analysis of frequency modulation (fm) transmitter and receiver
DESCRIPTION
With FM and PM modulators, the carrier at the output is generally somewhat lower than the desired frequency of transmission.TRANSCRIPT
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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