at input signal

50& is 130mV, (Data sheet TDA 1072) so there will be

need for amplification, because a normal audio line is 0.5 to

2 Volts. (Kennick, 2001). LM 386 is used to amplify the

audio signal before fed to speaker.

The AGC amplifier provides a control voltage which is

proportional to the carrier amplitude. Second-order filtering

of the AGC voltage achieves signals with very little

distortion, even at low audio frequencies. The AGC voltage

is fed to the RF and IF stages via suitable AGC delays.

The data sheet of IC TDA 1072 has a graph of field

strength voltage indicator in volts against RF input signal in

dBµV, this graph has good linearity for logarithmic input

signals over the whole dynamic range. A buffered voltage

source from the IC TDA 1072 provides a high-level field

strength output signal. At maximum voltage of 28V, the

corresponding RF input is 120dBµ& and this was used in

calibrating the field strength of the received signal. Philips

semiconductors (1989)

2.2.2. Controller Circuit with User Input and

Output Interface (Microcontroller) and

LCD

The pic16f877 is a controller used in this paper. It is an 8

bit RISC (reduced instruction set computer), central

processing unit. It is a complete computer on a chip, and

entire processor, memory and the I/O interfaces are located

on a single piece of silicon. For this reason, it takes less time

to read and write to external devices. Pic16f877 U1 has three

types of memories which are: flash program memory,

Electrical erasable memory and static random access memory.

Pic16f877 is also has 33 inputs and output pins that can be

individually configured as inputs or outputs

These ports have been assigned to the various tasks in the

circuit;

PORTA bit 0 is configured via firmware as an analogue

input for analogue to digital conversion. One channel is used

in this project to convert received signal strength from pin 9

of TDA1572T, in analogue form to digital, which will be

then be displayed on the LCD unit,

PORTC bits 6 and 7 were used as transmit and receive pin

for asynchronous communication with a user PC (personal

computer) using RS232. PORTA bits 1, 2, 3, 4, 5 and

PORTE bit 2 are used for the user keypad. Each of these pins

was configured as inputs. A 10 kΩ (R1 to R6) pull-up

resistor was used for each pin, while a de-bounced button

serves as the keypad. Via firmware the PIC monitors the state

of each pin. When not depressed the PIC sees logic 1 and

does nothing and when depressed the PIC sees logic 0 and

performs the associated routine attached to it. The keypad

allows channel selection from 500 kHz to 1699 kHz.

PORTB and the lower 4 bits of PORTC were configured

as output and were used to give data for the programmable

divider used in the frequency synthesizer

PICs generally have internal oscillator buffer configuration

option. In the circuit design, the HS (high speed crystal)

option is used with a 16 MHz crystal X1, because 16 MHz

crystals are widely available in the market. Also the circuit

clock is the time base for the generation of the serial

communication baud rate with the PC

2.2.3. Frequency Synthesizer

A frequency synthesizer is a circuit design that generate a

new frequency from a single stable reference frequency. For

frequency span 535 KHz to 1605 KHz, the standard divide

by N value and a fixed divide by M value of 2 are used.

Since N needs to be between 535 and 1605, The ICs required

for the programmable divider is LS74161. The LS74161 is a

4 bit synchronous, pre-settable counter. This counter is fully

programmable; the output may be preset to logic 0 or to

logic1. Pre-setting is synchronous. To achieve the

programmability from 990 to 2060 for this work a 12 bit

counter is needed. Three LS74161 (U33, U32 and U31) are

cascaded to make a 12 bit counter. The output from the VCO

is coupled to the programmable counter via a 2.2uf capacitor

C7 and converted to square wave using the 7414 Schmitt

trigger NOT gate U34.

AASCIT Journal of Physics 2017; 3(4): 18-27 22

Figure 2. Block diagram of frequency synthesizer.

