HIGH SPEED ROM FOR DIRECT DIGITAL SYNTHESIZER

APPLICATIONS IN INDIUM PHOSPHIDE

DHBT TECHNOLOGY

By

Sanjeev Manandhar

B.S. University of Maine, 2004

A THESIS

Submitted in Partial Fulfillment of the

Requirements for the Degree of

Master of Science

(in Electrical Engineering)

The Graduate School

The University of Maine

August, 2006

Advisory Committee:

David E. Kotecki, Associate Professor of Electrical and Computer Engineering,

Advisor

Donald M. Hummels, Professor of Electrical and Computer Engineering

Richard O. Eason, Associate Professor of Electrical and Computer Engineering

LIBRARY RIGHTS STATEMENT

In presenting this thesis in partial fulfillment of the requirements for an advanced

degree at The University of Maine, I agree that the Library shall make it freely available

for inspection. I further agree that permission for “fair use” copying of this thesis for

scholarly purposes may be granted by the Librarian. It is understood that any copying

or publication of this thesis for financial gain shall not be allowed without my written

permission.

Signature:

Date:

HIGH SPEED ROM FOR DIRECT DIGITAL SYNTHESIZER

APPLICATIONS IN INDIUM PHOSPHIDE

DHBT TECHNOLOGY

By Sanjeev Manandhar

Thesis Advisor: Dr. David E. Kotecki

An Abstract of the Thesis Presentedin Partial Fulfillment of the Requirements for the

Degree of Master of Science(in Electrical Engineering)

August, 2006

An increasing demand for multi-GHz direct digital synthesizers has prompted research

for high-speed phase to amplitude converters. A high-speed read-only memory (ROM)

look-up table operating at 20-40 GHz range clock frequency can be used as a high-speed

phase to amplitude converter. A 16x6-bit ROM, employing an architecture suitable for

use as a phase to amplitude converter for DDS, has been implemented in InP double

heterojunction bipolar transistor (DHBT) technology. The ROM uses a -3.8 V power

supply and dissipates 1.13 W of power. The ROM is implemented in a test circuit that

includes an 8-bit accumulator and a 6-bit digital to analog converter (DAC) to facilitate

demonstration of high-speed operation. The maximum operating clock frequency is

measured to be 36 GHz. To increase the bit size of the ROM two 32x6-bit ROMs were

designed. The first 32x6-bit ROM is designed for low power consumption; the second

32x6-bit ROM is designed to operate at maximum clock frequency. The schematic

simulation results showed these ROMs operated at 20GHz and 46GHz clock frequency

respectively. The low power design consumes 1.95 W and high-speed design consumes

7.07 W of power.

ACKNOWLEDGMENTS

This work has been supported by the U.S. Army Research Lab and Defense

Advanced Research Projects Agency (DARPA) under contract DAAD17-02-C-0115. I

would like to thank Mr. Frank Stroili and Mr. Richard Elder at BAE Systems, Dr. Steve

Pappert at DARPA MTO, and Dr. Alfred Hung at U.S. Army Research Laboratory for

their guidance and support.

I would like to thank my mom and dad for their support and guidance through

out my academic endeavor.

Thank you, Professor Kotecki for your guidance, suggestions and advices dur-

ing this research work. Thank you, Professor Hummels and Professor Eason for your

valuable advices. Finally, thank you, Alma, Steve, Ryan, Zhineng, Bingxin for you help

and happy hours during these past two years.

ii

TABLE OF CONTENTS

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

LIST OF TABLES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

Chapter

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Purpose of the Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3. Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. ROM LUT and Bipolar ROM .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1. ROM Look-Up Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2. Bipolar ROM Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1. The Emitter-Coupled Logic Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.2. ROM memory cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.2.1. Schottky cell ROM model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2.2. Transistor cell ROM model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.3. Row Decoder using Bipolar Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3.1. Schottky Barrier diode decoder combined with

a pull-up circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3.2. ECL-based Decoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.4. Bipolar Sense Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3. Bipolar ROM in DDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.4. ROM Compression Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3. 16x6-Bit High Speed Bipolar ROM in InP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1. Circuit Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1. Row Decoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.2. ROM Memory Cell Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.1.3. Sense Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2. Test Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.3. Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.4. Measurement Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4. 32x6-bit ROM designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.1. ROM Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

iii

4.1.1. ROM with 32x6-bit Memory core. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.1.2. 32x6-bit ROM with 2:1 MUX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2. Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.1. Summary of Accomplishment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.2. Recommendation for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

BIOGRAPHY OF THE AUTHOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

iv

LIST OF TABLES

Table 2.1. The comparison of different ROM compression techniquesfor ROM LUT in DDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Table 3.1. The bit pattern in the ROM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 4.1. The bit pattern in Bit 4 of accumulator, Bit 5 of ROM A andBit 5 of ROM B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Table 4.2. The power dissipated by ROMs with different memory core sizes. . . . . 43

Table 4.3. Power and speed comparison of three different ROMs . . . . . . . . . . . . . . . . 44

v

LIST OF FIGURES

Figure 1.1. Block diagram of a Sine-output DDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 2.1. Basic ECL gate consisting differential pair and emitter follower. . . . . . . 8

Figure 2.2. Schottky diode memory cell array. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 2.3. Transistor memory cell array. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 2.4. Schottky Barrier Diode Decoder with Pull-up Circuit. . . . . . . . . . . . . . . . . . 12

Figure 2.5. Predecoder design in ECL-based Decoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 2.6. RAM Cell Sense Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 3.1. Block diagram of 16x6-bit ROM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 3.2. The schematic of a single row of the row decoder circuit. . . . . . . . . . . . . . . 21

Figure 3.3. The simulation showing only one row of decoder output islogic high while others stay low. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 3.4. The schematic of the ROM memory cell array with sense amplifier. . . 23

Figure 3.5. The schematic of the sense amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 3.6. Functional schematic of the ROM test circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 3.7. Simulated differential output voltage from the ROM underfour different simulation settings: (1) schematic of the rowdecoder, the ROM memory cell array, and the sense am-plifier without layout parasitics: (2) the row decoder withlayout parasitics, schematic of the ROM memory cell arrayand sense amplifier without layout parasitics: (3) schematicof the decoder without layout parasitics, the ROM memorycell array and the row decoder with layout parasitics: and(4) the row decoder, the ROM memory cell array, and thesense amplifier with layout parasitics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 3.8. The skew in the row 16 of MSB when the ROM test circuitis clocked at 36 GHz and FCW set to 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 3.9. Microphotograph of ROM test circuit.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 3.10. The DAC output data plot of the ROM test circuit clockedat 36 GHz and FCW set to 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 3.11. The DAC output data plot of the ROM test circuit clockedat 36 GHz and FCW set to 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

vi

Figure 3.12. The output of MSB of ROM programmed with bit pattern1111110111111110 when the accumulator is clocked at 36GHz and the accumulator FCW is set to 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 3.13. The output of MSB of ROM programmed with bit pattern1111110111111110 when the accumulator is clocked at 36GHz and the accumulator FCW is set to 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 4.1. Block diagram of 32x6-bit ROM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 4.2. Functional schematic diagram of 32x6-bit ROM with 2:1 MUX. . . . . . . 36

Figure 4.3. The schematic diagram of 2:1 multiplexer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 4.4. The MUX output with glitch that occurs when MUX isswitching ROM inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 4.5. The schematic diagram of D-latch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 4.6. Buffered glitch-free output from D-latch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Figure 4.7. The buffered and latched MUX output of Bit 5 from simu-lation of 32x6-bit ROM with 2:1 MUX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Figure 4.8. Simulated differential output voltage of ROMs: (a) schematicof 32x6-bit ROM with 2:1 MUX and D-latch: (b) schematicof ROM with 32x6-bit memory core: and (c) schematic of16x6-bit ROM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

vii

LIST OF ABBREVIATIONS

BVceo transistor break-down voltageCML current mode logicCMOS complementary metal-oxide-semiconductorCORDIC co-ordinate rotation digital computerDAC digital to analog converterDDS direct digital synthesizerDHBT double heterojunction bipolar transistorECL emitter coupled logicfclk clock frequencyfout output frequencyfmax maximum oscillation frequencyft transit frequencyFCW frequency control wordHBT heterojunction bipolar transistorInP Indium PhosphideLSB least significant bitLUT look up tableMSB most significant bitMSFOM mixed-signal figure of meritMUX multiplexersPLL phase-locked loopRAM random access memoryROM read-only memorySBD Schottky barrier diodeSFDR spurious-free dynamic range

viii

CHAPTER 1

Introduction

1.1 Background

Frequency Synthesizers, capable of operating in the microwave range, are in

growing demand in areas of communication. Direct digital synthesizer (DDS) and

phase-locked loop (PLL) circuits can be used for generating many different frequen-

cies from a single frequency source. DDSs are widely used to generate frequencies

from DC to multi-GHz and offer advantages over other methods. Unlike conventional

PLL frequency synthesizers, which suffer from an inherent inability to simultaneously

provide both fast frequency switching and high purity [1], the DDS can simultaneously

achieve both fast switching and high spectral purity.

