Analysis Of Frequency Modulation (fm) Transmitter And Receiver 271jf

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.

iv

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.

v

Table of Contents

APPROVAL LETrER

APPROVAL SHEET

PROJECT TITLE

DEDICATION

ii

ACKOWLEGMENT

III

ABSTRACT

IV

ABSTRAK

V

TABLE OF CONTENTS

VI

LIST OF TABLES

x

LIST OF FIGURES

Xl

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

18

3.2.2 Indirect Method

19

3.3 Project Transmitter

19

3.3.1 Circuit Description 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- 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 and Dc Stabilisation

53

6.6.1 Collector and Emitter

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

62

7.3 Receiver 7.3.1 Pre-amplifier

62

7.3.2 Dual stage Current 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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I

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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I

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 by 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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I

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.

2

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 + c)

(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 kVs COSWst

fc =deviation of carrier frequency 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,

J

0= m(t)dt

4

I

9i (t) - 9(0) =

J(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, 9i = instantaneous angle of carrier

Vi (t) = A sin 9i(t) = A sin «(i)et + mrsiu(i)st )

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

5

L

I

Using Fourier Series, following can be expanded with coefficients of Bessel Function. cos( mrsincost)

=Jo(mr) + 2 L

sin( mrsincost)

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

J2n(mr)cos2ncost

where the Bessel Function is defined by:

Vi (t)

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

=A{sincoct (Jo(mr) + 2 L (2

L

J2n(mf)cos2ncost) + COSCOct

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

(2.4)

Applying the following equations into Equation 2.4,

Thus,

cos x sin y

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

sin x cos y

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

Vi (t)

2

2

=A { Josincost + Jl[ sin(coc+ COs)t - sin(coc - COs)t] + Ja [ sin(coc+3cos)t - sin(coc-3cos)t ] + .......}

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I

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

-~t---+--I--t--~"'---I--t---+-,+--I--I -10

-4

-2.

0

2.

.Q

,i)

"

mr

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

m,

Jo

J1

0.00

1.00

0.25

J1

J3

J4

Js

J6

-

-

-

-

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

4.0

- 0.40

-0.07

0.36

0.43

0.28

5.0

- 0.18

- 0.33

0.05

0.36

6.0

0.15

-0.28

-0.24

0.11

J7

J8

J9

-

-

J 10 ­

­

-

-

­

-

-

­ -

-

-

­

-

­

-

-

0.01

-

-

0.13

0.05

0.02

0.39

0.26

0.13

0.05

0.02

­

0.36

0.36

0.25

0.13

0.06

0.02

Table 2.1: Bessel Functions of the First Kind.

7

­

­ ­

­

I'h f =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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1

stations. Following are the layout of commercial FM broadcast band allocation and a table of differences between wideband and narrowband FM.



IE 88

200kHz

~ . /Guard Band .~

3 ll--_--i--1__I.. . .

u

75kHz-.+_- 75kHz 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

Pre-amplifier L -_ _ _ _

.. 111----1......

~

I

..

FM Oscillator Modulator .~--I..." L -_ _ _

....

Frequency Multiplier

- - I... ~

Power Amplifier

~

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