Telecommunication
Circuit Design
Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije
Copyright # 2002 John Wiley & Sons, Inc.
ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic)
Telecommunication
Circuit Design
Second Edition
Patrick D. van der Puije
A Wiley-Interscience Publication
JOHN WILEY & SONS, INC.
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CONTENTS
Preface xiii
Chapter 1 The History ofTele communications 1
1.1 Introduction 1
1.2 Telecommunication Before the Electric Telegraph 1
1.3 The Electric Telegraph 2
1.4 The Facsimile Machine 4
1.5 The Telephone 6
1.6 Radio 8
1.7 Television 9
1.8 The Growth of Bandwidth and the Digital Revolution 10
1.9 The Internet 11
1.10 The World Wide Web 13
References 15
Bibliography 16
Chapter 2 Amplitude Modulated Radio Transmitter 17
2.1 Introduction 17
2.2 Amplitude Modulation Theory 18
2.3 System Design 21
2.3.1 Crystal-Controlled Oscillator 22
2.3.2 Frequency Multiplier 22
2.3.3 Amplitude Modulator 22
2.3.4 Audio Amplifier 22
2.3.5 Radio-Frequency Power Amplifier 23
2.3.6 Antenna 23
v
2.4 Radio Transmitter Oscillator 23
2.4.1 Negative Conductance Oscillator 24
2.4.2 Classical Feedback Theory 26
2.4.3 Sinusoidal Oscillators 28
2.4.4 General Form of the Oscillator 28
2.4.5 Oscillator Design for Maximum Power Output 31
2.4.6 Crystal-Controlled Oscillator 35
2.5 Frequency Multiplier 37
2.5.1 Class-C Amplifier 40
2.5.2 Converting the Class-C Amplifier into a Frequency
Multiplier 40
2.6 Modulator 47
2.6.1 Square-law Modulator 47
2.6.2 Direct Amplitude Modulation Amplifier 49
2.6.3 Four-Quadrant Analog Multiplier 51
2.7 Audio-Frequency Amplifier 54
2.7.1 Basic Device Characteristics 54
2.7.2 Class-A Amplifier 55
2.7.3 Class-B Amplifier 60
2.8 The Radio-Frequency Amplifier 67
2.9 The Antenna 71
2.9.1 Radiation Pattern of an Isolated Dipole 72
2.9.2 Monopole or Half-Dipole 72
2.9.3 Field Patterns for a Vertical Grounded
Antenna 74
2.10 Classification of Amplitude Modulated Radio-Frequency
Bands 75
References 76
Problems 76
Chapter 3 The Amplitude Modulated Radio Receiver 79
3.1 Introduction 79
3.2 The Basic Receiver: System Design 79
3.3 The Superheterodyne Receiver: System Design 82
3.4 Components of the Superheterodyne Receiver 85
3.4.1 Receiver Antenna 85
3.4.2 Low-Power Radio-Frequency Amplifier 86
3.4.3 Frequency Changer or Mixer 90
3.4.4 Intermediate-Frequency Stage 99
3.4.5 Automatic Gain Control 101
vi CONTENTS
3.4.6 Demodulator 103
3.4.7 Audio-Frequency Amplifier 105
3.4.8 Loudspeaker 105
3.5 Short-Wave Radio 107
References 108
Bibliography 108
Problems 108
Chapter 4 Frequency Modulated Radio Transmitter 111
4.1 Introduction 111
4.2 Frequency Modulation Theory 111
4.3 The Parameter Variation Method 115
4.3.1 Basic System Design 115
4.3.2 Automatic Frequency Control of the FM
Generator 117
4.3.3 Component Design with Automatic Frequency
Control 118
4.4 The Armstrong System 122
4.4.1 Practical Realization 125
4.4.2 Component Circuit Design 127
4.5 Stereophonic FM Transmission 137
4.5.1 System Design 137
Bibliography 138
Problems 139
Chapter 5 The Frequency Modulated Radio Receiver 143
5.1 Introduction 143
5.2 Component Design 146
5.2.1 Antenna 146
5.2.2 Radio-Frequency Amplifier 146
5.2.3 Local Oscillator 147
5.2.4 Frequency Changer 147
5.2.5 Intermediate-Frequency Stage 147
5.2.6 Amplitude Limiter 148
5.2.7 Frequency Discriminator 148
5.3 Stereophonic Frequency Modulated Reception 156
5.3.1 Synchronous Demodulation 158
5.3.2 Stereophonic Receiver Circuit 158
References 158
Problems 159
CONTENTS vii
Chapter 6 The Television Transmitter 161
6.1 Introduction 161
6.2 System Design 162
6.3 Component Design 163
6.3.1 Camera Tube 163
6.3.2 Scanning System 167
6.3.3 Audio Frequency and FM Circuits 169
6.3.4 Video Amplifier 170
6.3.5 Radio-Frequency Circuits 179
6.3.6 Vestigial Sideband Filter 179
6.3.7 Antenna 179
6.3.8 Color Television 180
References 184
Bibliography 184
Problems 185
Chapter 7 The Television Receiver 187
7.1 Introduction 187
7.2 Component Design 189
7.2.1 Antenna 189
7.2.2 Superheterodyne Section 189
7.2.3 Intermediate-Frequency Amplifier 189
7.2.4 Video Detector 190
7.2.5 The Video Amplifier 191
7.2.6 The Audio Channel 191
7.2.7 Electron Beam Control Subsystem 191
7.2.8 Picture Tube 202
7.3 Color Television Receiver 203
7.3.1 Demodulation and Matrixing 203
7.3.2 Component Circuit Design 205
7.3.3 Color Picture Tube 207
7.4 High-Definition Television (HDTV) 209
Bibliography 210
Problems 210
Chapter 8 The Telephone Network 213
8.1 Introduction 213
8.2 Technical Organization 213
8.3 Basic Telephone Equipment 216
viii CONTENTS
8.3.1 Carbon Microphone 216
8.3.2 Moving-Iron Telephone Receiver 218
8.3.3 Local Battery – Central Power Supply 220
8.3.4 Signalling System 220
8.3.5 The Telephone Line 221
8.3.6 Performance Improvements 222
8.3.7 Telephone Component Variation 224
8.4 Electronic Telephone 224
8.4.1 Microphones 225
8.4.2 Receiver 225
8.4.3 Hybrid 226
8.4.4 Tone Ringer 229
8.4.5 Tone Dial 230
8.5 Digital Telephone 243
8.5.1 The Codec 244
8.6 The Central Office 253
8.6.1 Manual Office 253
8.6.2 Basics of Step-by-Step Switching 253
8.6.3 The Strowger Switch 256
8.6.4 Basics of Crossbar Switching 257
8.6.5 Central Office Tone Receiver 259
8.6.6 Elements of Electronic Switching 261
References 263
Problems 263
Chapter 9 Signal Processing in the Telephone System 267
9.1 Introduction 267
9.2 Frequency Division Multiplex (FDM) 267
9.2.1 Generation of Single-Sideband Signals 268
9.2.2 Design of Circuit Components 269
9.2.3 Formation of a Basic Group 269
9.2.4 Formation of a Basic Supergroup 270
9.2.5 Formation of a Basic Mastergroup 270
9.3 Time-Division Multiplex (TDM) 272
9.3.1 Pseudodigital Modulation 273
9.3.2 Pulse-Amplitude Modulation Encoder 273
9.3.3 Pulse-Amplitude Modulation Decoder 279
9.3.4 Pulse Code Modulation Encoder=Multiplexer 286
9.3.5 Pulse-Code Modulation Decoder=Demultiplexer 286
CONTENTS ix
9.3.6 Bell System T-1 PCM Carrier 286
9.3.7 Telecom Canada Digital Network 290
9.3.8 Synchronization Circuit 290
9.3.9 Regenerative Repeater 291
9.4 Data Transmission Circuits 295
9.4.1 Modem Circuits 298
References 300
Problems 301
Chapter 10 The Facsimile Machine 305
10.1 Introduction 305
10.2 Systems Design 307
10.2.1 The Transmit Mode 308
10.2.2 The Receive Mode 308
10.3 Operation 310
10.3.1 ‘‘Handshake’’ Protocol 310
10.4 The Transmit Mode 311
10.4.1 The CCD Image Sensor 311
10.4.2 The Binary Quantizer 315
10.4.3 The Two-Row Memory 317
10.5 Data Compression 318
10.5.1 The Modified Huffman (MH) Code 318
10.5.2 The Modified READ Code 318
10.6 The Modem 319
10.7 The Line Adjuster 319
10.8 The Receive Mode 319
10.8.1 The Power Amplifier 319
10.8.2 The Thermal Printer 321
10.9 Gray Scale Transmission: Dither Technique 322
References 323
Bibliography 323
Glossary 323
Problems 324
Chapter 11 Personal Wireless Communication Systems 325
11.1 Introduction 325
11.2 Modulation and Demodulation Revisited 326
11.3 Access Techniques 327
11.3.1 Multiplex and Demultiplex Revisited 327
11.3.2 Frequency-Division Multiple Access (FDMA) 328
x CONTENTS
11.3.3 Time-Division Multiple Access (TDMA) 328
11.3.4 Spread Spectrum Techniques 329
11.4 Digital Carrier Systems 331
11.4.1 Binary Phase Shift Keying (BPSK) 333
11.4.2 Quadrature Phase Shift Keying (QPSK) 334
11.5 The Paging System 338
11.5.1 The POCSAG Paging System 338
11.5.2 Other Paging Systems 341
11.6 The Analog Cordless Telephone 343
11.6.1 System Design 343
11.6.2 Component Design 343
11.6.3 Disadvantages of the Analog Cordless
Telephone 347
11.7 The Cellular Telephone 347
11.7.1 System Overview 348
11.7.2 Advanced Mobile Phone System (AMPS) 348
11.8 Other Analog Cellular Telephone Systems 355
11.8.1 Disadvantages of Analog Cellular Telephone
Systems 357
11.9 The CDMA Cellular Telephone Systems 358
11.9.1 System Design of the Transmit Path 358
11.9.2 Component Circuit Design for the Transmit
Path 359
11.9.3 System Design of the Receive Path 361
11.9.4 Component Circuit Design for the Receive
Path 362
11.10 Other Digital Cellular Systems 362
References 363
Bibliography 363
Problems 364
Abbreviations 364