A DDS can output arbitrary waveforms but this research focuses on DDS pro-

ducing Sine-output. A simple block diagram of a sine-output DDS is shown in Fig-

ure 1.1. The DDS system is composed of a phase accumulator, a phase to amplitude

converter, a digital to analog converter (DAC) and a low pass filter. The accumulator is

a phase counter whose phase increment can be controlled by the ”k” input. The phase in-

crement is added to previous output value of the accumulator on every clock cycle. The

phase output of the accumulator is used as an input to the phase to amplitude converter.

The phase to amplitude converter acts as a look-up table (LUT) and is usually imple-

mented with a read only memory (ROM). It generates the digital output representing the

desired output voltage of the waveform. This digital output is converted into an analog

sine wave using a DAC. The low pass filter eliminates high frequency components of

the discrete sine wave and provides a clean sine waveform.

The phase to amplitude converter is one of the critical parts of the DDS. Some

of the important characteristics of DDS systems such as speed, spurious free dynamic

1

Phase Accumulator

k

Phase to Sine Converter

DACLow Pass

Filter

Figure 1.1: Block diagram of a Sine-output DDS.

range (SFDR), power and chip area are dependent on phase to amplitude converter.

Vankka [2] has compared various methods and architectures that can be employed for

phase to amplitude converters. A ROM LUT is one of the most widely used methods to

implement the phase to amplitude converter because it has better SFDR and a simpler

circuit design than other architectures. The ROM is often the limiting factor for the

speed of the DDS [3, 4]. When the ROM is made larger to attain good SFDR, it slows

down, occupies a larger area and consumes significant percentage of DDS power. A

tradeoff optimized high speed ROM LUT can give both good SFDR and high operating

frequency.

CMOS technologies become faster with each generation, but CMOS devices

are an order of magnitude slower than state of the art bipolar transistor technology.

The increasing demand for higher speed DDS circuits and the frequency limitations in

CMOS technologies have necessitated the development of ROMs implemented in dou-

ble heterojunction bipolar transistor (DHBT) technology. ROMs designed in CMOS

technologies typically utilize NAND or NOR architectures [5] and provide operation

in the 1-2 GHz range because of limitations in the CMOS technologies. The fastest

recently reported CMOS ROM by Takahashi et al. operates at 1.1 GHz [6]. In compari-

son, recently reported InP DDS circuits have been shown to operate at clock frequencies

2

from 12 GHz to 28 GHz [7]-[9] and accumulators have been reported operating over

40 GHz [10].

The Vitesse VIP-2 InP DHBT process [11] provide transistors with ft and fmax

both over 300 GHz and a BVceo over 4 V. This is a self-aligned process with good yield.

The new technology performs as good as, if not better than, other technologies like

SiGe and InP HBT in terms of speed. The new technology provides current mode logic

(CML) ring-oscillator gate delay of 1.95 ps [11] and emitter coupled logic (ECL) static-

divider frequency of 152 GHz [11], compared to ring oscillator gate delay of 3.6 [12] and

3.21 ps [13] and static-divider frequencies of 96 [14] and 100 GHz [15] from SiGe and

InP HBT technology respectively. The high frequency and high break-down voltage

transistor of this process is advantageous when designing high speed circuits. Mixed

signal figure of merit (MSFOM) is one of the measures of evaluating the performance

of HBT mixed signal circuits. MSFOM has been reported to be directly related to BVceo

by Zolper [16]; MSFOM improves with higher BVceo. Higher BVceo in InP DHBT

technology also provides a higher dynamic range and linearity. Despite these advantages

ROM designs operating at 30-40 GHz remains a challenge. A high speed ROM design

implemented in InP DHBT process is investigated in this thesis.

1.2 Purpose of the Research

The purpose of this research is to design a ROM, suitable for LUT, operating at

clock frequency of 30-40 GHz range in InP DHBT technology. This research investi-

gates whether InP DHBT technology can provide a high-speed ROM suitable for DDS

application. There has been very little reported prior research work in HBT ROMs in

general. Likewise, InP DHBT processes are costly and not easily accessible, so very lit-

tle research has been reported for ROMs in InP DHBT technology. This thesis provides

the first reported research on ROMs designed and fabricated in an InP DHBT process.

3

The InP DHBT process is different from CMOS and other bipolar technologies.

The ROM designs from other technologies can not be directly adapted into InP DHBT

technology. In this research, the prior work done in bipolar ROM designs has been

used as a starting point for the new design. A 16x6-bit ROM has been designed in InP

DHBT technology. The simulation and the test results from the ROM were analyzed.

Further two 32x6-bit ROMs have been designed to increase the bit size of the ROM.

The simulation results from these two ROMs has been analyzed.

1.3 Thesis Organization

This thesis is structured to provide some background on a ROM designs, fol-

lowed by the circuit designs, simulation and hardware results of 16x6-bit ROM. Then

it describes two design approaches taken to increase the bit size of ROM to 32x6 bits

followed by simulation results.

Chapter 2 gives an overview of prior works on bipolar ROM cell, row decoder,

sense amplifier designs. The chapter also explains optimization of ROM space by using

compression techniques.

Chapter 3 describes the design blocks of a 16x6-bit ROM design implemented

in InP DHBT technology. The chapter compares simulation and test results of ROM test

circuits.

Chapter 4 describes two design approaches taken to increase ROM bit size. The

simulations results from two 32x6-bit ROM designs are compared in terms speed and

power.

Chapter 5 gives the summary of the thesis and recommendations for future work.

4

CHAPTER 2

ROM LUT and Bipolar ROM

This chapter describes the ROM LUT and its advantages over other phase to

amplitude conversion techniques. The chapter gives an overview of the bipolar ROM

memory cell, row decoder and sense amplifier designs done in the past. The later part

of the chapter describes ROM compression techniques to optimize ROM memory, and

summaries prior work in these areas.

2.1 ROM Look-Up Table

In a DDS, the ROM is used as a LUT to convert digital phase input from the

accumulator to output amplitude. The accumulator output represents the phase of the

wave as well as an address to a word, which is corresponding amplitude of the phase,

in the LUT. This phase amplitude from ROM LUT drives the DAC to provide an analog

output.

The output frequency of a DDS is defined by the frequency control word (FCW)

input to the accumulator and the clock frequency. The FCW is a phase increment that is

added in the accumulator every clock cycle. Each period of sine wave is an overflow of

the phase in the accumulator. Frequency resolution 4f of DDS is dependent on word

length of the phase accumulator and is given by

∆f =fclk

2N, (2.1)

where fclk is the clock frequency and N is the accumulator word length. The DDS output

frequency is given by

fout =fclk × FCW

2N. (2.2)

5

Using a ROM LUT as a phase to amplitude converter has some advantages over

other ROMless architectures. The simplicity of the ROM circuit makes the ROM LUT

easy to implement. The ROMless architectures, such as co-ordinate rotation digital com-

puter (CORDIC), depend on iterations of computation to obtain amplitude of a phase.

These computations are done using circuit cells like adders, shifters, multiplexers and

registers. In HBT technology, these cells are known for consuming a lot of power and

space. A 2-bit adder in InP DHBT reported by Turner et al. [10] consumes 0.9 W. In a

CORDIC algorithm each iteration increases the accuracy of the result by approximately

one bit [2]. In order to reduce the truncation error and have higher accuracy the number

of iterations is required to be high. This contributes to higher power dissipation and

larger area. The advantages of ROMless architectures are seen only when the higher bit

accuracy is desired. At this point the ROM becomes very large, consumes high power

and becomes slow compare to ROMless architecture. Even in CMOS technology, when

the desired phase amplitude is less than 14-bits a ROM LUT with compression is better

than CORDIC algorithm technique in terms of both speed and space [2]. The ROM

LUT stores the values of phase amplitudes while ROMless architectures compute phase

amplitudes. Inherently, a ROM LUT provide better SFDR than any ROMless architec-

ture for same bit width. In an HBT technology, a ROM LUT can provide better SFDR,

lower power, and smaller area than ROMless architecture.