The reference oscillator is 500 Hz, A crystal oscillator of 8

MHz, X2 was obtained and divided down to 500 Hz. The

circuit is shown in figure 2. The NOT gate 7404, (U15, U16

and U34) serves as amplifier (buffer) for the crystal to

oscillate and the two 1kΩ resistors (R9 and R10) are the

feedback resistors for the amplifier. LS74161, a 4 bit counter

divides 8 MHz, X2 by 16 to get 500 kHz, Also three 4017

decade counters (U21, U22 and U23) are cascaded to form a

divide by 1000 counter, the result is that 500 KHz is divided

by 1000 to get 500 Hz, on pins 12 of the 3rd 4017, U38

2.2.4. Phase Locked Loop

The integrated circuit CD4046, U36 is the phase locked

loop used in the circuit. The reference oscillator output signal

of 500Hz is fed on phase comparator II of PLL. The output

of VCO (from TDA1572T receiver) is fed back to the phase

comparator II second input, through a programmable divider

that has been programmed to divide by any number from 990

to 2100. The loop will lock when the frequency fed back

through programmable divider is equal to 500Hz of reference

oscillator frequency. A simple RC filter is employed as the

loop filter. Where C12 is fixed at 1000µf, the value of R

varies via a digital switch between 10 kΩ, 100 kΩ and 330

kΩ. The CD4066 analogue switch is controlled by the PIC

controller. The need for using different loop is to ensure fast

lock and smooth lock. The centre frequency of the low pass

filter is given as

= 12( (4)

This gives a frequency cut-off of 0.015Hz for R = 10 kΩ,

0.0015Hz for R = 100 kΩ and 0.00048Hz for 330 kΩ. The

output of the phase comparator/detector is amplified using

LM 741 OP-AMP. A non-inverting amplifier configuration is

used. The gain of the stage is;

) 1 (*(++

(5)

Where RF is 2.2 kilo-ohms (R17) and 1 kilo-ohms (RV2)

variable in series

R18 is 10 kilo ohms, Therefore,

) 1 2.2 110 = 1 3.2

10 = 1.325

This gives a gain of less than 2

2.2.5. Audio Amplifier

The LM386 is a power amplifier designed for use in low

voltage consumer applications. The gain is internally set to

20 to keep external part count low, but the addition of an

external resistor and capacitor between pins 1 and 8 will

increase the gain to any value from 20 to 200. The inputs are

ground referenced while the output automatically biases to

one-half the supply voltage. The quiescent power drain is

only 24 milli-watts when operating from a 6 volt supply,

making the LM386 ideal for battery operation.

2.2.6. RS232 Serial Port

Serial Ports come in two sizes; there are D-Type 25 pin

connector and the D Type 9 pin connector both of which are

male on the back of the PC, thus you will require a female

connector on your device. RS-232 communication is

asynchronous, a clock signal is not sent with the data. Each

word is synchronized using its start bit, and an internal clock

on each side, keeps tabs on the timing. The first step in

connecting a device to the RS-232 port is to transform the

RS-232 levels back into 0 and 5 Volts. This is done by RS-

232 Level Converters. An example of such a device is the

MAX-232 use in this work.

2.2.7. Power Supply

The circuit is powered from an 18 volts rechargeable

battery source. A transformer is provided to charge the

battery. The voltage is then regulated to 12 volts and 5 volts

for each stage using voltage regulator 7805.

23 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter

Figure 3. Complete Circuit Diagram of AM Field strength meter.

A firmware program was developed to test the working of this circuit, below is the flow chart used.

AASCIT Journal of Physics 2017; 3(4): 18-27 24

Figure 4. Flow Chart for the Microcontroller.

Figure 5. PC graphic user interface.

Plate 1. Real-Time Data Logging with Laptop.

25 Babalola Micheal Toluwase et al.: Development of Am Field Strength Meter

The flow chart is then converted to the source codes using

ASSEMBLY program. The Real-Time Data Logging with

Laptop is shown in plate 1. The PIC programmer is used to

“burn” the source codes into the PIC. The serial port program

is written using visual basic dot net 2008 edition. This

program reads the serial port data and displays in a text box

with the TIME information shown in figure 5. The complete

work is shown in plate 2

Plate 2. Complete work of FSM.