Chapter 12 Telecommunication Transmission Media 367
12.1 Introduction 367
12.2 Twisted-Pair Cable 367
12.2.1 Negative-Impedance Converter 370
12.2.2 Four-Wire Repeater 374
12.3 Coaxial Cable 375
12.4 Waveguides 376
CONTENTS xi
12.5 Optical Fiber 376
12.6 Free Space Propagation 377
12.6.1 Direct Wave 379
12.6.2 Earth-Reflected Wave 379
12.6.3 Troposphere-Reflected Wave 379
12.6.4 Sky-Reflected Wave 379
12.6.5 Surface Wave 380
12.7 Terrestrial Microwave Radio 380
12.7.1 Analog Radio 381
12.7.2 Digital Radio 384
12.8 Satellite Transmission System 384
References 389
Bibliography 389
Problems 390
Appendix A The Transformer 391
A.1 Introduction 391
A.2 The Ideal Transformer 392
A.3 The Practical Transformer 394
Appendix B Designation ofFre quencies 397
VHF Television Frequencies 397
UHF Television Frequencies 398
Appendix C The Electromagnetic Spectrum 399
Appendix D The Modified Huffman Data Compression Code 401
Appendix E Electronic Memory 405
E.1 Introduction 405
E.2 Basics of S-R Flip-Flop Circuits 405
E.3 The Clocked S-R Flip-Flop 407
E.4 Initialization of the S-R Flip-Flop 409
E.5 The Shift Register 409
E.5 Electronic Memory 411
E.6 Random Access Memory (RAM) 411
Appendix F Binary Coded Decimal to Seven-Segment
Decoder 415
Index 417
xii CONTENTS
PREFACE
The first edition of this book was published in 1992. Nine years later it had become
clear that a second edition was required because of the rapidly changing nature of
telecommunication. In 1992, the Internet was in existence but it was not the
household word that it is in the year 2001. Cellular telephones were also in use
but they had not yet achieved the popularity that they enjoy today. In the current
edition, Chapter 1 has been revised to include a section on the Internet. Chapter 10 is
new and it covers the facsimile machine; I had overlooked this important telecommunication
device in the first edition. Chapter 11 is also new and it describes the
pager, the cordless telephone and the cellular telephone system. These are examples
of a growing trend in telecommunications to go ‘‘wireless’’.
This book is about telecommunications: the basic concepts, the design of subsystems
and the practical realization of the electronic circuits that make up
telecommunication systems. The aim of this book is to fill a gap that exists in the
teaching of telecommunications and electronic circuit design to electrical engineering
students. Frequently, courses on electronic circuits are taught to students without
a clear indication of where these circuits may be used. Later in their career, students
may take a course in communication theory where the usual approach is to treat
subjects such as modulation, frequency changing and detection as mathematical
concepts and to represent them in terms of ‘‘black boxes’’. Thus the connection
between the function ‘‘black boxes’’ and the design of an electronic circuit that will
perform the function is glossed over or is completely missing.
The approach followed in this book is to take a specific communication system,
for example the amplitude modulated (AM) radio system, and describe in mathematical
terms how and why the system is designed the way it is. The system is then
broken down into functional blocks. The design of each functional block is
examined in terms of the electronic devices to be used, the circuit components
and requirements for power. The effectiveness of each functional block is determined.
In most cases, more than one circuit is presented, starting from the very
elementary which usually illustrates the principles of operation best, to more
sophisticated and practical varieties. The order in which the signal encounters the
xiii
functional blocks determines the order of the presentation, so new information is
presented at an opportune moment when the interest of the student is optimal.
Examples are provided to emphasize the link from concept, to design and realization
of the circuits. Systems examined in this text include commercial radio broadcasting,
television and telephone, with sections devoted to personal wireless communication,
satellite communications and data transmission circuits.
This book was written with the final-year engineering undergraduate student in
mind, so a clear and explicit style of writing has been used throughout. Illustrative
examples have been given whenever possible to promote the active participation of
the student in the learning process. However, the practical approach to electronic
circuit design will no doubt be useful to people involved in the telecommunications
industry for updating, review and as a reference.
Prerequisites to the course in telecommunication circuit design are university
mathematics, basic electronics and some familiarity with communication theory
(although this is not strictly necessary). In every case where communication theory
has a direct impact on the design, enough background has been given to gain an
understanding of the topic.
A number of specialized topics have been excluded in the interest of brevity.
These include antennas, filters and loudspeakers. Many books are available on these
subjects. Rather than presenting a cursory treatment of these very important subjects,
I opted for a qualitative description of the operation and design of antennas, filters
and loudspeakers in the hope that the reader can develop an appreciation of the
outstanding features of these devices which can be built upon if they are of special
interest. A list of available reading material has been given in the appropriate
chapters.
Although most of the circuits discussed in this book can be found in integrated
circuit form, I have, in general, avoided detailed discussion of integrated circuit
design. This is because the ‘‘rules’’ of integration aim to reduce the area of the chip
to a minimum and thus tend to increase the number of transistors or active
components, which take up little area, at the expense of passive components such
as resistors and capacitors, which take up relatively large areas. Furthermore,
because the integrated circuit process can produce very closely matched transistors,
the integrated circuit designer often uses symmetry to achieve circuit functions not
possible with discrete devices. An explanation of how an integrated circuit works is
therefore more complicated than its discrete counterpart. In every case where the
simple and the modern have clashed, I have chosen the simple. However, integrated
circuit design techniques have been discussed whenever they are relevant and do not
distract the reader from a good understanding of the basic principles of circuit
design.
In Chapter 1, a brief history of telecommunication is given. The last 150 years has
been a time of tremendous growth and change in telecommunications, more than
enough change to qualify as a ‘‘revolution’’, perhaps the greatest revolution in the
history of mankind — the Information Revolution.
Chapters 2 and 3 describe the amplitude modulated (AM) radio system and the
electronic circuits that make it possible, from the design of the crystal-controlled
xiv PREFACE
oscillator in the transmitter to the loudspeaker in the receiver. Chapters 4 and 5
repeat the process with the frequency modulated (FM) radio and include sections on
stereophonic commercial broadcast and reception.
Television—the transmission and reception of images—is discussed in Chapters
6 and 7. The design of the circuits involved in the acquisition of the video signals,
the processing, transmission, coding, broadcasting, reception and decoding are
described.
In Chapters 8 and 9, the growth of the telephone system is traced from its humble
beginnings to the world-wide network that it is today. The need to open up the
system to an increasing number of subscribers has led to the development of
sophisticated signal processing techniques and circuits with which to implement
them.
Chapter 10 describes the facsimile machine as a system, as well as the design of
its component parts.
In Chapter 11 the pager, the cordless telephone and cellular telephone systems are
described.