The InP DHBT process [11] used in this thesis has transistor with ft and fmax

both over 300 GHz simultaneously and a high of breakdown voltage BVceo of over 4 V.

A ROM implemented in InP DHBT technology will inherently provide high speed oper-

ation. Prior work on bipolar ROM designs and BiCMOS random access memory (RAM)

were researched to come up with a design that can get as much leverage as possible from

high speed devices in InP DHBT process. The next section describes emitter-coupled

logic (ECL) gate, which is the building block of high-speed digital logic, along with

bipolar ROM components such as memory cell, row decoder and sense amplifier.

6

2.2 Bipolar ROM Designs

2.2.1 The Emitter-Coupled Logic Gate

The ECL gate is designed for high speed operation. The ECL gate consists of

a differential pair and emitter followers, as shown in Figure 2.1. The differential pair

serves as a fast current steering switch. The transistors Q1 and Q2 are identical and are

biased symmetrically. When inputs Vin1 and Vin2 are equal, current ID/2 flows through

both legs of the differential pair. The current source ID and resistor RS are chosen

to provide a voltage drop of about 300 mV across RS . The low voltage swing of the

differential pair is one of the reasons why ECL gates have high performance. When

there is difference in voltage between Vin1 and Vin2 one leg has more current steered

through than the other. The ratio of the collector currents flowing through transistors

Q1 and Q2 is exponentially related to the voltage difference between Vin1 and Vin2.

A difference of 120 mV between Vin1 and Vin2 will practically turn off transistor Q2

allowing most of the current ID to flow through Q2. This results in two output states,

logic low and logic high, at the collectors of Q1 and Q2 which are given by Equations 2.3

and 2.4 .

Vc1 = VCC − ID ×RS (2.3)

Vc2 = VCC (2.4)

The differential pair output of an ECL gate cannot drive other multiple gates

directly because the collector current used in driving the load will cause a reduction

in the voltage swing. Thus an emitter follower is connected to each of the differential

outputs to drive other gates. This causes current to be drawn from emitter follower

instead of collector of differential pair. The outputs Vout1 and Vout2 are a diode drop

7

ID

Vc1 Vc2

VEE

VCC

RS RS

Vin1 Vin2

VCCVCC

Vout1 Vout2

Q2Q1

Q4Q3

IFIF

Figure 2.1: Basic ECL gate consisting differential pair and emitter follower.

lower than Vc1 and Vc2, so emitter follower also act as a level-shifter. The ECL gate

provides differential outputs, so it serves as both an inverter and a buffer simultaneously.

The differential pair with an emitter follower is a building block for ECL multiplexers,

XORs, latches and other logic gates. The ECL gate is the most versatile circuit in

high-speed bipolar digital circuit family.

2.2.2 ROM memory cell

The memory cell is the most crucial building block of ROM memory. Each cell

represents a logic high or low and an array of memory cells forming a word represents

the digital amplitude of a sine wave in the DDS. The memory cell should be designed

such that it is easy to decode and read. In this section the two memory cell architectures

most commonly used in bipolar ROM design are described.

2.2.2.1 Schottky cell ROM model

The ROM cell designed with a Schottky barrier diode (SBD) as a memory cell

is described by Gunn et al. [17] in 1977. A SBD serves as good choice for a memory

8

Bit Line

Word Line Bit High

Bit Low

Figure 2.2: Schottky diode memory cell array.

cell because it is small and can be fabricated free of the emitter-collector leakage using

conventional bipolar IC technology. Figure 2.2 shows an array of SBD memory cells.

A bit is addressed by the coincidence of sinking current from a bit line and forcing a

voltage on a word-line. The bit line is held high if a SBD is present at an intersection;

otherwise the bit line is pulled low. The bipolar ROM fabricated by Gunn et al. in [17]

used 2 metal layers and had an access time of 150 ns. The ROM design by Ludwig [18]

using SBD cell similar to [17] was reported to have bit access time of 36 ns. This type

of cell can be implement in ROM design only in technologies that provide SBD, such

as SiGe HBT, but the design has to be adapted to the low break down voltage of SiGe

HBT process. This memory cell design is not applicable for technologies which do not

provide SBD such as Vitesse VIP-2 InP DHBT.

2.2.2.2 Transistor cell ROM model

A ROM designed with a transistor as a memory cell is described by Barret et

al. [19]. Figure 2.3 shows the memory array using bipolar transistors for the memory

cell. The row line has horizontal base-diffused stripe. This strip has n+ emitter blocks

9

Word Line

Bit Line

GND

Bit High

Bit Low

Figure 2.3: Transistor memory cell array.

diffused at regular intervals serving as memory cells. If a metal connection to an emitter

is present, the bit is high; otherwise it is low. The ROM consists of a buffer-inverter

circuit with high fan out capable of driving high capacitance. The address decoder

and bit detector are also integrated with the memory array which are not explained by

authors. The ROM access time was reported to be in range or 15 to 40 ns [19].

Comparison of two memory cells in terms of speed can be difficult as technolo-

gies progress, but the Schottky cell is easier to fabricate and takes less space than the

transistor cell. In general, the SBD has less capacitance, making it easier to charge and

discharge quickly. This contributes to high speed performance. The transistor cell has

benefit of having low current drive because it is driven from a base input. In the case of

Schottky cell, the word driver must be able to drive more current as the SBDs are used

for pulling the bits high in the word. A word driver design has been reported by [18]

that can sink up to 25mA of current.

Neither of these ROM memory cell designs are implemented in the ROM design

for this thesis. Rather a differential ROM memory cell baed on the work of Padoan et

10

al [20], which provide higher differential signal at sense amplifier input, is implemented.

This ROM memory cell design is described in Chapter 3, Section 3.1.2.

2.2.3 Row Decoder using Bipolar Transistors

The decoder circuit takes an input address of the word to be decoded and it

selects a word line from the ROM memory core. The access time of ROM is dependent

on the decoder delay time. The ROM access time can be reduced by having a high

speed decoder. BiCMOS and Bipolar RAM designs have taken advantage of high-speed

bipolar decoder designs to get fast access time. In this section some high-speed bipolar

decoders used in BiCMOS and Bipolar RAM are described.

2.2.3.1 Schottky Barrier diode decoder combined with a pull-up circuit

Homma et al. [21] designed SBD decoder with pull-up circuit. This decoder

provides fast discharge and pull-up of decoder lines. The decoder uses an active pull-

up circuit instead of the pull-up resistor as shown in the Figure 2.4. The pull-up is

performed by means of emitter follower therefore sufficiently high pull-up current can

flow transiently when the voltage of the decoder line switches from low to high. This

provides very fast rise time of the decoder lines. The SBDs have low capacitance due to

their small area which provides short delay time. The decoder design is not applicable

for the technologies that do not provide SBD.

2.2.3.2 ECL-based Decoder

The ECL-based decoder uses the ECL gate configuration to decode memory ad-

dresses. The ECL-based decoder by Douseki et al. [22] used for a high-speed BiCMOS

SRAM is composed of a predecoder, a level shifter, and a main decoder. The prede-

coder selects the block of memory and does the most of the decoding, while the main

decoder does decoding within that block and buffers the signal to the MOSFET level.

11

ID

aVEE

VCC

RS RSVCC

VCC

Three–level VSS Generator

Row 1Q4Q3

ID

VEE

Q2Q1

Ac_Bit1

Clk

VEE

a

VCCVCC

SBD Decoder

VSS

QE

Pull-up Circuit

RDE

Row 2

VCC

RDE

Figure 2.4: Schottky Barrier Diode Decoder with Pull-up Circuit.

12

VCC

RS

RS

Ac_Bit0

Vref

VCC

VCC

Ac_Bit1

Ac_Bitn

Ac_Bitn

Ac_Bit0

RS

VCC

Ac_Bit1

VEE

VEE

Row 1

Row 2n

Figure 2.5: Predecoder design in ECL-based Decoder

The predecoder consists of ECL NOR gates, that take the address input, and emitter

followers as shown in the Figure 2.5. The main decoder is designed to drive MOSFET

gates, which is not required for bipolar ROM. This decoder design has multiple stages

which contribute to more power consumption and greater delay.

