FREQUENCY MODULATION (FM)

TRANSMITTER AND RECEIVER

GOHHANSHIN

Tesis Dikemukan Kepada Fakulti Kejuruteraan, Universiti Malaysia Sarawak

Sebagai Memenuhi Sebahagian daripada Syarat Penganugerahan Sarjana Muda Kejuruteraan

Dengan Kepujian (Kejuruteraan Elektronik dan Telekomunikasi) September 1998

DEDICATION

To my beloved parents, family and friends.

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ACKNOWLEDGMENT

First and foremost, I would like to express my high gratitude to my

supervisor Madam Park Young Soon for everything she had done. Without her

priceless advice, encouragement and guidance, this thesis would be an

extremely hard task for me.

I would also like to convey my gratitude to the Faculty of Engineering

which provided the necessary facilities for this thesis project, and also to the

lecturers, tutors and lab assistance for their information, help and guidance.

To my co-lab mates, Darshan Singh slo Gurbax Singh, Kismet Hong Ping

and Grace Quak, I feel proud to have them gone through with me the hard time

of completing this thesis. Their advice, comments and guidance would not be

forgotten.

Finally, I would like to thank my "White House" housemates, namely

Alexander Siew, Chow Ow Wei, Hoh Hoong Koan, Teoh Sim Keat, Teoh Poh

Hian and Wong Kiung Chung, who has gone through with me the hard time of

preparing the thesis.

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ABSTRACT

FM transmitter-receiver is indeed an electronic project that places great

emphasis on practical work. The project enhances one's practical skill and it

involves both the electronics and telecommunications fields. Theoretical

knowledge such as circuit theory, amplifier and principles of telecommunication

learned through several courses offered by the Electronics and

Telecommunications program is applied in the project. A set of handset-size

transmitter and receiver is constructed. This wireless communication system is

operating at 90 MHz, using Frequency Modulation (FM) techniques and limited

at simplex communication only.

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ABSTRAK

Pemancar-penerima FM adalah satu projek elektroink yang lebih

mengutamakan kerja praktikal. Project tersebut menambah kemahiran

praktikal seseorang dan ia melibatkan kedua-dua bidang elektronik dan

telekomunikas1, Pengetahuan teori seperti teori litar, pengekuat dan prinsip­

prinsip telekomunikasi yang belajar daripada pelbagai kursus yang ditawarkan

oleh program Electronics and Telecommunications telah digunakan dalam

projek ini. Satu set pemancar dan penerima yang bersaiz handset telah dibina.

Dalam project ini. Sistem komunikasi tanpa wayar ini beroperasi pada 90 MHz,

menggunakan teknik Frequency Modulation dan ianya terhad dalam satu arah

komunikasi sahaja.

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Table of Contents

APPROVAL LETrER

APPROVAL SHEET

PROJECT TITLE

DEDICATION ii

LIST OF FIGURES Xl

ACKOWLEGMENT III

ABSTRACT IV

ABSTRAK V

TABLE OF CONTENTS VI

LIST OF TABLES x

CHAPTER 1: INTRODUCTION

1.1 Project Description 1

1.2 Objective 1

1.3 Background 2

CHAPTER 2: THEORY OF FM

2.1 Introduction 3

2.2 Modulation Index, Deviation Ratio, Bessel Function 3

2.3 Wideband and Narrowband FM 9

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CHAPTER 3: TRANSMITTER

3.1 Introduction 11

3.2 FM Generation Method 12

3.2.1 Direct Method 13

3.2.1.1 Varactor Modulator 14

3.2.1.2 Reactance Modulator 16

3.2.2 Indirect Method 18

3.3 Project Transmitter 19

3.3.1 Circuit Description 19

3.3.1.1 Pre-amplifier 20

3.3.1.2 Pre-emphasis 21

3.3.1.3 Audio Amplifier 21

3.3.1.4 FM Modulator 22

CHAPTER 4: RECEIVER

4.1 Introduction 25

4.2 Sensitivity and Selectivity 25

4.3 Project Receiver 26

4.3.1 Circuit Description 27

4.3.1.1 Pre-amplifier 27

4.3.1.2 Demodulator 28

4.3.1.3 Low-pass Filter 28

4.3.1.4 Multistage Amplifier 29

CHAPTER 5: AMPLIFIER

5.1 Introduction 31

5.2 Voltage, Current and Power Amplifier 31

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5.3 Class of Power Amplifier 33