Chapter 12 covers the development of channels in new transmission media, such
as satellites and fiber optics, as well as improvements to hard wire connections made
possible because circuit designers have produced the hardware at the right time and
at the right cost. The growing traffic of ‘‘conversations’’ between machines of
various descriptions has accelerated the trend towards ‘‘digitization’’ of signals in the
telephone network. The design of circuits capable of accepting data corrupted by
noise, restoring and retransmitting them is discussed.
This book started off as lecture notes for a senior college course in electrical
engineering called ‘‘Telecommunication Circuits’’. At the time I proposed the
course, it was becoming increasingly clear that the knowledge of our graduating
students of communication systems left much to be desired. Students seemed to
think that anything analog (including radio, television and the telephone) was passe´.
Digital circuits (computers and software development), on the other hand, were
considered ‘‘cutting edge’’. It was necessary to bring some balance into this situation
and I hope that this book helps to restore some semblance of symmetry. The
seemingly simple task of changing a set of lecture notes into a textbook turned out
not to be quite as simple as I had imagined. However, I have learned a lot from it and
I hope the reader does, too.
The material contained in this book is more than can be presented in the normal
13-week term. However, the organization of chapters is based on the three major
telecommunication networks: radio, television, and the telephone. It is therefore
convenient to organize such a course around a group of chapters with minimal
rearrangement of the material and still maintain coherence.
PATRICK D. VAN DER PUIJE
Ottawa, Ontario, Canada
September 2001
PREFACE xv
WILEY SERIES IN TELECOMMUNICATIONS
AND SIGNAL PROCESSING
John G. Proakis, Editor
Northeastern University
Introduction to Digital Mobil Communications
Yoshihiko Akaiwa
Ditigal Telephony, 3rd Edition
John Bellamy
ADSL, VDSL, and Multicarrier Modulation
John A. C. Bingham
Biomedical Signal Processing and Signal Modeling
Eugene N. Bruce
Elements of Information Theory
Thomas M. Cover and Joy A. Thomas
Practical Data Communications
Roger L. Freeman
Radio System Design for Telecommunications, 2nd Edition
Roger L. Freeman
Telecommunication System Engineering, 3rd Edition
Roger L. Freeman
Telecommunications Transmission Handbook, 4th Edition
Roger L. Freeman
Introduction to Communications Engineering, 2nd Edition
Robert M. Gagliardi
Optical Communications, 2nd Edition
Robert M. Gagliardi and Sherman Karp
Active Noise Control Systems: Algorithms and DSP Implementations
Sen M. Kuo and Dennis R. Morgan
Mobile Communications Design Fundamentals, 2nd Edition
William C. Y. Lee
Expert System Applications for Telecommunications
Jay Liebowitz
Polynomial Signal Processing
V. John Mathews and Giovanni L. Sicuranza
Digital Signal Estimation
Robert J. Mammone, Editor
Digital Communication Receivers: Synchronization, Channel Estimation, and Signal
Processing
Heinrich Meyr, Marc Moeneclaey, and Stefan A. Fechtel
Synchronization in Digital Communications, Volume I
Heinrich Meyr and Gerd Ascheid
Business Earth Stations for Telecommunications
Walter L. Morgan and Denis Rouffet
Wireless Information Networks
Kaveh Pahlavan and Allen H. Levesque
Satellite Communications: The First Quarter Century of Service
David W. E. Rees
Fundamentals of Telecommunication Networks
Tarek N. Saadawi, Mostafa Ammar, with Ahmed El Hakeem
Meteor Burst Communications: Theory and Practice
Donald L. Schilling, Editor
Digital Communications over Fading Channels: A Unified Approach to Performance Analysis
Marvin K. Simon and Mohamed-Slim Alouini
Digital Signal Processing: A Computer Science Perspective
Jonathan (Y) Stein
Vector Space Projections: A Numerical Approach to Signal and Image Processing, Neural
Nets, and Optics
Henry Stark and Yongyi Yang
Signaling in Telecommunication Networks
John G. Van Bosse
Telecommunication Circuit Design, 2nd Edition
Patrick D. van der Puije
Worldwide Telecommunications Guide for the Business Manager
Walter H. Vignault
1
THE HISTORY OF
TELECOMMUNICATIONS
1.1 INTRODUCTION
According to UNESCO statistics, in 1997, there were 2.4 billion radio receivers in
nearly 200 countries. The figure for television was 1.4 billion receivers. During the
same year, it was reported that there were 822 million main telephone lines in use
world-wide. The number of host computers on the Internet was estimated to be 16.3
million [1]. In addition to this, the military in every country has its own communication
network which is usually much more technically sophisticated than the
civilian network. These numbers look very impressive when one recalls that
electrical telecommunication is barely 150 years old. One can well imagine the
number of people employed in the design, manufacture, maintenance and operation
of this vast telecommunication system.
1.2 TELECOMMUNICATION BEFORE THE ELECTRIC TELEGRAPH
The need to send information from one geographic location to another with the
minimum of delay has been a quest as old as human history. Galloping horses,
carrier pigeons and other animals have been recruited to speed up the rate of
information delivery. The world’s navies used semaphore for ship-to-ship as well as
from ship-to-shore communication. This could be done only in clear daylight and
over a distance of only a few kilometres. The preferred method for sending messages
over land was the use of beacons: lighting a fire on a hill, for example. The content
of the message was severely restricted since the sender and receiver had to have
previously agreed on the meaning of the signal. For example, the lighting of a
beacon on a particular hill may inform one’s allies that the enemy was approaching
from the north, say. In 1792, the French Legislative Assembly approved funding for
the demonstration of a 35 km visual telegraphic system. This was essentially
1
Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije
Copyright # 2002 John Wiley & Sons, Inc.
ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic)
semaphore on land. By 1794, Lille was connected to Paris by a visual telegraph [3].
In England, in 1795, messages were being transmitted over a visual telegraph
between London and Plymouth – a return distance of 800 km in 3 minutes [4].
North American Indians are reputed to have communicated by creating puffs of
smoke using a blanket held over a smoking fire. Such a system would require clear
daylight as well as the absence of wind, not to mention a number of highly skilled
operators.
A method of telecommunication used in the rain forests of Africa was the
‘‘talking drum’’. By beating on the drum, a skilled operator could send messages
from one village to the next. This system of communication had the advantage of
being operational in daylight and at night. However, it would be subject to operator
error, especially when the message had to be relayed from village to village.
1.3 THE ELECTRIC TELEGRAPH
The first practical use of electricity for communication was in 1833 by two
professors from the University of Goettingen, Carl Friedrich Gauss (1777–1855)
and Wilhelm Weber (1804–1891). Their system connected the Physics Institute to
the Astronomical Observatory, a distance of 1 km, and used an induction coil and a
mirror galvanometer [4].
In 1837, Charles Wheatstone (1802–1875) (of Wheatstone Bridge fame) and
William Cooke (1806–1879) patented a communication system which used five
electrical circuits consisting of coils and magnetic needles which deflected to
indicate a letter of the alphabet painted on a board [5]. The first practical use of
this system was along the railway track between Euston and Chalk Farm stations in
London, a distance of 2.5 km. Several improvements were later made, the major one
being the use of a coding scheme which reduced the system to a single coil and a
single needle. The improvement of the performance, reliability and cost of communication
has since kept many generations of engineers busy.
At about the time when Wheatstone and Cooke were working on their system,
Samuel Morse (1791–1872) was busy doing experiments on similar ideas. His major
contribution to the hardware was the relay, also called a repeater. By connecting a
series of relays as shown in Figure 1.1, it was possible to increase the distance over
Figure 1.1. The use of Morse’s relay to extend the range of the telegraph.
2 THE HISTORY OF TELECOMMUNICATIONS
which the system could operate [5]. Morse also replaced the visual display of
Wheatstone and Cooke with an audible signal which reduced the fatigue of the
operators. However, he is better known for his efficient coding scheme which is
based on the frequency of occurrence of the letters in the English language so that
the most frequently used letter has the shortest code (E: dot) and the least frequently
used character has the longest code (‘–apostrophe: dot-dash-dash-dash-dash-dot).
This code was in general use until the 1950s and it is still used by amateur radio
operators today.
In 1843, Morse persuaded the United States Congress to spend $30,000 to build a
telegraph line between Washington and Baltimore. The success of this enterprise
made it attractive to private investors, and Morse and his partner Alfred Vail (1807–
1859), were able to extend the line to Philadelphia and New York [6]. A number of
companies were formed to provide telegraphic services in the east and mid-west of
the United States. By 1851, most of these had joined together to form the Western
Union Telegraph Company.