The ECL-based decoder reported by Essl et al. [23] is very similar in design to

the predecoder stage in [22]. The design has two stages; the first stage is a level shifter

changing the input to ECL, and the second stage is a NOR gate configuration like that

shown in Figure 2.5. The output of the NOR gate drives the wordline in the ROM.

Both SBD decoder and ECL-based decoder designs consume more power than

multi-emitter transistor AND gate decoder reported by Kawarada et al. [24, 25]. The

SBD decoder uses three more current sources per bit than multi-emitter transistor AND

gate decoder. The SDB decoder is considered to be faster than multi-emitter transistor

AND gate decoder because of its pull-up circuit and low capacitance SBD. However,

SBD decoder is not applicable in this InP DHBT technology [11] as it does not provide

SBD devices. The ECL-based decoder design has one more current source per row

13

than multi-emitter transistor AND gate decoder. When the number of row increases,

the power dissipation through these current sources becomes significant. This thesis

work utilizes multi-emitter transistor AND gate decoder which is described in detail in

Chapter 3, Section 3.1.1.

2.2.4 Bipolar Sense Amplifier

The sense amplifier detects the value of a bit stored in the memory cell and

outputs it value. A fast sense amplifier design is essential for fast ROM access time. A

sense amplifier design reported by Miyanaga et al. [26] is shown in the Figure 2.6. The

bases of the differential gate take inputs from the bit lines. In next stage, the current

signal is converted to a voltage signal. The sense amplifier provides a single-ended data

output line. Since the sense amplifier has few stages, it is fast and low power. In this

thesis this sense amplifier design has been implemented with modifications to provide

fewer stages and a differential output to drive the DAC. The details are described in

Chapter 3, Section 3.1.3

2.3 Bipolar ROM in DDS

There have been few reported designs of the bipolar ROM in DDS circuits.

Kwok et al. [3] reported a sine ROM for DDS in AlGaAs/GaAs HBT technology with

300 ps bit access time. The design of the bipolar ROM is described briefly and vaguely.

The ROM is based on current mode logic modified as appropriate in different circuit

configuration to achieve high speed and low power. The memory cell consists of one

transistor per cell with the collector grounded. Bits are assigned by connecting the emit-

ter metal to the output bit line through a via. The description of the memory cell in [3]

resembles to ROM cell by Barrett et al. [19].

14

VCC

VR

ID

VEE

VCC

IRIR

RS

VCC

VR

Cell Array

VEE

VCC

Data oVref

Figure 2.6: RAM Cell Sense Amplifier

2.4 ROM Compression Techniques

If a full sine wave is mapped in the ROM, it will increase the size of ROM

which results in increased area and power. For DDS designs operating at low clock

frequencies, memory size is not a big problem. However, DDSs operating at clock

frequency above couple GHz, memory size becomes very crucial because bigger ROM

gets slower. When using bipolar ROM, power and chip area becomes crucial. ROM

space can be optimized by exploiting the sine wave symmetry nature and using different

compression techniques.

Tierney et al. [27] in 1971 recognized the need for ROM compression. For

example, the quadrature sine-wave symmetry technique uses the symmetric nature of

sine wave separated in four quadrants. The phase amplitude of only one quadrant of

sine wave is mapped in the ROM LUT. The phase amplitude of a single quadrant is used

for mapping the sine wave in the other three quadrants by changing the address sign

and the amplitude sign to obtain the full sine output. The memory size is reduced to

15

one fourth using just the quadrature sine-wave symmetry technique. The benefit of the

reduced ROM size comes without any loss in the SFDR.

There are other ROM compression techniques which are used along with sine-

wave quadrature symmetry. ROM compression techniques generally require additional

circuit block such as adders, subtractors, and multipliers. A comparison of differ-

ent ROM compression techniques is shown in the Table 2.1. The table illustrates the

ROM size and additional logic circuits required for different compression techniques

to achieve nearly equal SFDR. Inherently, smaller ROMs operate at higher frequency

than the bigger ROMs. This is another benefit of reducing the ROM size by using

compression techniques.

Table 2.1 shows that Hutchison’s techniques, Sunderland’s, Nicholas’ and Bel-

laouar’s architectures provide different compression ratios at no loss of SFDR. All these

four techniques require two ROMs; a coarse ROM provides low resolution samples, and

a fine ROM gives additional resolution by interpolating between the low resolution sam-

ples [2]. The ROM compression techniques have tradeoffs in ROM size, compression

ratio and number of required additional circuit blocks. The linear addressing technique

reported by Ghosh et al. [30] requires a single ROM and provides little loss in SFDR.

The non linear addressing technique reported by Ghosh et al. [30] requires two smaller

ROMs and provides a high compression ratio with little loss of SFDR. The modified

Bellaouar’s architecture with improved compression and considerable reduction in ROM

size for little loss in SFDR has been reported by El Said et al. [31].

The ROM compression techniques compared in Table 2.1 provide SFDR be-

tween 71 dB and 72.245 dB. Bellaouar’s architecture [29], non linear addressing tech-

nique [30] and modified Bellaour’s architecture [31] can be implemented in DHBT

technology to achieve a smaller ROM size with little loss of SFDR. It can be seen from

Table 2.1 that the size of the ROM required for these three compression techniques are

between 25x9 bit to 24x8 bit, which can be implemented in InP DHBT technology [32].

16

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17

Applying ROM compression techniques is not the first phase goal of this re-

search, but it provides an idea of the size of the ROM required if ROM compression

techniques were to be applied. There has been very little work on ROM compression

techniques implemented in bipolar ROM compared to low frequency DDS in CMOS

technology. Kwok et al. [3] have implemented Nicholas architecture [33] compression

technique in their HBT sine ROM to achieve compression ratio of 1:70. Some of the

bipolar ROM LUT work done are mentioned in the section Bipolar ROM in DDS, 2.3.

18

CHAPTER 3

16x6-Bit High Speed Bipolar ROM in InP

Chapter 2 gave an overview of previously reported bipolar ROM memory cells,

decoders and sense amplifier designs. This chapter describes a 16x6-bit ROM imple-

mented in InP DHBT technology. The ROM implementation draws from prior work

on bipolar ROMs to achieve a good tradeoff between power consumption and speed in

InP DHBT technology. Simulation and measured results from the ROM test circuit are

discussed.

3.1 Circuit Design

The ROM consists of a 16 row decoder, a 16x6-bit memory cell array, and an

array of sense amplifiers, as illustrated in Figure 3.1. The row decoder receives its input

from the 4 most significant bits (MSBs) of an 8-bit differential accumulator, based on

the accumulator reported by Turner et al. [9]. The bit width of the accumulator feeding

the decoder determines the word length and hence the resolution of the system. During

each clock cycle, the row decoder reads one word line and the bit values stored in the

ROM cells are detected by the sense amplifier. The sense amplifier outputs drive a DAC

which converts several high speed digital outputs from the ROM into single analog

output. This facilitates testing by providing a single high-speed analog output instead of

several high-speed digital outputs.

3.1.1 Row Decoder

Minimizing the row decoder delay time is key to high speed bit access. A single

row of the row decoder is shown in Figure 3.2 and it contains a multi-emitter transistor

AND gate with the base biased by feedback from the collector circuit and a word drive

transistor (QW ) based on [24, 25]. The multi-emitter transistor AND gates in the decoder

19

16x6

Memory Cell Array

Sense Amplifier

Row

Dec

oder

Row 1

Row 2

Row 3

Row 16

Col

1

Col

2

Col

3

Col

6

Bit

0

Bit

1

Bit

2

Bit

5

Inpu

t fr

om A

ccum

ulat

or

Output to DAC

Figure 3.1: Block diagram of 16x6-bit ROM.

take single ended inputs. The differential output from the accumulator provides both the

single-ended signal and its complement, eliminating the need for a separate inverter cir-

cuit between the accumulator and the multi-emitter transistor AND gate. When all of

the bits connected to the multi-emitter transistor AND gate are high, the multi-emitter

transistor (QM ) is near the cut-off region. With a minimal amount of current flowing

through the collector, the voltage drop across resistor R2 is small. If one or more bits

are low at the multi-emitter transistor AND gate, QM is turned on. The collector cur-

rent produces about 250 mV of potential drop across resistor R2. The potential drop is

also seen at the word drive transistor QW . R1 and R2 connecting base and collector of

transistor QM , biases QM at saturation region when QM is on and close to cutoff region

when QM is off. When the output of a particular QW is low, the corresponding row is

deselected. A row is selected when all the input bits to its AND gate are high making the

output of QW high. A simulated output of the 16-row decoder is shown in Figure 3.3.