5.4 Coupling and De-coupling Capacitor 35

5.5 Amplifier Configuration 37

5.6 Dc Analysis of an Amplifier 39

5.6.1 Q-point and Dc Load Line 39

5.6.2 Dc Biasing Circuit 40

5.6.3 Dc analysis 42

CHAPTER 6: NOISE, DISTORTION IN FM AND REDUCTION

6.1 Introduction 46

6.2 Noise 46

6.3 Distortion 47

6.4 Noise Effect in FM 48

6.5 Pre-emphasis and De-emphasis 50

6.6 Negative Feedback and Dc Stabilisation 53

6.6.1 Collector Feedback and Emitter Feedback 53

6.6.2 Bootstrapping 55

CHAPTER 7: CIRCUIT ANALYSIS

7.1 Introduction 57

7.2 Transmitter 57

7.2.1 Pre-amplifier 57

7.2.2 Audio Amplifier 59

7.2.3 Power amplifier 60

7.3 Receiver 62

7.3.1 Pre-amplifier 62

7.3.2 Dual stage Current Feedback Amplifier 65

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7.3.3 Other Circuits 66

7.3 Problem encountered in Project Implementation 70

CONCLUSION 73

SUGGESTIONS FOR FUTURE WORK 74

APPENDIX 1: Transistor Specification Sheet

APPENDIX 2: The Schematic Diagram of Project Transmitter

APPENDIX 3: The Schematic Diagram of Project Receiver

APPENDIX 4: List of Components

APPENDIX 5: Photographs of the Project Transmitter and Receiver

REFERENCES

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LIST OF TABLES

Table Page

2.1 Bessel Function of the First Kind 7

2.2 The comparison between wideband and narrowband 10

5.1 The distinction between current and voltage amplifier 33

5.2 Three typical classes of power amplifier 34

5.3 Typical amplifier configuration 38

7.1 Measurement value and calculation value for transmitter 62

7.2 Measurement value and calculation value for receiver 68

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LIST OF FIGURES

Figure Page

2.1 A plot of Bessel functions for n=I,2,3 7

2.2 FM spectrogram 8

2.3 Commercial FM bandwidth allocation for two adjacent stations 10

3.1 Block diagram of standard FM transmitter 11

3.2 The functional blocks diagram of FM generation 13

3.3 The simple microphone modulator for FM generation 13

3.4 Direct FM modulator using varactor diode 15

3.5 The relationship between junction capacitance and reverse 15

bias voltage

3.6 The JFET reactance modulator 17

3.7 Armstrong phase modulator 19

3.8 The block diagram of the project transmitter 20

3.9 The schematic diagram of FM transmitter 24

4.1 The functional blocks of the project receiver 27

4.2 Schematic diagram of project receiver 30

5.1 Thevenin model for voltage amplifier 31

5.2 The function of bypass capacitor CE 36

5.3 The Q-point and the maximum swing of Ic at saturation and 40

VCE at cut-off

5.4 The Q-point location for class of amplifier 40

5.5 The dc biasing circuit 41

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5.6 The dc analysis of common emitter amplifier 43

5.7 The Q-point for a common emitter amplifier 44

6.1 The distortion and its reasons in amplifier 48

6.2 The relationship between noise vector and frequency shift 48

6.3 Noise sideband distribution for FM (tringle) and AM 50

(rectangular)

6.4 The pre-emphasis and de-emphasis network 51

6.5 Origin signal strength before pre-emphasis 51

6.6 Signal strength after pre-emphasis 52

6.7 The signal strength after de-emphasis 52

6.8 The compensation of ac gain through capacitor Cr 55

7.1 The pre-amplifier with 9 V dc supply 58

7.2 The audio amplifier 59

7.3 The power amplifier 61

7.4 The pre-amplifier of receiver 62

7.5 Transmitter and its measurement pins location 64

7.6 Simplified dual stage amplifier 65

7.7 The simplified dc biasing circuit for Q4 67

7.8 Receiver and its measurement pins location 69

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CHAPTER 1

INTRODUCTION

1.1 Project Description

To explain the title of project clearer, frequency modulation transmitter­

receiver (transceiver) actually refers to a hand-sized amateur transmitter­

receiver, which operates at radio FM range. The transmitter operating

frequency is fixed to around 90 MHz while the receiver is tuned to the desired

signaL

The project is to create a FM transmitter-receiver as described above.

