FREQUENCY MODULATION (FM) TRANSMITTER AND RECEIVER GOHHANSHIN
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1 FREQUENCY MODULATION (FM) TRANSMITTER AND RECEIVER GOHHANSHIN Tesis Dikemukan Kepada Fakulti Kejuruteraan, Universiti ...
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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.
it
I
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.
iii
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
vi
I
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-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
vii
I
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
62
7.3 Receiver 7.3.1 Pre-amplifier
62
7.3.2 Dual stage Current Feedback Amplifier
65
viii
I
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
ix
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
x
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 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
xi
I
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
xii
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.
1
I
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 + cpc)
(2.1)
3
I
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 terms 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 ] + .......}
6
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.
8
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
9
1
stations. Following are the layout of commercial FM broadcast band allocation and a table of differences between wideband and narrowband FM.
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