20

Row Driver

VCC

R1

R2

IS

Ac_Bit 0

Ac_Bit 1Ac_Bit 2

Ac_Bit 3

QW

QM

Figure 3.2: The schematic of a single row of the row decoder circuit.

The output of only one row is logic high at any time. This row decoder design balances

speed and power [24, 25] as there are few stages. The simulation result showed shortest

delay through the decoder to be about 7 ps. This design approach performs better than

the ECL decoder of base input type by Essl et al. [23], which uses extra stages that

increase delay and power consumption.

3.1.2 ROM Memory Cell Array

The ROM memory cell array with pull up transistors and sense amplifier circuits

is illustrated in Figure 3.4. Single-ended inputs from the row decoder drive the row

selection lines of the ROM. Each column of the ROM has differential signal lines that

feed into the sense amplifiers. When a row line is not selected, both of these lines are

at same the potential and have the same amount of current flowing through them. If a

transistor is connected to the high line, it is a high bit (QH). If it is connected to the low

21

1 1.1 1.2 1.3 1.4−1350

−1300

−1250

−1200

−1150

−1100

−1050

−1000

Time (ns)

Out

put V

olta

ge (m

V)

ROW1ROW2

ROW3ROW4

ROW5ROW6

ROW11ROW12

ROW7ROW8

ROW9ROW10

ROW13ROW14

ROW15ROW16

Figure 3.3: The simulation showing only one row of decoder output is logic high whileothers stay low.

line, it is a low bit (QL). A diode (QD) is connected in series with the transistor QL or

QH to maintain the transistor bias in the safe operating regime.

A word line is selected when its potential is raised by the row decoder. The row

decoder selects only one row at a time. When the word line is selected, the transistors

for each bit of that word are turned on producing a voltage difference between the high

line and low line. This small voltage difference is detected by the sense amplifier which

outputs either a logic high or a logic low.

The ROM cell uses a transistor to assign both a high and low bit value to each

cell. This provides a larger signal at the sense amplifier input than obtained from a

ROM utilizing transistors only for logical high [19]. In this differential ROM memory

cell array design, there are on average twice as many transistors, since both high and low

bits have transistors. However, the overall layout area is not substantially increased [20],

because the transistors added for low bits are located in the space that would otherwise

have been left empty between the differential signal lines.

22

Bit 0 Bit 1 Bit 5

+ -

Sense Amplifier

Row 1

Row 16

VCC

+ - + -

Hi g

h L

i ne

Lo w

Lin

e

16x6 ROM

QL

QH

QD

IH IL

Rc Rc

QD

QC QC

-+ -+ -+

Figure 3.4: The schematic of the ROM memory cell array with sense amplifier.

3.1.3 Sense Amplifier

The sense amplifier used in the ROM circuit is modification of a sense amplifier

reported by Miyanaga et al. [26]. The sense amplifier used in [26] has current mode

logic (CML) gate to detect the current in the bit line. A stage of emitter input type is used

for converting current signal to voltage signal followed by emitter follower. The sense

amplifier used in this ROM is a simple differential amplifier, as shown in Figure 3.5. The

high and low signal lines from each column of the ROM memory cell array are input to

the sense amplifier. When a row is selected, the transistor connected to the high or low

line will pull it down by about 250 mV. The current flowing through the collectors of the

differential pair is exponentially related to the difference in voltage between high and

low line. The voltage difference created by row selection is amplified as a differential

output. The 0.5 mA current difference between the high and low lines of the ROM is

23

Low Line

ID

High Line

Out N Out P

VEE

VCC

IC IC

IH IL+ -

RS RS

Figure 3.5: The schematic of the sense amplifier.

generated due to the difference in their voltage. The sense amplifier output is buffered

by an inverter and emitter follower to provide full differential outputs to the DAC.

3.2 Test Circuit

A test circuit was designed and sent out for fabrication. A block diagram of the

ROM test circuit is shown in Figure 3.6. It consists of an 8-bit accumulator, a 16x6-bit

ROM, and a 6-bit DAC. The accumulator generates a predictable high speed input to the

ROM which can be controlled easily. The DAC is used to convert several high speed

digital outputs from the ROM into single analog output. This facilitates testing, as it is

not practical to measure several ROM outputs separately. The accumulator and ROM

use a -3.8 V power supply, and the DAC uses a -4.5 V power supply.

In the test circuit, the four MSBs from the accumulator are input to the ROM.

The bit pattern in the ROM is shown in Table 3.1. The five least significant bits (LSBs)

from the ROM are input to the DAC. The four LSBs of the ROM are programmed as a

decreasing counter, and bit 4 is programmed as 0s. The DAC uses this input to generate

24

FCW

Clk in

ROMBit(7:4)

Bit 5 output

Bit(4,4:0) Analog OutputWaveform

DAC

AccumulatorThe Worst Case Bit Pattern

Figure 3.6: Functional schematic of the ROM test circuit.

a falling ramp. To further analyze the proper functioning of the ROM, the MSB from

the ROM is programmed with the bit pattern (1111110111111110). This bit pattern is

intended to act as a worst case switching. The bit pattern uses a long string of logic

high and then a single instance of logic low followed by a long string of logic high. The

high line of the MSB column has more bit transistors connected to it than the low line,

so it has a greater parasitic capacitance than low line due to the additional base-emitter

capacitance from the high bit transistors. This causes the MSB column to retain logic

high value for a longer time than logic low value. Also, at high clock frequency, the

parasitic capacitor will not have enough time to discharge in response to the short time

of changed bit, causing the ROM to fail.

3.3 Simulation Results

Figure 3.7 shows the simulated output voltage differential of the MSB from the

ROM circuit as a function of the input clock frequency under four different settings of

simulation conditions:

1. schematic of the row decoder, the ROM memory cell array, and the sense ampli-

fier, all without layout parasitics

25

Row # Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 01 1 0 1 1 1 12 1 0 1 1 1 03 1 0 1 1 0 14 1 0 1 1 0 05 1 0 1 0 1 16 1 0 1 0 1 07 0 0 1 0 0 18 1 0 1 0 0 09 1 0 0 1 1 1

10 1 0 0 1 1 011 1 0 0 1 0 112 1 0 0 1 0 013 1 0 0 0 1 114 1 0 0 0 1 015 1 0 0 0 0 116 0 0 0 0 0 0

Table 3.1: The bit pattern in the ROM.

2. the row decoder with layout parasitics, schematic of the ROM memory cell array

and sense amplifier without layout parasitics

3. schematic of the decoder without layout parasitics, the ROM memory cell array

and the row decoder with layout parasitics

4. the row decoder, the ROM memory cell array, and the sense amplifier with layout

parasitics

The results indicate that the maximum operating clock frequency of the ROM is

limited by the ROM memory cell array and not by the row decoder or the sense amplifier.

The plot of the results from the simulation of the layout of the row decoder, the ROM

memory cell array and the sense amplifier with layout parasitics shows a differential

output from the ROM of about 160 mV at a clock frequency at 36 GHz. The differential

signal of 150 mV from the ROM is sufficient to drive the DAC after being buffered. The

performance of the ROM is dependent on the decoding time since each row does not

have the same decoding time. The worst case delay occurs when 3 or 4 of the input bits

26

20 25 30 35 40 45 50 55 600

50

100

150

200

250

300

350

Operating Clock Frequency in GHz

Diff

eren

tial O

utpu

t Vol

tage

in m

V 1. All Schematic

4. All Layout with Parasitics

3. Decoder Schematic and ROM Layout

2. Decoder Layout and ROM Schematic

Figure 3.7: Simulated differential output voltage from the ROM under four differentsimulation settings: (1) schematic of the row decoder, the ROM memory cell array, andthe sense amplifier without layout parasitics: (2) the row decoder with layout parasitics,schematic of the ROM memory cell array and sense amplifier without layout parasitics:(3) schematic of the decoder without layout parasitics, the ROM memory cell array andthe row decoder with layout parasitics: and (4) the row decoder, the ROM memory cellarray, and the sense amplifier with layout parasitics.