Theory of FM and operation of circuit are studied before any implementation of

hardware. Circuit analysis, testing and trouble-shooting are done for circuit

optimisation. Further improvement is concentrated on the half-duplex or full

duplex communication.

1.2 Objective

The primary purpose of the project is to understand the operation of basic

wireless telecommunication. By going through the project, theoretical knowledge

is transferred into practice. During the hardware implementation, practical

skills such as soldering, printed circuit board (PCB) implementation and circuit

testing can be enhanced.

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1.3 Background

Frequency modulation (FM) is one of the angle modulation techniques. This

modulation technique is so common nowadays that it can be found in any kind of

commercial radios. Several projects and researches have been carried out on this

topic since its introduction in 193 1.

Due to the rapid development of integrated circuit (IC), most of the FM

transmitters and receivers nowadays are constructed and designed using modulator

and demodulator IC chips. The use of ganged inductors and capacitors can also be

easily found in modern radio set. However, to understand the basic theory of

frequency modulation, this project makes use of only transistors to form the heart of

modulator and demodulator. Capacitors and hand-made inductors are used to provide

generation of carrier frequency.

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CHAPTER 2

THEORY OF FREQUENCY MODULATION

2.1 Introduction

Voice or information that is going to be transferred is termed as

information signal. If the distance between communication parties is too large,

direct voice communication is impossible. The method of message sender is

needed. The message sender could be a dove, servant or an arrow. The function

of message sender is just to carry the information to the desired destination.

Thus the message sender can be said to be a carrier. The carrier merely sends

the information and needs not to be intelligent. The information signal is

sometimes called the intelligence signal.

In telecommunications, the mechanism of putting the information signal

into a carrier for it to be transmitted farther is called modulation. Since the

characteristic of the carrier signal is being altered by the information signal, the

carrier is also a modulated signal. Therefore, the information signal, intelligence

signal and modulating signal representing the same thing.

For the carrier to carry information, at least one of the carrier signal's

characteristics (amplitude, phase or frequency) must be modified. Frequency

Modulation (FM) is a method of modifying frequency of carrier signal in order

that the receiver can obtain the desired transmitted information.

2.2 Modulation Index, Deviation Ratio and Bessel Function

For a simple mathematical evaluation, carrier signal can be expressed in

term of:

Vc =A sin e=A sin (mct + cpc) (2.1)

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where Vc =instantaneous value of carrier (in voltage or current)

A = maximum amplitude of carrier

e = angle of sinusoidal carrier wave

We =angular velocity, radians per second (radJs)

4> = phase angle, rad

Changing the value of A corresponding to the amplitude of information

signal, this will induce the Amplitude Modulation (AM). Changing the e will

give us the Angle Modulation. Frequency Modulation (FM) can be achieved by

varying the value of We while alteration of 4> will produce Phase Modulation

(PM).

In frequency modulation, the frequency of carrier swings at certain

amount of frequency that is proportional to the instantaneous amplitude of

information signal. The instantaneous frequency of carrier, £ can be expressed

as:

~ fc =deviation of carrier frequency

= £, +fc kVs COSWst k = proportionality constant

Vs coswst = instantaneous information signal

Thus, the instantaneous angular velocity of carrier is given by,

Wi We + We kVscoswst (2.2)

The relationship between phase angle and angular velocity is given as:

dO - =w(t)dt

By integration,

0= Jm(t)dt

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9i (t) - 9(0) = JI (i)c + (i)e kVsCOS(i)st dt o

By substituting (i) = 2nf and setting initial value of angle to zero, 9(0) = O.