3. Operation and Performance of the

System

3.1. Calibration of the Field Strength Meter

The AM field strength meter was calibrated against a

laboratory standard. The calibration was made to prove the

performance of the equipment after the construction. The

meter was also calibrated with the standard and imported

field strength meter lag-gear, model. TC805C. The two

meters were used together for measurement at the same time

at different locations in Ibadan Oyo state. Before the

measurement the two meters were switched on, the

constructed meter was adjusted to give the same reading with

the standard meter at initial reading of 30dBµV before tuning

to the station. On tuning the two meters to the AM broadcast

transmitting station, broadcasting at frequency of 756 kHz,

readings were recorded for five different locations. The result

shown in figure 6, gives 95% accuracy with imported Model

TC805C.

3.2. Cost Analysis

One of the advantages of using the newly constructed AM

field strength meter is cost efficiency of the meter. By

analysing the cost of the constructed AM field strength meter,

the total cost is N35,000.00 ($175), but the minimum cost of

acquiring a field strength meter of the same frequency range

from abroad ranges from N100,000.00 to N300,000.00 ($280

to $840), the rate of conversion was $1 to N360. This clearly

indicates that the newly developed FSM is cheaper since part

were sourced locally. Table 2 shows the cost analysis of the

field strength meter.

Table 2. Cost analysis of FSM.

Materials Quantity Cost (N) Cost ($)

AM receiver 1 2,000 6.0

Battery 3 2,400 7.0

Transformer/ crystal Oscillators 1 1,200 3.3

PIC 16f877. Microcotroller 1 1,000 3.0

Other components 20,000 56.0

Casing 1 3,000 8.0

Transportation 3,000 8.0

Total N35,000 $91.30

3.3. Testing of Am Field Strength Meter

On completion of the design and construction work,

certain performance tests were carried out with the meter and

the available one (imported field strength meter lag-gear,

model TC805C) from the Electrical Engineering Department,

University of Ibadan in Ibadan, Oyo State. The test took

place in five different locations in Ibadan; the two

instruments were used to measure the AM field strength from

AM transmitter of the Broadcasting Corporation of Oyo State

in Ile Akede, Bashorun, Ibadan, transmitting at the frequency

of 756kHz.

4. Result and Discussion

The constructed meter performance was evaluated in terms

of sensitivity and against standard imported FSM. Figure 6

shows the result of the field strength values in decibel-

microvolt -.& against distance of various locations

where the measurement took place in kilometres (km) for

two meters (imported and constructed FSM). It was observed

that, as the two meters were moving away from the

transmitter in kilometres, there was gradual decreased in the

field strength measurements. At Bodija market located 2.9m

away from Akede AM Transmission station, the variation

percentage in the values of the two meters was about 0.1%.

Also at UI (5.8km away), the variation percentage in the two

meters was 0.05%, at N. B (7.0km away) the variation in the

values of the two meters was 0.001%. But at Ibadan airport

(8.0km away) and IITA (10km away) there were no

significant variations in the value recorded in the two meters,

i.e the meters read the same value in decibel-microvolt

((dBµV)). The variations in values of the FSM measurement

of the two meters were as a result of minor errors that might

occur during the development of the constructed meter.

AASCIT Journal of Physics 2017; 3(4): 18-27 26

Figure 6. Graph of field strength (dBµV) against distance (Km).

5. Conclusion

An AM field strength meter capable of measuring AM

signals from any AM broadcasting transmitters with centre

frequency of 500kHz to 1650kHz, bandwidth of 10khz and

gain control from 0 – 120dBµV was designed, constructed

and tested. The system consist of an antenna, AM

demodulation unit with signal indicator output, pre-amplifier,

frequency divider, tuning circuit comprising of a phase lock

loop (PLL), integrator, differential amplifier, microcontroller

output with serial communication interface and a varactor.

The AM field strength meter integrates a digital tuning using

frequency synthesisers, and data logger for interface with a

computer. Frequency synthesiser helps to overcome some

problem of accuracy and stability of signals.

A comparison test was conducted with this meter using the

signal level meter, lag-gear, model TC805C as a standard

meter. The work testing exercise was conducted in five

different locations in Ibadan city, Oyo State. Comparing the

readings of the two meters (imported and constructed FSM),

the result shows that the constructed field strength meter was

specify with accuracy of 99%

Hence, the use of this system is limited to area where AM

Transmission is available

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