27

to the row decoder are switching simultaneously. Row 1 and row 16 have the longest

delay since all of the address bits to the row decoder inputs must switch when going from

row 16 to row 1, or vice versa. The skew in the row 16 of the MSB is simulated to be

10 ps compared to clock signal when the ROM test circuit is clocked at 36 GHz and with

frequency control word (FCW) set to 16. This is due to the high parasitic capacitance

in the high line and the leakage current from the high bit transistors of unselected rows

in the MSB column. The high line in the MSB column has more leakage current than

the low line. When the sense amplifier senses the high bit, the differential voltage at

the input is higher than when sensing the low bit. This offset in the voltage causes fast

high bit sensing and slow low bit sensing. As illustrated in Figure 3.8, when decoding

row 16 the high line does not get discharged completely to the potential of logic low.

When the row decoder switches and selects row 1, the high line gets back to logic high

potential quickly generating the shortest delay. The skew in row 16 and the MSB are

also impaired by parasitics on the long input interconnections from the accumulator

and the fact that the MSB column is the farthest column from the row decoder output.

The parasitic extracted simulation results predicted a bit access time of 15.43 ps for the

ROM. The ROM test circuit simulation showed the accumulator and the ROM dissipates

6.76 W of which 1.49 W was dissipated by the ROM circuit alone.

3.4 Measurement Results

The microphotograph of the ROM test circuit is shown in the Figure 3.9. The

ROM test circuit is 2700 µm by 1450 µm and contains 2242 transistors. The circuits

were tested on wafer using a probe station and high-frequency probes. The output wave-

forms were measured with a high-frequency sampling oscilloscope.

Figure 3.10 shows the measured output from the DAC when the accumulator is

clocked at 36 GHz and the accumulator FCW is set to 1. When the FCW is set to 1,

the output at DAC changes every 16 clock cycles because the four MSBs of the 8-bit

28

2.31 2.315 2.32 2.325 2.33 2.335 2.34 2.345 2.35−350

−300

−250

−200

−150

−100

−50

0

Time (ns)

Out

put V

olta

ge (m

V)Row16, Bit5

Row16, Bit0

Figure 3.8: The skew in the row 16 of MSB when the ROM test circuit is clocked at36 GHz and FCW set to 16.

Accumulator Row Decoder

Sense Amplifier

DAC

Memory Core

Figure 3.9: Microphotograph of ROM test circuit. The chip is 2700 µm by 1450 µmand has 2242 transistors.

29

0 2 4 6 8 10−30

−20

−10

0

10

20

30

40

Time (ns)

Out

put V

olta

ge (m

V)

Figure 3.10: The DAC output data plot of the ROM test circuit clocked at 36 GHz andFCW set to 1. With this FCW the output changes every 16 clock cycles.

accumulator are used for the row decoder. The measured result shows a falling ramp as

expected, with each step equal to 443.2 ps. Figure 3.11 shows the measured result for

FCW set to 16 with a 36 GHz clock. In this state, the ROM word lines change every

clock cycle, hence the output of the DAC changes every clock cycle. A glitch can be

seen in the 8th step where the ROM data switches from 1000 to 0111. This is due to

the DAC not functioning properly when all four inputs switch at high frequency. This

problem in the DAC has been addressed by Tierney et al. [27] as a DAC noise. It can

be seen that the DAC also has problem when inputs switch from 0000 to 1111. Smaller

glitches can be seen at when inputs switch from 0100 to 0011 and 1100 to 1011, but

these are more subtle. However, the DAC shows the proper output after the glitch, so it

is seeing the correct inputs from the ROM at a 36 GHz clock frequency.

Figure 3.12 shows the measured single-ended output of the MSB of the ROM

when the circuit is clocked at 36 GHz and the accumulator FCW is set to 1. Figure 3.13

shows the measured single-ended output of the MSB from the ROM when FCW is set to

16. The output bits match the programmed bits from the Bit 5 of the ROM. This shows

that the worst case bit pattern in the ROM works at 36 GHz.

30

0 0.1 0.2 0.3 0.4 0.5−30

−20

−10

0

10

20

30

Time (ns)

Out

put V

olta

ge (m

V)

Figure 3.11: The DAC output data plot of the ROM test circuit clocked at 36 GHz andFCW set to 16. With this FCW the output changes every clock cycle. The glitch at thecenter of the wave form is caused from the DAC switching from 1000 to 0111.

0 2 4 6 8 10−150

−100

−50

0

50

Time (ns)

Out

put V

olta

ge (m

V)

1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0

Figure 3.12: The output of MSB of ROM programmed with bit pattern1111110111111110 when the accumulator is clocked at 36 GHz and the accumulatorFCW is set to 1. With this FCW the output is low for 16 clock cycles at a time.

31

0 0.1 0.2 0.3 0.4 0.5 0.6−80

−60

−40

−20

0

20

Time (ns)

Out

put V

olta

ge (m

V)

1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0

Figure 3.13: The output of MSB of ROM programmed with bit pattern1111110111111110 when the accumulator is clocked at 36 GHz and the accumulatorFCW is set to 16. With this FCW the output is low for a single clock cycle at a time.

The entire test circuit dissipates 7.22 W of power, with 2.12 W from the DAC

and 5.1 W from the combination of the accumulator and the ROM. The ROM alone

consumes 1.13 W of power. The measured power dissipation is 0.36 W or 24.2% less

than simulated power dissipation.

32

CHAPTER 4

32x6-bit ROM designs

4.1 ROM Designs

For a high speed DDS, the ROM needs to be fast, large in terms of memory

size and have low power consumption. These three goals are conflicting and must be

considered while making design decisions. The ROM design described in Chapter 3

has high operating frequency but is relatively small for DDS applications. This chapter

describes two design approaches for increasing the size of the ROM. In the first design

approach, the bit size is increased by increasing rows in the ROM memory core. In this

design, low power consumption is considered as goal. In the second design approach,

the bit size is increased by integrating two high speed 16x6-bit ROMs using high-speed

2:1 multiplexer (MUX) and D-latch. High speed operation is the primary goal of second

design approach.

4.1.1 ROM with 32x6-bit Memory core

The memory size of the ROM can be increased by increasing the size of the

memory core. The memory core can be increased by increasing the number of rows

and column in the memory. Figure 4.1 illustrates a 32x6 bit ROM design where the

bit-size of the ROM is increased by doubling the number of rows in the memory core

of the 16x6-bit ROM. A new 32-row decoder is designed which takes 5 input bits from

the accumulator. A single row of a row decoder consist of a 5-input multi-emitter AND

gate and a word drive transistor. The inputs from the accumulator have to drive twice as

many gates than in 16-row decoder so twice as many inverter-buffer are used for driving

32 rows. The sense amplifier design remains unchanged as the number of columns is

same as in the 16x6-bit ROM.

33

32x6

Memory Cell Array

Sense Amplifier

Row

Dec

oder

Row 1

Row 2

Row 3

Row 32

Col

1

Col

2

Col

3

Col

6

Bit

0

Bit

1

Bit

2

Bit

5

Inpu

t fr

om A

ccum

ulat

or

Output to DAC

Figure 4.1: Block diagram of 32x6-bit ROM.

34

All the design aspects of this ROM are the same as that of 16x6-bit ROM. It

has a larger memory core and row decoder than the design in Chapter 3. There is no

additional new circuit added for the proper functioning of the ROM. The larger memory

core and decoder increases the area of the ROM by about two times. The 5-input multi-

emitter transistor AND gate is bigger than 4-input multi-emitter transistor AND gate

hence has higher capacitance. The 32-row decoder is twice as big as 16 row decoder

which means more parasitic capacitance. The accumulator has to drive twice as much

capacitance in 32-row decoder which makes it slower than the 16-row decoder. The

increase in memory core size added more parasitic capacitance in both low and high

line of the ROM slowing down the sense amplifier. The 32x6-bit ROM is slower than

16x6-bit ROM not just because of added parasitics in both decoder and memory core

but also due to leakage current through unselected bit transistor. The bit transistor in

the unselected rows in the memory core are not in cutoff and a small amount of current

of about 20 µA is always flowing through them. In the 32x6-bit ROM there are more

unselected rows adding up to more leakage current than in the 16x6-bit ROM. In a

column with the number of logic high is much greater than the number of logic low, or

vice-versa, it gives rise to uneven current in high line and low line. This uneven amount

of current in the lines at sense amplifier will slow down ROM.