By 1847, several improvements had been made to the Wheatstone invention by
the partnership of Werner Siemens (1816–1892) and Johann Halske (1814–1890) in
Berlin. This was the foundation of the Siemens telecommunication company in
Germany.
The next major advance came in 1855 when David Hughes (1831–1900) invented
the printing telegraph, the ancestor of the modern teletype. This must have put a lot
of telegraph operators out of work (a pattern which was to be repeated over and over
again) since the machine could print messages much faster than a person could
write. Another improvement which occurred at about this time was the simultaneous
transmission of messages in two directions on the same circuit. Various schemes
were used but the basic principle of all of them was the balanced bridge.
In 1851, the first marine telegraphic line between France and England was laid,
followed in 1866 by the first transatlantic cable. The laying of this cable was a major
feat of engineering and a monument to perseverance. A total of 3200 km of cable
was made and stored on an old wooden British warship, the HMS Agamemnon. The
laying of the cable started in Valentia Bay in western Ireland but in 2000 fathoms of
water, the cable broke and the project had to be abandoned for that year. A second
attempt the following year was also a failure. A third attempt in 1858 involved two
ships and started in mid-ocean and it was a success. Telegraphic messages could then
be sent across the Atlantic. The celebration of success lasted less than a month when
the cable insulation broke down under excessively high voltage. Interest in
transatlantic cables was temporarily suspended while the American Civil War was
fought and it was not until 1865 that the next attempt was made. This time a new
ship, the Great Eastern, started from Ireland but after 1900 km the cable broke.
Several attempts were made to lift the cable from the ocean bed but the cable kept
breaking off so the project was abandoned until the following year. At last in 1866,
the Great Eastern succeeded in laying a sound cable and messages could once more
traverse the Atlantic. By 1880, there were nine cables crossing the ocean [6].
The telegraph was and remained a communication system for business, and in
most European countries it became a government monopoly. Even in its modernized
1.3 THE ELECTRIC TELEGRAPH 3
form (telex) it is essentially a cheap long-distance communication network for
business.
1.4 THE FACSIMILE MACHINE
In 1843, the British Patent Office issued a patent with the title ‘‘Automatic
electrochemical recording telegraph’’ to the Scottish inventor Alexander Bain
(1810–1877). The essence of the invention is shown in Figure 1.2. Two identical
pendulums are connected as shown by the telegraph line. For simplicity, we assume
the ‘‘message’’ to be sent is the letter H and it is engraved on a metallic plate and
shaped to the appropriate radius so that the ‘‘read’’ stylus makes contact with the
raised parts of the plate as the pendulum sweeps across it. On the far end of the
telegraph line, the stylus of the second pendulum maintains contact with the
electrosensitive paper which rests on an electrode shaped to the same radius as
before. The electrosensitive paper has been treated with a chemical which produces a
dark spot when electric current flows through it.
To operate the system, both pendulums are released from their extreme left
positions simultaneously. Since they are identical, it follows that they will travel at
the same speed, one across the ‘‘message’’ plate and the other across the electrosensitive
paper. At first no current flows, but as the transmitter pendulum makes
contact with the raised portion of the plate, the circuit is complete and the resulting
current causes the electrosensitive paper to produce a dark line of the same length as
the raised metal segment. The original patent included the functions: (a) an
electromagnetic device to keep the pendulums swinging at a constant amplitude
Figure 1.2. The configuration of the Bain ‘‘Automatic electrochemical recording telegraph.’’ To
keep the diagram simple, additional circuits required for synchronization, phasing and scanning
are not shown.
4 THE HISTORY OF TELECOMMUNICATIONS
(synchronization), (b) a second electromagnetic arrangement to ensure that the two
pendulums start their swings at the same instant (phasing), and (c) a mechanism to
move the message plate and the electrosensitive paper simultaneously one step at a
time after each sweep at right angles to the direction of the pendulum swing
(scanning). When several sweeps have occurred, the lines produced will form an
exact image of the raised metal parts of the ‘‘message’’ plate. Figure 1.3(a) shows the
letter H scanned in 20 lines and Figure 1.3(b) shows the corresponding current
waveforms. Figure 1.3(c) shows the reproduced image.
All the facsimile machines since the Bain patent have the three functions listed
above. In modern facsimile machines, the first two functions have been replaced by
electronic techniques which ensures that the transmitter and the receiver are
‘‘locked’’ to each other at all times. The mechanism for scanning the message is
also largely electronic, although in most machines it is still necessary to move the
page mechanically as it is scanned.
In 1848, Frederick Bakewell, an Englishman, produced a new version of the fax
machine in which the ‘‘message plate’’ as well as the image were mounted on
Figure 1.3. (a) Shows the letter H scanned in 20 lines, (b) shows the current waveforms for
each line scanned and (c) shows the reproduced H. Note the effect of the finite width of the
receiver stylus on the image.
1.4 THE FACSIMILE MACHINE 5
cylinders which were turned by falling weights, similar to a grandfather clock. To
ensure that the cylinders turned at the same speed he used a mechanical speed
governor. The scanning head (stylus) was propelled on an axis parallel to that of the
cylinder by a lead-screw. This was an example of ‘‘spiral scanning’’. Unlike Bain, he
used an insulating ink to write the message on a metallic surface. But, as before, the
paper in the receiver was chemically treated to respond to the flow of electric current
and it was mounted on an identical cylinder with the ‘‘write’’ head driven by an
identical lead-screw. The main difficulty with this design was the necessity to keep
the two clock motors in remote locations starting and running at the same speed
during the transmission.
In 1865, Giovanni Caselli (1815–1891), an Italian living in France, patented an
improved version of Bain’s machine which he called the ‘‘Pantelegraph’’. He then
established connections between Paris and a number of other French cities. His
machine was a combination of the insulating ink message plate of Bakewell, the
pendulum of Bain’s transmitter, and the Bakewell cylindrical receiver. The pantelegraph
was a commercial success and it was used in Italy and Britain for many years.
By the end of the 1800s it was possible to send photographs by fax. The picture
had to be etched on a metallic plate in the form of raised dots (similar to the
technique used for printing pictures in newspapers). The size of the dots represented
the different shades of gray; small for light and large for dark gray. The transmitter
stylus traced lines across the picture making contact with the raised dots and thus
producing corresponding large and small dots at the receiver.
In 1902, Arthur Korn demonstrated a scanning system which used light instead of
physical contact with a metallic plate and the resultant flow of current. His method
was far superior to all the previous techniques, especially in the transmission of
photographs. He wrapped the photographic film negative of the picture on the
outside of a glass cylinder which was turned at a constant rate by an electric motor.
An electric lamp provided the light and a system of lenses were used to focus the
light onto the negative. The light that passed through the film was reflected by a
mirror onto a piece of selenium whose resistance varied according to how much light
reached it. The selenium cell was used to control the current flowing in the receiver.
The receiver recorded the image directly onto film. To ensure that the transmitter and
receiver cylinders were in synchronism at all times, he used a central control system
with a tuning fork generating the control signal.
In the 1920s the large American telecommunication companies, American
Telephone and Telegraph (AT&T), Radio Corporation of America (RCA) and
Western Union, became interested in fax machine development and they used new
techniques, materials and devices such as the vacuum tube, phototubes and later
semiconductors to produce the modern fax machine.
1.5 THE TELEPHONE
In 1876, Alexander Graham Bell (1847–1922) was conducting experiments on a
‘‘harmonic telegraph’’ system when he discovered that he could vary the electric
6 THE HISTORY OF TELECOMMUNICATIONS
current flowing in a circuit by vibrating a magnetic reed held in close proximity to an
electromagnet which formed part of the loop. By connecting a second electromagnet
together with its own magnetic reed in the circuit, he could reproduce the vibration
of the first reed. Using a human voice to excite the magnetic reed led to the first
telephone for which he was granted a patent later that year. He went on to
demonstrate his invention at the International Centennial Exhibition in Philadelphia
and before the year ended, he transmitted messages between Boston, Massachusetts,
and North Conway, New Hampshire, a distance of 230 kilometres. Few people
realized the potential of the new invention and in 1878, when Bell tried to sell his
patent to the Western Union Telegraph Company, he was turned down [7].
The early telephone system consisted of two of Bell’s magnetic reed-electromagnet
instruments in series with a battery and a bell. Bell’s instrument worked very
well as a receiver, in fact so well that it has survived almost unchanged to this day.