9(t) = (i)et + (fJfs)kVssin(i)st fs =frequency of information signal

= (i)et + mr sin(i)st (2.3)

The maximum frequency deviation of carrier is given as:

8 = kVsfc

The Modulation Sensitivity is expressed as:

Kr= kfc

Thus, the modulation index, mr is given as:

mr= 8/fs

maximum deviation of carrier = ------------------------~--~

modulating frequency

Note that when the modulating frequency is at its maximum value, the

modulation index is known as the deviation ratio. Thus, the deviation ratio is

the minimum value of modulation index of a system. By substituting Equation

2.3 into Equation 2.1, the instantaneous amplitude of carrier becomes,

Vi (t) = A sin 9i(t) 9i = instantaneous angle of carrier

= A sin «(i)et + mrsiu(i)st )

= A [ sin(i)ct .cos( mrsin(i)st ) + COS(i)ct .sin(mrsin(i)st) ]

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Using Fourier Series, following terms can be expanded with coefficients of

Bessel Function.

cos( mrsincost) =Jo(mr) + 2 L J2n(mr)cos2ncost

sin( mrsincost) =2 L J2n+1(mf)sin(2n+ l)cost

where the Bessel Function is defined by:

Vi (t) =A[sincoct .COS( mrsincost ) + COSCOct .sin(mrsincost) ]

=A{sincoct (Jo(mr) + 2 L J2n(mf)cos2ncost) + COSCOct

(2 L J2n+1(mf)sin(2n+ l)cost)} (2.4)

Applying the following equations into Equation 2.4,

cos x sin y =.! [sin(x+y) - sin(x-y)] 2

sin x cos y =.! [sin(x+y) + sin(x-y)] 2

Thus, Vi (t) =A { Josincost + Jl[ sin(coc+ COs)t - sin(coc - COs)t] +

Ja [ sin(coc+3cos)t - sin(coc-3cos)t ] + .......}

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1.2r---.,..-...,..-.....---,r---.,....--r--...,..---r--.---"I

-~t---+--I--t--~"'---I--t---+-,+--I--I ,i)-10 -4 -2. 0 2. .Q "

mr

Figure 2.1: A plot of Bessel functions for n == 0, 1,2 and 3. [1]

m, Jo J 1 J1 J3 J4 J s J6 J 7 J 8 J 9 J 10

0.00 1.00 - - - - - ­

0.25 0.98 0.12 - - - - - - ­

0.5 0.94 0.24 0.03 - ­

1.0 0.77 0.44 0.11 0.02 - - ­

1.5 0.51 0.56 0.23 0.06 0.01 - - - ­

2.0 0.22 0.58 0.35 0.13 0.03 - - - ­

2.5 -0.05 0.50 0.45 0.22 0.07 0.02 - - ­

3.0 -0.26 0.34 0.49 0.31 0.13 0.04 0.01 - - ­

4.0 - 0.40 -0.07 0.36 0.43 0.28 0.13 0.05 0.02 ­

5.0 - 0.18 - 0.33 0.05 0.36 0.39 0.26 0.13 0.05 0.02 ­

6.0 0.15 -0.28 -0.24 0.11 0.36 0.36 0.25 0.13 0.06 0.02 ­

Table 2.1: Bessel Functions of the First Kind.

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I'hf =2.5

Constant fs, varying () Constant (), varying fs

Figure 2.2: FM spectrogram [2].

Several observations can be obtained from above evaluation, table and graph

of Bessel Function as well as graphical representation of FM spectrograms.

1. FM has an infinite number of sidebands (sum and difference between carrier

frequency and information signal). Thus in theory, FM has the endless

bandwidth. However, from the table of Bessel Function the amplitudes of the

sidebands (In) decrease as n increases. Therefore, the In will become less and

less significant as the number of sidebands (n) increases.

2. The modulation index IIlf determines the number of significant sidebands

since the In is function of mf. The greater modulation index, the greater the

number of significant sidebands will be.

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3. From the spectrogram of FM signal, the sidebands distribution of FM is

symmetric about carrier frequency fe. Every sidebands is allocated from the

carrier frequency fe at the distance of ± fs, ± 2f., ± 3f., ± 4f., ± 5f•.... The upper

and lower sidebands with the same distance from fc will have the same value

of amplitude.