4.1.2 32x6-bit ROM with 2:1 MUX

The functional schematic in Figure 4.2 illustrates the second design approach

where two high-speed 16x6-bit ROMs, ROM A and ROM B, are integrated together

using 2:1 MUX and D-latch to form a 32x6-bit ROM. MUX designs in InP DHBT

technology have been reported to operate at 80 Gbit/s [34]. This ROM leverages the fast

ECL MUX shown in Figure 4.3. There are no design changes within the 16x6-bit ROM.

A 2:1 MUX takes input from Bit4 of the accumulator for selecting bits from ROM A or

ROM B for output.

35

16x6 Bit

ROM A

2:1 Mux

Input From

A

ccumulator

AC_Bit<3:0>

AC_Bit<4>

16x6 Bit

ROM B

Bit<0:5>

Bit<0:5>

Bit<0:5> D

Clk

Clock

Q

QBuffer

Delay

Delay Inv

Figure 4.2: Functional schematic diagram of 32x6-bit ROM with 2:1 MUX. The ROMis designed by integrating a 2:1 MUX, D-latch and 2 16x6-bit ROMs.

The ECL 2:1 MUX takes two differential inputs, A and B, 6 inputs each from

two 16x6-bit ROMs and a differential input, C, a single input from the accumulator. The

bit from the accumulator is delayed by using the series of ECL inverter-buffer to match

the access time of 16x6-bit ROM. The input from the accumulator determines which

ROM, A or B is seen at the output by controlling the current through transistors QC1

and QC2. The emitter follower is connected to differential pair output so that current is

not drawn from the collector of differential pair to drive the other gates.

If values from 16x6-bit ROMs are switching or are different from each other

at the time when the MUX is switching the output, then a glitch occurs. Figure 4.4

illustrates the glitch that occurs when the MUX switches the inputs from ROMs whose

logic levels are changing as well. A D-latch shown in the Figure 4.5 is used for removing

the glitch from the output of the MUX. The D-latch takes outputs VP and VN from the

2:1 MUX as inputs D and D̄, and a clock signal. The output value of the latch Q and Q̄ is

latched until the next rising cycle of the clock. The clock signal to the D-latch is delayed

by the amount of access time of the 16x6-bit ROM and then by another half clock cycle

by using an inverter. This makes D-latch to trigger on the second half of the clock cycle

36

IS

VEE

VC

C

RSRS

AA B B

VC

C

C C

VEEVEE

VP

VN

IL

IL

QC1 QC2

Figure 4.3: The schematic diagram of 2:1 multiplexer.

or falling edge of the real clock cycle. Delayed D-latch triggering eliminates the glitch

in the MUX output that occurs in the first half of the clock cycle. Figure 4.6 shows the

glitch-free buffered output from the D-latch when the clock signals are appropriately

delayed. The output of the D-latch is delayed by a half clock cycle from its input.

This design approach requires additional digital logic gates such as 2:1 MUX and

D-latch. These additional circuits contribute to higher power consumption and greater

layout area. The results from both ROMs are compared in the Simulation Results section

below.

4.2 Simulation Results

Firstly, simulation results from the operation of the ROM with 2:1 MUX is dis-

cussed. Table 4.1 shows Bit 4 of the accumulator, the programmed Bit 5 of ROM A,

Bit 5 of ROM B and the latched Bit 5 of the 32x6-bit ROM with 2:1 MUX. ROM A

and ROM B are the two 16x6-bit ROMs in 32x6 bit ROM with 2:1 MUX as shown in

Figure 4.2. When accumulator Bit 4 is logic high MUX selects output from ROM A and

from ROM B when it is logic low, which is illustrated in Table 4.1. Figure 4.7 shows

37

0.4 0.45 0.5 0.55−1400

−1300

−1200

−1100

−1000

−900

−800

Time (ns)

Out

put V

olta

ge (m

V)

1 0 0 1

Glitch

Figure 4.4: The MUX output with glitch that occurs when MUX is switching ROMinputs .

ID

VEE

VC

C

RMRM

DD

VC

C

Clock Clock

VEE

VEE

Q

Q

ILIL

Figure 4.5: The schematic diagram of D-latch.

38

0.4 0.45 0.5 0.55−1400

−1300

−1200

−1100

−1000

−900

−800

Time (ns)

Out

put V

olta

ge (m

V)

1 0 0 1

Figure 4.6: Buffered glitch-free output from D-latch.

Bit 4 from the accumulator, Bit 5 from ROM A and ROM B, and the output from the

ROM with 2:1 MUX simulated at 40 GHz. It can be clearly seen from Figure 4.7 that

the ROM output bit matches with the expected output bit from the Table 4.1. It also can

be seen from Figure 4.7 that the 32x6-bit ROM output is delayed by about a clock cycle

compared to the output from ROM A and ROM B. This is due to the half cycle delayed

clock input to the D-latch used for removing glitch caused by the MUX and buffer after

the D-latch output.

Both design approaches taken to increase the bit size of the ROM have their

own advantages in terms of power and speed. Figure. 4.8 shows a comparison of the

simulated output voltage differential of the MSB based on schematic simulation of the

32x6-bit ROM with 2:1 MUX, and the ROM with 32x6-bit memory core along with

16x6-bit ROM plotted against clock frequency. The outputs from the ROMs are not

loaded. It can be seen from the comparison that the ROM with 2:1 MUX design has

faster operating clock frequency and higher differential output than the ROM with the

32x6-bit memory core. It also can be seen that the speed of the ROM with the 2:1

MUX is comparable with the speed of 16x6-bit ROM. The output of the ROM with 2:1

39

Row # Accumulator Bit 4 ROM A Bit 5 ROM B Bit 5 Output1 0 1 0 02 1 0 0 03 0 0 0 04 1 0 0 05 0 0 0 06 1 0 0 07 0 1 1 18 1 1 0 19 0 0 0 0

10 1 1 0 111 0 0 0 012 1 1 0 113 0 0 0 014 1 1 0 115 0 0 0 016 1 1 1 1

Table 4.1: The bit pattern in Bit 4 of accumulator, Bit 5 of ROM A and Bit 5 of ROMB. Bit 4 from the accumulator switches the outputs in the 2:1 MUX.

MUX has a higher output voltage than the output from 16x6-bit ROM because the 2:1

MUX takes the buffered output from 16x6-bit ROMs and the output from the 2:1 MUX

is latched and buffered again. The 32x6-bit ROM with 2:1 MUX has higher operating

clock frequency but it suffers from slow access time. The outputs from the two 16x6

bit ROMs in the 32x6-bit ROM with 2:1 are delayed by series of 2:1 MUX, D-latch

and buffers. The access time will depend on the ROM layout with parasitics, hence

it is not extracted from the simulation based on schematic. The ROM with 2:1 MUX

does not function correctly once the output from the 16x6-bit ROM differential output is

below 50 mV. The ROM with 32x6-bit memory core operates slower because of longer

parasitic capacitance in its decoder and memory core. The access time in the ROM with

2:1 MUX is higher than the ROM with 32x6-bit memory core although it has higher

operation frequency. This is due to the propagation delay caused by buffers, 2:1 MUX

and D-latch. Considering 300 mV differential output as the least required level of output

after the buffer, the ROM with 32x6-bit memory core simulated at clock frequency of

40

20 GHz and the ROM with 2:1 MUX simulated frequencies up to 46 GHz. The ROM

with 2:1 MUX does not give correct output bit at frequencies higher than 46 GHZ.

Both ROM designs use a power supply of -3.8 V. The schematic simulation

result showed that the ROM with 32x6-bit memory core draws 0.515 A of current and

consumes 1.96 W. Results in Chapter 3 showed that the power consumed by the 16x6-bit

ROM from simulation result overestimates the measured power by 24.2%. The 1.96 W

consumed by 32x6-bit memory core ROM is comparable to the 1.49 W consumed by

16x6-bit ROM. Increase in the power is primarily due to increase in size of the row

decoder and buffers. To further analyze the power dissipation caused by changing the

size of memory core, the ROM with 16x8-bit memory core and the ROM with 32x9-

bit memory core were designed and simulated. The 16x8-bit ROM provides the effect

on power consumption due to an increase in the number of memory core columns and

32x9-bit ROM provides the effect on power consumption due to the increase in numbers

of both memory core rows and columns. Table 4.2 shows current drawn by the row

decoder and memory core, and the total power consumed by the ROMs with different

memory core sizes. It can be seen from the table that current drawn by the memory

core of 16x6 bit and 32x6 bit ROM are equal but the row decoder current is almost

twice as much. Table 4.2 also illustrates that when the number of memory core columns

is increased, keeping the rows constant, the memory core current increases but row

decoder current remains same. This indicates that increasing the number of rows in

memory core does not increase power consumption in memory core because power

consumption in memory core is dependent on the number of bit columns in the memory

core. The row decoder power consumption increases when the row in the row decoder

is increased.