As a transmitter, however, it left a lot to be desired. It was soon replaced by the
carbon microphone (one of the many inventions of Thomas Edison (1847–1931))
which was, until recently, the most widely used microphone in the telephone
system.
In the early telephone system, each subscriber was connected to a central office
by a single wire with an earth return. This led to cross-talk between subscribers. At
about this time, electric traction had become very popular which resulted in
increased interference from the noise generated by the electric motors. The earthreturn
system was gradually replaced by two-wire circuits which are much less
susceptible to cross-talk and electrical noise. The rapid growth of the telephone
system was based almost entirely on the fact that the subscriber could use the system
with the minimal amount of training. The ease of operation of the telephone
outweighed the disadvantages of having no written record of conversations and
the requirement that both parties have to be available for the call at the same time.
The basic central office responds to a signal from the subscriber (calling party)
indicating that he wants service. A buzzer excited by current from a hand-cranked
magneto was the standard. The telephone operator answers and finds out whom
(called party) the calling party wants to talk to. The operator then signals to the
called party by connecting his own hand-cranked magneto to the line and cranking it
to ring a bell on the called party’s premises. When and if the called party responds,
he connects the lines of the two parties together and withdraws until the conversation
is over, at which point he disconnects the lines. In order to carry out his function, the
operator had to have access to all the lines connected to the exchange. This was not a
problem in an exchange with less than fifty lines but as the system grew, more
operators were required for each group of fifty subscribers. If the calling party and
called party belong to the same group of fifty, the above sequence was followed. If
they belong to two different operators, it was necessary for the two operators to have
a verbal consultation before the connection could be made. The errors, delays and
misunderstandings in large central offices led to a re-organization whereby each
operator responded to only fifty incoming lines but had access to all the outgoing
lines. Another improvement in the system was to replace all the batteries on the
subscribers’ premises with one battery in the central office.
1.5 THE TELEPHONE 7
The motivation for the changeover from manual switching to the automatic
telephone exchange was not, as one would expect, the inability of the central office
to cope with the increasing volume of traffic. It was because the operators could
listen to the conversations. The inventor of the automatic exchange, Almon B.
Strowger (1839–1902), after whom the system was named, was an undertaker in
Kansas City around the 1890s. There was another undertaker in the city whose wife
worked in the local telephone exchange; whenever someone died in the city, the
telephone operators were the first to know and the wife would pass a message to the
husband, giving him a head-start on his competitor [8]. The automatic exchange
certainly improved the security of telephone conversations; it was also one more
example of machines replacing people.
The success of the telephone system led to a large number of small telephone
companies being formed to service the local urban communities. Pressure to interconnect
the various urban centres soon grew and techniques for transmission over
longer distances had to be developed. These included amplification and inductive
loading. Since these transmission lines (trunks) were expensive to construct and
maintain, techniques for transmission of more than one message (multiplex) over the
trunk at any one time became a matter of great concern and an area of rapid
advancement.
1.6 RADIO
In 1864, James Maxwell (1831–1879), a Scottish physicist, produced his theory of
the electromagnetic field which predicted that electromagnetic waves can propagate
in free space at a velocity equal to that of light [9]. Experimental confirmation of this
theory had to wait until 1887 when Heinrich Hertz (1857–1894) constructed the first
high-frequency oscillator. When a voltage was induced in an induction coil
connected across a spark gap, a discharge would occur across the gap setting up a
damped sinusoidal high-frequency oscillation. The frequency of the oscillation could
be changed by varying the capacitance of the gap by connecting metal plates to it.
The detector that he used consisted of a second coil connected to a much shorter
spark gap. The observation of sparks across the detector gap when the induction coil
was excited showed that the electromagnetic energy from the first coil was reaching
the second coil through space. These experiments were in many ways similar to
those carried out in 1839 by Joseph Henry (1797–1878). Several scientists made
valuable contributions to the subject, such as Edouard Branly (1844–1940) who
invented the ‘‘coherer’’ for wave detection, Aleksandr Popov (1859–1906) and
Oliver Lodge (1851–1940) who discovered the phenomenon of resonance.
In 1896, Guglielmo Marconi (1874–1937) left Italy for England where he worked
in cooperation with the British Post Office on ‘‘wireless telegraph’’. A year later, he
registered his ‘‘Wireless Telegraphy and Signal Co. Ltd’’ in London, England to
exploit the new technology of radio. On the 12th of December 1901, Marconi
received the letter ‘‘S’’ in Morse code at St, Johns, Newfoundland on his receiver
whose antenna was held up by a kite, the antenna which he had constructed for the
8 THE HISTORY OF TELECOMMUNICATIONS
purpose having been destroyed by heavy winds. He had confounded the many
skeptics who thought that the curvature of the earth would make radio transmission
impossible [10].
Up to this point, no use had been made of ‘‘electronics’’ in telecommunication:
high-frequency signals for radio were generated mechanically. The first electronic
device, the diode, was invented by Sir John Ambrose Fleming (1849–1945) in 1904.
He was investigating the ‘‘Edison effect’’ that is, the accumulation of dark deposits
on the inside wall of the glass envelope of the electric light bulb. This phenomenon
was evidently undesirable because it reduced the brightness of the lamp. He was
convinced that the dark patches were formed by charge particles of carbon given off
by the hot carbon filament. He inserted a probe into the bulb because he had the idea
that he could prevent the charged particles from accumulating by applying a voltage
to the probe. He soon realized that, when the probe was held at a positive potential
with respect to the filament, there was a current in the probe but when it had a
negative potential no current would flow: he had invented the diode. He was granted
the first patent in electronics for his effort. Fleming went on to use his diode in the
detection of radio signals – a practice which has survived to this day.
The next major contribution to the development of radio was made by Lee
DeForest (1873–1961). He got into legal trouble with Marconi, the owner of the
Fleming diode patent, when he obtained a patent of his own on a device very similar
to Fleming’s. He went on to introduce a piece of platinum formed into a zig-zag
around the filament and soon realised that, by applying a voltage to what he called
the ‘‘grid’’, he could control the current flowing through the diode. This was, of
course, the triode – a vital element in the development of amplifiers and oscillators.
1.7 TELEVISION
Shortly after the establishment of the telegraph, the transmission of images by
electrical means was attempted by Giovanni Caselli (1815–1891) in France. His
technique was to break up the picture into little pieces and send a coded signal for
each piece over a telegraph line. The picture was then reconstituted at the receiving
end. The system was slow, even for static images, but it established the basic
principles for image transmission; that is, the break up of the picture into some
elemental form (scanning), the quantization of each element in terms of how bright it
is (coding), and the need for some kind of synchronization between the transmitter
and the receiver. Subsequent practical image transmission schemes, whether
mechanical or electronic, had these basic units.
The discovery in 1873 by Joseph May, a telegraph operator at the Irish end of the
transatlantic cable, that when a selenium resistor was exposed to sunlight its
resistance decreased, led to the development of a light-to-current transducer.
Subsequently, various schemes for image transmission based on this discovery
were devised by George Carey, William Ayrton (1847–1908), John Perry and others.
None of these was successful because they lacked an adequate scanning system and
1.7 TELEVISION 9
each element of the picture had to be sent on a separate circuit, making them quite
impractical.
In 1884, Paul Nipkow (1860–1940) was granted a patent in Germany for what
became known as the Nipkow Disc. This consisted of a series of holes drilled in the
form of spirals in a disc. When an image is viewed through a second disc with
similar holes driven in synchronism with the first, the observed effect was scanning
point-to-point to form a complete line and line-by-line to cover the complete picture.
This was a practical scheme since the point-to-point brightness of the picture could
be transmitted and received serially on a single circuit. The persistence of an image
on the human eye could be relied on to create the impression of a complete scene
when, in fact, the information is presented point-by-point. Nipkow’s scheme could
not be exploited until 1927 when photosensitive cells, photomultipliers, electron
tube amplifiers and the cathode ray tube had been invented and had attained
sufficient maturity to process the signals at an acceptable speed for television.
Several people made significant contributions to the development of the components
as well as to the system. However, two people, Charles Jenkins (1867–1934) and
John Baird (1888–1946), are credited with the successful transmission of images at
about the same time. They both used the Nipkow disc. Mechanical scanning
methods of various forms were used with reasonable success until about 1930
when Vladimir Zworykin (1889–1982) invented the ‘‘iconoscope’’ and Philo Farnsworth
(1906–1971) the electronic camera tube, which he called the ‘‘image
dissector’’. These inventions finally removed all the moving parts from television
scanning systems and replaced them with electronic scanning [11]. The application
of very-high-frequency carriers and the use of coaxial cables have contributed
significantly to the quality of the pictures. The use of color in television had been
shown to be feasible in 1930 but would not be available to the general public until
the mid-1960s. By the 1980s, satellite communication systems brought a large
number of television programs to viewers who could afford the cost of the dish
antenna. By the beginning of the 21st century, the dish antennas had shrunk in size
from over 3m to less than 70 cm and the signal had changed into digital form.