4. Increasing the modulation index will finally increase the required FM

bandwidth. By approximation, Carson's rule for bandwidth calculation can

be used to calculate 98 % level of the Bessel functions. Thus, the

approximation for desired FM bandwidth could be written as:

FM Bandwidth ~ 2(0 + fs )

2.3 Wideband and Narrowband FM

Previous observations described that FM has infinite bandwidth and thus

the approximation of conserving the significant sidebands is done. However, in

real practical world, the proper range of FM bandwidth usually depends on its

application. For broadcasting, the wideband is used. Meanwhile the narrowband

FM is applied in television sound and mobile communication systems such as

police, aircraft, taxicabs and private industry network. Narrowband uses

smaller modulation index that the signal fidelity is no so critical factor as long

as the received voice is understandable, although sometime it is not

recognisable.

Wideband FM has standard broadcast bandwidth of 200 kHz for each

station. Under Federal Communications Commission (FCC) rules, the maximum

deviation is restricted to ± 75 kHz with the extra band (guard band) of 25 kHz.

The main purpose of guard band is to avoid signal overlapping from 2 adjacent

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stations. Following are the layout of commercial FM broadcast band allocation

and a table of differences between wideband and narrowband FM.

~ ./Guard Band <III 200kHz .~

IE 75kHz-.+_- 75kHz 3u ll--_--i--1__I........ Carrier I (88.1) Carrier II (88.3) Frequency (MHz)

Figure 2.3: Commercial FM bandwidth allocation for two adjacent stations.

Wideband FM Narrowband FM

FCC Bandwidth allocation 200 kHz 10 - 30 kHz

Modulating signal used 30 Hz - 15 kHz 30 - 3kHz

Maximum deviation ± 75 kHz ±5kHz

Table 2.2: The comparison between wideband and narrowband FM.

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CHAPTER 3

TRANSMITTER

3.1 Introduction

The FM transmitter mainly consists of pre-amplifier, FM modulator,

oscillator, frequency multiplier and power amplifier. Basically common FM

transmitter contains following functional blocks.

Audio signal

PowerPre-amplifier .. FM I ... Oscillator Frequency --I...111----1...... Modulator .~--I..." .. ... Multiplier ~ Amplifier

L-____~ L-___~

Figure 3.1: Block diagram of standard FM transmitter.

The pre-amplifier boosts the audio signal levels from several milli-volts to

higher enough stage for feeding into the modulator. Usually a high pass filter

network is added between pre-amplifier and modulator stage. This high pass

filter acts as pre-emphasis network to improve the signal to noise level of FM

transmission at higher frequency. The pre-emphasis network is optional.

However, the receiver will suffer from distortion at higher frequency of audio

signal if this stage is ignored. With the carrier signal generated from oscillator,

the modulator modulates the carrier with input signal from pre-amplifier stage.

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The operating frequency of the generated FM output is still not high

enough to be transmitted through free space. Thus, several stages of frequency

multiplier are put to increase the operating frequency. After going through a

number of multipliers, the attenuation of signal level is compensated by the

final stage power amplifier. Power amplifier restored the FM signal strength to

the desired level.

3.2 FM Generation Method

Basically there are two types of FM generation. In the first method, the

intelligence signal varies the carrier frequency directly, so it is called direct FM.

The second method is the use of Armstrong indirect method.

In the indirect method, the Phase Modulation (PM) is generated instead

of FM. Since changing phase of a signal (PM) indirectly causes its frequency to

be changed simultaneously, the generation of PM indirectly produces FM.

However the FM generated in method is lack in bass. Thus, the modulating

signal must be bass-boosted before it is fed into PM modulator to produce the

same quality of FM.

The major reason of using indirect FM method is to improve the

frequency stability of carrier oscillator. In the direct FM method, due to the

inherent variations of electronic components (inductor L and capacitor C) in

manufactured value and the inevitable drift caused by temperature changes and

component ageing, it can not provide the precise carrier frequency. Thus crystal

oscillator with extremely high frequency stability is deployed to replace the LC

tank circuit in generating carrier frequency. Following is the block diagram for

direct method and indirect method.

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