The ROM with 32x6-bit ROM with 2:1 MUX draws 1.86 A of current and con-

sumes power of 7.068 Watt of power. The power consumed by 32x6-bit ROM with 2:1

MUX is more than 3 times that of ROM with 32x6-bit memory core. The high power

41

2.2

2.25

2.3

2.35

2.4

2.45

2.5

2.55

2.6

−1.

2

−1

−0.

8

Tim

e (n

s)

ACC<4>(V)

2.2

2.25

2.3

2.35

2.4

2.45

2.5

2.55

2.6

−1.

2

−1

−0.

8

Tim

e (n

s)

ROMA<5>(V)

2.2

2.25

2.3

2.35

2.4

2.45

2.5

2.55

2.6

−1.

2

−1

−0.

8

Tim

e (n

s)

ROMB<5>(V)

2.25

2.3

2.35

2.4

2.45

2.5

2.55

2.6

−1.

2

−1

−0.

8

Tim

e (n

s)

Output(V)

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

1

0

0

0

0

0

0

0

1

1

0

1

0

1

0

1

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

1

1

0

1

0

1

0

1

Figu

re4.

7:T

hebu

ffer

edan

dla

tche

dM

UX

outp

utof

Bit

5fr

omsi

mul

atio

nof

32x6

-bit

RO

Mw

ith2:

1M

UX

.

42

10 15 20 25 30 35 40 45 5050

100

150

200

250

300

Dif

fere

nti

al O

utp

ut

Vo

ltag

e m

V

Frequency in GHz

a. 32x6−bit ROM with 2:1 MUX

b. ROM with 32x6−bit Memory Core

c. 16x6−bit ROM

Figure 4.8: Simulated differential output voltage of ROMs: (a) schematic of 32x6-bit ROM with 2:1 MUX and D-latch: (b) schematic of ROM with 32x6-bit memorycore: and (c) schematic of 16x6-bit ROM. The outputs from the ROMs are not drivinganything. The 32x6-bit ROM with 2:1 MUX has a higher output voltage than the outputfrom the 16x6-bit ROM because the 2:1 MUX takes the buffered output from 16x6-bitROMs and the output from the 2:1 MUX is latched and buffered again.

ROM Memory Row Decoder Memory Core Total Total PowerCore Size (bit) Current (A) Current (A) Current (A) (W)

16x6 0.17 0.22 0.39 1.4916x8 0.15 0.29 0.44 1.6532x6 0.29 0.22 0.51 1.9632x9 0.29 0.32 0.61 2.33

Table 4.2: The power dissipated by ROMs with different memory core sizes.

43

ROM Design Power Operation Clock Frequency(W) (GHz)

16x6-bit ROM 1.49 36ROM with 32x6-bit core 1.957 20

32x6-bit ROM with 2:1 MUX 7.068 46

Table 4.3: Power and speed comparison of three different ROMs

dissipation in the ROM with the 2:1 MUX is due to having two fully functional 16x6-bit

ROM with additional circuits like 2:1 MUXs, D-latches and buffers. The circuit block of

6 2:1 MUXs, 6 D-latches and delay buffers for 6-bits dissipate most of the power. Low

power MUX and D-latch techniques have been reported by Razavi et al. [35]. These

designs are modification of stacked differential pairs in conventional ECL to adapt in

low power supply. Low voltage supply reduce the power consumption but this comes at

the cost of having multiple power supply because ROM and decoder has higher voltage

rail. The designs of MUX and latch reported by Razavi et al. [35] operate power supply

voltage as low as 1.5 V.

The design approach of increasing ROM bit size by increasing the rows in mem-

ory core slows down the ROM but this design approach is better if the power and access

time is the prime goal and not the very high frequency operation. If a very high fre-

quency operation is the design goal, and not low power, then the ROM with 2:1 MUX is

a better approach. Table 4.3 shows the speed and power analysis for the 16x6-bit ROM,

ROM with 32x6-bit memory core and 32x6-bit ROM with 2:1 MUX.

44

CHAPTER 5

Conclusion

The goal of the research was to explore designs of bipolar ROMs in InP DHBT

technology suitable for use as a ROM LUT and capable of operating at 30-40 GHz. The

ROM suitable for DDS LUT was designed, fabricated and tested to work at 36 GHz.

The simulation results were compared with test results. Further research was done to

increase the ROM bit size without losing speed and power. The SFDR of a DDS system

is associated with the size of ROM LUT. Based on the need for low power and high

speed, two design approaches were explored. One design is for low power and the other

provides high frequency clock operation. The simulation results from both of these

designs were discussed. It would be good to realize these two designs in hardware and

verify the simulation results with the test results in the future.

5.1 Summary of Accomplishment

A 16x6-bit ROM test circuit in InP DHBT technology operating at maximum

clock frequency of 36 GHz was demonstrated. The ROM bit access time of 15.43 ps is

estimated from the simulation result. The entire test circuit consumes power of 7.22 W,

while the ROM circuit alone dissipates 1.13 W. The ROM test circuit has 2242 transis-

tors in an area of 2700 µm by 1450 µm. Further more, two different 32x6-bit ROMs are

designed to increase the bit size. The first ROM design increased the bit size by increas-

ing the bits in the memory core. The schematic simulation showed that ROM operates

at 20 GHz and consumes 1.96 W of power. The second ROM design integrates two

smaller high-speed ROMs together using 2:1 MUX. The schematic simulation result

showed that ROM operates in the range of 46 GHz and consumes 7.06 W of power.

45

5.2 Recommendation for Future Work

Power dissipation is always higher in bipolar circuits than in CMOS counter-

parts. This makes bipolar circuits less attractive and limits their application. Some low

voltages techniques for high speed digital bipolar circuits have been proposed by Razavi

et al. [35]. These techniques have modified stacked differential pairs in conventional

ECL to use low power supply to reduce the power. Some digital logic blocks such as

MUX and D-latch can be designed using these techniques to save power, but it comes at

the cost of multiple different level power supply and slower speed. Additional research

is required to improve the ROM design regarding both speed and power.

The first recommendation for future work is to duplicate and extend this work

in SiGe technology. Despite lower BVCEO, SiGe technology has some advantages that

this project can benefit from. The first advantage is that SiGe process provides SBD, so

the memory cell and decoder in the ROM design could benefit from smaller and faster

SBD. A SBD memory cell and a decoder design is described in Chapter 2, Section 2.2

can be implemented in SiGe process to achieve better performance. Another advantage

in SiGe process is it provides PMOS and NMOS because it is built on CMOS process.

The ROM design can benefit from high impedance NMOS current source to reduce

the power consumption. SiGe process also has thin copper interconnect which provide

lower capacitance compare to thick aluminium interconnect in InP. Lower parasitic ca-

pacitance in the interconnect can help improve the performance of the ROM. On top

of these, SiGe process is more easily manufacturable than InP process with better yield

and lower cost.

The next step needs to be implementing ROM compression technique in LUT of

DDS system after having high speed ROM designed. The ROM compression techniques

such as Bellaouar’s architecture [29], non linear addressing technique [30] and modified

Bellaour’s architecture [31] can be implemented in the ROM designed in this thesis. The

additional circuit blocks required for ROM compression such as multipliers and adders

46

need to be explored. Finally, the future work showing the comparison in terms of SFDR,

speed, power and size of DDS with ROM LUT to ROM-less DDS architecture in InP

DHBT technology is recommended.

47

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BIOGRAPHY OF THE AUTHOR

Sanjeev Manandhar was born in Kathmandu, Nepal on November 13, 1979. He

received his high school education from St. Xavier’s Campus in Kathmandu.

He entered The University of Maine in 2000 and obtained his Bachelor of Sci-

ence degree in Computer Engineering in 2004.

In September 2004, he was enrolled for graduate study in Electrical Engineering

at The University of Maine and served as a Graduate Research Assistant. His current re-

search interests include digital and mixed-signal circuit design. He is a member of IEEE,

and his interests include playing soccer, photography and hiking. Sanjeev is a candidate

for the Master of Science degree in Electrical Engineering from The University of Maine

in August 2006.

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