1.8THE GROWTH OF BANDWIDTH AND THE DIGITAL REVOLUTION
Electrical telecommunication started with a single wire with a ground return, but, as
the system grew, the common ground return had to be replaced with a return wire,
hence the advent of the open-wire telephone line. The open-wire system with its
forests of telegraph poles along city streets strung with an endless array of wires
eventually gave way to the twisted pair cable. The twisted pair cable owes its
existence to improved insulating materials, mainly plastics, which reduced the space
requirements of the cable. The bandwidth of an unloaded twisted pair is approximately
4 kHz and it decreases rapidly with length. This can be improved by
connecting inductors (loading coils) in series with the line at specific distances
and by various equalization schemes to about 1MHz. However, the twisted pair has
found a niche in the modern telephone system where its bandwidth approximately
10 THE HISTORY OF TELECOMMUNICATIONS
matches that required for analog audio communication. This is still the dominant
mode of telephone communication up to the central office. Beyond the central office
the network of inter-office trunks use a variety of conduits for the transmission of the
signal.
Increased bandwidth alone was not an answer to the expanding telecommunication
traffic. High-frequency carriers had to be developed in order to exploit fully the
bandwidth capability of new telecommunication media such as coaxial cables,
terrestrial microwave networks and fiber optics. The development of the coaxial
cable, which confines the electromagnetic wave to the annular space between the two
concentric conductors, reduced significantly the radiation losses that would otherwise
occur. As a result the bandwidth was increased to approximately 1 GHz and
attenuation was reduced. Terrestrial as well as satellite microwave communication
systems have further expanded the bandwidth into the terrahertz range and, for those
who can afford the dish antenna and its associated equipment, it has increased the
number of television channels available to over 800. The application of fiber optics
to telecommunication has extended the channel bandwidth to that of visible light
(1
1012 Hz). It is now possible for one optical fiber to carry as many as 300
109
telephone channels at the same time.
An increasingly dominant factor in telecommunication is the enormous popularity
of digital techniques. The information is reduced to a train of pulses (binary
digits; 1s and 0s) and sent over the channel. The limited bandwidth, phase change
and the noise in the channel cause the signal to deteriorate so it is necessary to
‘‘refresh’’ or regenerate the signal at various points along the channel. This is
accomplished by using repeaters whose function is to determine whether the digit
sent was a 1 or a 0 and to generate the appropriate new digits and transmit them. At
the receiving end, the digits are converted back into an analog signal. The compact
disc music recording system is a common example of this technique. Although the
need for information transfer between computers spurred on the development of
digital communication, speech signals increasingly are being converted into digital
form for telephone transmission.
1.9 THE INTERNET
The use of personal computers as a means of communication gained enormous
popularity in the last decade of the 20th century. However, computer science experts
have used the ARPANET (Advanced Research Projects Agency of the U.S.
Department of Defense) for communication between computers since 1969. The
basic idea was to enable scientists in different geographic locations to share their
research results [12] and also, as a money saving scheme, their computing resources.
The first four sites to be connected were the Stanford Research Institute, the
University of California at Los Angeles, the University of California at Santa
Barbara, and the University of Utah in Salt Lake City. The messages traveling
between these centers were over 50 kbps telephone lines. In 1962 when the
ARPANET was being designed, the Cold War was in full swing and so one of the
1.9 THE INTERNET 11
specifications for the design was that the network should survive a nuclear attack in
which parts of it were knocked out [13]. Needless to say, this feature of the design
was never tested! The ideal structure was a network with every node connected to
every other node (high redundancy) so that, if a part of the network went down for
whatever reason, the traffic could be routed around the trouble spot. Moreover, in
such a design all nodes are of equal importance, hence there is no one node the
destruction of which would cripple the network. The configuration of the network is
shown in Figure 1.4. This is similar to the electric power grid which was designed to
provide electric power to consumers with a maximum reliability service.
Another design feature of the ARPANETwhich further improved its robust nature
was the use of ‘‘packet’’switching. In packet switching, the incoming message is first
divided into smaller packets of binary code. Each packet is labeled with a number
and the address of its destination and then transmitted to the next node when the
local router can accommodate the packet. Each packet, in theory, can travel from the
source to the destination by a different route and arrive at different times. At its
destination the packets are re-assembled in the proper order ready for the recipient.
The strength of packet switching is the fact that, if a number of nodes are put out of
operation, the packets will still find their way to their proper destination by way of
the remaining operational nodes, in perhaps a longer time. Moreover, errordetection
codes can be included with each packet and, when errors are present in
a packet, that packet can be re-sent.
The ARPANET grew so that by 1983 there were 562 sites connected. By 1992,
the number of ‘‘host’’ or ‘‘gateway’’ computers connected to it had reached one
million. Four years later, the number was 12 million. It has been estimated that by
the year 2000 the number with access to the Internet worldwide will be 100 million
[14]. The term ‘‘Internet’’ came into use in 1984, and this was also the time when the
Figure 1.4. Each node of the Internet is connected to all the neighboring nodes. The increased
redundancy implies a high level of reliability. The design of the electric power grid follows the
same principle for the same reason.
12 THE HISTORY OF TELECOMMUNICATIONS
United States Department of Defense handed over the oversight of the network to the
National Science Foundation. The Internet is currently run in a very loose fashion by
a number of volunteer organizations whose membership is open to the public. Their
main activity is centered around the registration of names, numbers and addresses of
the users of the system.
The Internet is a collection of a large number of computers connected together by
telephone lines, coaxial cables, optical fiber cables and communication satellites
with set protocols to enable communication between them and also to control the
flow of traffic.
1.10 THE WORLD WIDE WEB
What factors have contributed to the unprecedented growth of the Internet?
Personal computers have been in common use in scientific laboratories and in
universities since the mid-1980s but they were mostly used for calculation,
information storage and retrieval. Many businesses acquired desk-top computers
for preparing invoices, word-processing and general book-keeping. Some enthusiasts
owned their own personal computers and some belonged to clubs for the
exchange of computer software which they had developed. The growth of computing
power of the personal computer was one of the pivotal developments that made the
Internet possible. In 1971, the Intel Corporation produced its first microprocessor,
the Intel 4004. It was used in a calculator and its clock frequency (an indication of
how fast it operates) was 108 kHz. The following year the Corporation produced the
Intel 8008 which was twice as fast (200 kHz) as the 4004 and it was used in 1974 in
a predecessor of the first personal computer. Also in 1974, Intel produced the 8080
which was clocked at 2MHz. The 8080 was marketed to computer enthusiasts as
part of a kit and it very quickly became the ‘‘brains’’ of the modern personal
computer. By the year 2000, the Intel Pentium III processor had achieved a clocking
speed of 1.13 GHz (over four orders of magnitude faster than the original 4004).
This phenomenal increase in speed was coupled to an equally incredible decrease in
price which made the personal computer affordable to the general public. A
hypothetical comparison with the automobile industry in 1983 was as follows:
‘‘If the automobile business had developed like the computer business, a Rolls-Royce
would now cost $2.75 and run 3 million miles on a gallon of gas’’ [15].
Even this comparison was considered conservative fifteen years later. In the Spirit
of the Web, Wade Rowland amends the statement as follows:
‘‘That Rolls-Royce would now cost twenty-seven cents and run 300 million miles on a
gallon of gas’’.
The telephone system was already in place, although its capacity would have to
be expanded to carry the digital data in addition to the voice signals for which it was
designed. Most of the main communication lines carrying Internet data have been
1.10 THE WORLD WIDE WEB 13
updated, for example the copper cables (coaxial) laid across continents and on the
sea-bed are capable of 2.5 Mbps and fiber optic cables can go as high as 40 Gbps.
The building of the throughways for large volumes of data was the easy part of the
problem. The more difficult part was to get the data to its destination in the
workplaces and into the living quarters of the owners of personal computers.
Unfortunately, the cost of wiring houses with optical fiber cable or even coaxial
cable cannot be justified on economic grounds. Currently, the speed limit to data
flow is determined by the analog telephone line (sometimes referred to as a ‘‘twisted
pair’’) between the central office (or its equivalent) and the wall socket to which the
most personal computers are connected. This is popularly known as the-last-mile
problem. New circuits have been developed to speed up data transfer on the existing
twisted pair cable. These include the T1, the Integrated Services Digital Network
(ISDN), the High-bit-rate Digital Subscriber Line (HSDL) and the Asymmetrical
Digital Subscriber Line (ASDL).
In the early 1990s, cable television service had reached into a large number of
homes in North America and Europe and there was talk of them providing highbandwidth
(mainly coaxial cable) conduits to subscribers for access to the Internet.
Unfortunately, the television cable network had millions of amplifiers and one-way
traps installed which restricted signal flow in only one direction. Connection to the
Internet required a bilateral flow of information and the cost of the conversion was
considered prohibitive. At the time this book was going to press, the cable television
companies were in the process of converting their networks for bilateral flow of
information (cable modems). The speed of transmission was predicted to be from
10 Mbps to 400 Mbps [16].
Another serious impediment to the free flow of data from one computer to another
was the almost incomprehensible commands required to effect computer communication.
The ‘‘spoken language’’ of the computers was UNIX and this was quite
unfriendly to the uninitiated. The ‘‘point-and-click’’ feature of the computer
‘‘mouse’’ and the development of browsers, such as Netscape Navigator and the
Internet Explorer, finally lowered the threshold to a level where even computer
neophytes could successfully access information from the Internet. These ‘‘facilitators’’
were all available by 1989 and they would have a very profound effect on the
popularity of the Internet. The Internet can be seen as a network connecting various
sites where information is stored. The stored information and the technology for
transferring the information back and forth is the World Wide Web.
The World Wide Web in its infancy carried only text. Later the transmission of
graphics, in color, was added. With the increasing speed and sophistication of the
personal computer, sound and video have been added subsequently. What made the
Web particularly useful is the ability of the Internet browsers and search engines to
provide a list of Internet sites where the requested information may be located. It is
necessary to prompt the system with a set of keywords for the search to begin. At the
Web site there are ‘‘links’’ to other sites so the search can ‘‘fan out’’ in very many
directions.
An important factor that stimulated the growth of the Internet was the decision of
the United States government to turn over the running of the Internet to commercial
14 THE HISTORY OF TELECOMMUNICATIONS
interests. This happened in stages. The National Science Foundation’s NSFnet was
superceded by the Advanced Networks and Services network, ANSnet, a non-profit
organization. However, in 1998 the Federal Communications Commission (FCC)
insisted that subscribers to the telephone system be billed at the same rate for voice
services as for data. The way was now open for commercial organizations to offer
their services on the Internet. The Internet Service Providers (ISP) collect money
from their subscribers for access to their servers. They in turn have to pay for access
to the long-distance, high-speed Internet backbone.
The decentralized nature of the Web and its ability to transfer information
bilaterally meant that its users could add their own contribution to the vast
amount already present in the form of their ‘‘personal home page’’. Commercial
organizations, special-interest groups and even governments would take advantage
of the possibilities offered by the Web. But so would people and groups with hidden
and not-so-hidden agendas to propagate their own distortions. As there is no
authority to monitor the content of the Web sites, only the criminal laws of the
country in which the Web site is registered can be used to control information on the
sites.
A number of services are available to people who have access to the Internet.
Newsgroups can be found on practically any topic. These newsgroups run roundthe-
clock and anyone can join in the discussion from his or her keyboard. The
participants are free to use assumed names and identities. The number of people in
any given newsgroup can vary from zero to several thousand, so it is possible to
reach a very large audience from one’s keyboard. Chat rooms are similar to
newsgroups except that the number of people ‘‘in’’ a chat room is likely to be
much smaller. It is essentially a conversational mode of communication from one’s
keyboard. One of the more popular features of the Internet is electronic mail
(e-mail). It is the nearest thing to mailing a letter to a correspondent, and although it
is not as secure as the service provided by the Post Office, it is much faster. To send
e-mail, one has to register a unique e-mail address and choose a password.
REFERENCES
1. Statistical Yearbook 1999, UNESCO, Paris.
2. Berto, C., Telegraphes et Telephones de Valmy au Microprocesseurs, Le Livre de Poche,
1981.
3. Stumpers, F. L. H. M., ‘‘The History, Development and Future of Telecommunications in
Europe’’, IEEE Comm. Magazine, 22(5), 1984.
4. Fraser, W., Telecommunications, MacDonald & Co, London, 1957.
5. Tebo, Julian D., ‘‘The Early History of Telecommunications’’, IEEE Comm. Soc. Digest,
14(4), pp. 12–21, 1976.
6. Osborne, H. S., Alexander Graham Bell, Biographical Memoirs, Nat. Acad. Sci., Vol. 23,
pp 1–29, 1945.
REFERENCES 15
7. Smith, S. F., Telephony and Telegraphy A, 2nd Ed., Oxford University Press, New York,
1974.
8. Bernal, J.D., Science in History, Vol. 2, Penguin Books Ltd, Middlesex, 1965.
9. Carassa, F., ‘‘On the 80th Anniversary of the First Transatlantic Radio Signal’’, IEEE
Antennas Propagat. Newsl., pp. 11–19, Dec., 1982.
10. Knapp, J. G. and Tebo, J. D., ‘‘The History of Television’’, IEEE Comm. Soc. Digest, 16(3),
pp 8–21, May 1978.
11. Licklider, J. C. R. and Clark, W., ‘‘On-line Man-Computer Communications’’, Massachusetts
Institute of Technology, Aug., 1962.
12. Baran, P., ‘‘On Distributed Communications Networks’’, RAND Corporation, Washington
DC, Sept., 1962.
13. Zakon, R. H., ‘‘Hobbes’ Internet Timeline v5.1’’ at www.isoc.org/zakon/internet, Oct.,
2000.
14. ‘‘The Computer Moves In’’, Time, January 3, 1983, 10.
15. Rowland, W., Spirit of the Web, Somerville House Pub., Toronto, 1997.
16. www.wired.com/news
BIBLIOGRAPHY
Jones, C. R., Facsimile, Murray Hill Books, New York, 1949.
Costigan, D. M., Fax: The Principles and Practice of Facsimile Communication, Chilton
Book Co., Philadelphia, 1971.
McConnell, K., Bodson, D., and Urban, S., Fax: Facsimile Technology and Systems, 3rd Ed.,
Artech House, Boston, 1999.
Dodd, Annabel, Z. The Essential Guide to Telecommunications, 2nd Ed., Prentice-Hall,
Englewood Cliffs, NJ, 2000.
Lehnert, Wendy, G., Internet 101: A Beginner’s Guide to the Internet and the World Wide Web,
Addison-Wesley, Reading, MA, 1998.
16 THE HISTORY OF TELECOMMUNICATIONS
2
AMPLITUDE MODULATED
RADIO TRANSMITTER
2.1 INTRODUCTION
A radio signal can be generated by causing an electromagnetic disturbance and
making suitable arrangements for this disturbance to be propagated in free space.
The equipment normally used for creating the disturbance is the transmitter, and the
transmitter antenna ensures the efficient propagation of the disturbance in free space.
To detect the disturbance, one needs to capture some finite portion of the electromagnetic
energy and convert it into a form which is meaningful to one of the human
senses. The equipment used for this purpose is, of course, a receiver. The energy of
the disturbance is captured using an antenna and an electrical circuit then converts
the disturbance into an audible signal.
Assume for a moment that our transmitter propagated a completely arbitrary
signal (that is, the signal contained all frequencies and all amplitudes). Then no other
transmitter can operate in free space without severe interference because free space is
a common medium for the propagation of all electromagnetic waves. However, if we
restrict each transmitter to one specific frequency (that is, continuous sinusoidal
waveforms) then interference can be avoided by incorporating a narrow-band filter at
the receiver to eliminate all other frequencies except the desired one. Such a
communication channel would work quite well except that its signal cannot
convey information since a sinusoid is completely predictable and information, by
definition, must be unpredictable.
Human beings communicate primarily through speech and hearing. Normal
speech contains frequencies from approximately 100 Hz to approximately 5 kHz
and a range of amplitudes starting from a whisper to very loud shouting. An attempt
to propagate speech in free space comes up against two very severe obstacles. The
first is similar to that of the transmitters discussed earlier, in which they interfere
with each other because they share the same medium of propagation. The second
obstacle is due to the fact that low frequencies, such as speech, cannot be propagated
17
Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije
Copyright # 2002 John Wiley & Sons, Inc.
ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic)
efficiently in free space whereas high frequencies can. Unfortunately, human beings
cannot hear frequencies above 20 kHz which is, in fact, not high enough for free
space transmission. However, if we can arrange to change some property of a
continuous sinusoidal high-frequency source in accordance with speech, then the
prospects for effective communication through free space become a distinct
possibility. Changing some property of a (high-frequency) sinusoid in accordance
with another signal, for example speech, is called modulation. It is possible to
change the amplitude of the high-frequency signal, called the carrier, in accordance
with speech and=or music. The modulation is then called amplitude modulation or
AM for short. It is also possible to change the phase angle of the carrier, in which
case we have phase modulation (PM), or the frequency, in which case we have
frequency modulation (FM).
2.2 AMPLITUDE MODULATION THEORY
In order to simplify the derivation of the equation for an amplitude modulated wave,
we make the simplification that the modulating signal is a sinusoid of angular
frequency os and that the carrier signal to be modulated (also sinusoidal) has an
angular frequency oc.
Let the instantaneous carrier current be
i ¼ A sinoct ð2:2:1Þ
where A is the amplitude.
The amplitude modulated carrier must have the form
i ¼ ½A þ gðtÞ sinoct ð2:2:2Þ
where
gðtÞ ¼ B sinost ð2:2:3Þ
is the modulating signal. Then
i ¼ ðA þ B sinostÞ sinoct ð2:2:4Þ
The waveform is shown in Figure 2.1.
The current may then be expressed as
i ¼ ðA þ kA sinostÞ sinoct ð2:2:5Þ
where
k ¼
B
A
: ð2:2:6Þ
18 AMPLITUDE MODULATED RADIO TRANSMITTER
The factor k is called the depth of modulation and may be expressed as a percentage.
Simplification of Equation (2.2.5) gives
i ¼ A sinoct þ
kA
2 ½cosoc  osÞt  cosðoc þ osÞt ð2:2:7Þ
The frequency spectrum is shown in Figure 2.2.
From Equation (2.2.7) it is evident that modulated carrier current has three
distinct frequencies present: the carrier frequency oc, the frequency equal to the
difference between the carrier frequency and the modulating signal frequency
Figure 2.1. Amplitude modulated wave: the carrier frequency remains sinusoidal at oc while
the envelope varies at frequency os.
Figure 2.2. Frequency spectrum of the AM wave of Figure 2.1. Note that there are three
distinct frequencies present.
2.2 AMPLITUDE MODULATION THEORY 19
(oc  os), and the frequency equal to the sum of the carrier frequency and the
modulating signal frequency (oc þ os). The difference and sum frequencies are
called the ‘‘lower’’ and ‘‘upper’’ sidebands, respectively.
To make the situation more realistic, let us assume that the modulating signal is
speech which contains frequencies between os1 and os2. Then it follows from
Equation (2.2.7) that the sum and difference terms will yield a band of frequencies
symmetrical about the carrier frequency, as shown in Figure 2.3.
Figure 2.4 shows how two audio signals which would normally interfere with
each other, when transmitted simultaneously through the same medium, can be kept
separate by choosing suitable carrier frequencies in a modulating scheme. This
method of transmitting two or more signals through the same medium simultaneously
is referred to as frequency-division multiplex and will be discussed in detail
in Chapter 9.
Figure 2.3. Frequency spectrum of the AM wave when the single frequency modulating signal
is replaced by a band of audio frequencies. Note that the information in the signal resides only in
the sidebands.
Figure 2.4. The diagram illustrates how two audio-frequency sources, which would normally
interfere with each other, can be transmitted over the same channel with no interaction.
20 AMPLITUDE MODULATED RADIO TRANSMITTER
2.3 SYSTEM DESIGN
The choice of carrier frequency for a radio transmitter is largely determined by
government regulations and international agreements. It is evident from Figure 2.4
that, in spite of frequency division multiplexing, two stations can interfere with each
other if their carrier frequencies are so close that their sidebands overlap. In theory,
every transmitter must have a unique frequency of operation and sufficient
bandwidth to ensure no interference with others. However, bandwidth is limited
by considerations such as cost and the sophistication of the transmission technique to
be used so that, in practice, two radio transmitters may operate on frequencies which
would normally cause interference so long as they propagate their signals within
specified limits of power and are located (geographically) sufficiently far apart. The
location as well as the power transmitted by each transmitter is monitored and
controlled by the government.
Once the carrier frequency is assigned to a radio station, it is very important that
it maintains that frequency as constant as possible. There are two reasons for this: (1)
if the carrier frequency were allowed to drift then the listeners would have to re-tune
their radios from time to time to keep listening to that station, which would be
unacceptable to most listeners; (2) if a station drifts (in frequency) towards the next
station, their sidebands would overlap and cause interference. The carrier signal is
usually generated by an oscillator, but to meet the required precision of the
frequency it is common practice to use a crystal-controlled oscillator. At the heart
of the crystal-controlled oscillator is a quartz crystal cut and polished to very tight
specifications which maintains the frequency of oscillation to within a few hertz of
its nominal value. The design of such an oscillator can be found in Section 2.4.6.
Figure 2.5 is a block diagram of a typical transmitter.
Figure 2.5. Block diagram showing the components which make up the AM transmitter.
2.3 SYSTEM DESIGN 21
2.3.1 Crystal-Controlled Oscillator
The purpose of the crystal oscillator is to generate the carrier signal. To minimize
interference with other transmitters, this signal must have extremely low levels of
distortion so that the transmitter operates at only one frequency. As discussed earlier,
the frequency must be kept within very tight limits, usually within a few hertz in
107 Hz. It is difficult to design an ordinary oscillator to satisfy these conditions, so it
is common practice to use a quartz crystal to enhance the frequency stability and to
reduce the harmonic distortion products.
The quartz crystal undergoes a change in its physical dimensions when a potential
difference is applied across two corresponding faces of the crystal. If the potential
difference is an alternating one, the crystal will vibrate and exhibit the phenomenon
of resonance. For a crystal, the range of frequency over which resonance is possible
is very narrow, hence the frequency stability of the crystal-controlled oscillators is
very high. In general, the larger the physical size of the crystal, the lower the
frequency at which it resonates. Thus a high-frequency crystal is necessarily small,
fragile, and has low reliability. To generate a high-frequency carrier, it is common
practice to use a low-frequency crystal to obtain a signal at a subharmonic of the
required frequency and to use a frequency multiplier to increase the frequency.
Figure 2.5 shows that the crystal-controlled oscillator is followed by a frequency
multiplier.
2.3.2 Frequency Multiplier
The purpose of the frequency multiplier is to accept an incoming signal of frequency
fc=n, where n is an integer, and to produce an output at a frequency fc. A frequency
multiplier can have a single stage of multiplication or it can have several stages. The
output of the frequency multiplier goes to the carrier input of the amplitude
modulator.
2.3.3 Amplitude Modulator
The amplitude modulator has two inputs, the first being the carrier signal generated
by the crystal oscillator and multiplied by a suitable factor, and the second being the
modulating signal (voice or music) which is represented in Figure 2.5 by the single
frequency fs. In reality, the frequencies present in the modulating signal are in the
audio range 20–20,000 Hz. The output from the amplitude modulator consists of the
carrier, the lower and upper sidebands.
2.3.4 Audio Amplifier
The audio amplifier accepts its input from a microphone and supplies the necessary
gain to bring the signal level to that required by the amplitude modulator.
22 AMPLITUDE MODULATED RADIO TRANSMITTER
2.3.5 Radio-Frequency Power Amplifier
The power level at the output of the modulator is usually in the range of watts and
the power required to broadcast the signal effectively is in the range of tens of
kilowatts. The radio-frequency amplifier provides the power gain as well as the
necessary impedance matching to the antenna.
2.3.6 Antenna
The antenna is the circuit element that is responsible for converting the output power
from the transmitter amplifier into an electromagnetic wave suitable for efficient
radiation in free space. Antennae take many different physical forms determined by
the frequency of operation and the radiation pattern desired. For broadcasting
purposes, an antenna that radiates its power uniformly to its listeners is desirable,
whereas in the transmission of signals where security is important (e.g. telephony),
the antenna has to be as directive as possible to reduce the possibility of its reception
by unauthorized persons.
2.4 RADIO TRANSMITTER OSCILLATOR
Perhaps the simplest way to introduce the phenomenon of oscillation is to describe a
common experience of a public address system going unstable and producing an
unpleasantly loud whistle. The system consists of a microphone, an amplifier and a
loudspeaker (or loudspeakers) as shown in Figure 2.6. The amplified sound from the
Figure 2.6. The diagram illustrates how acoustic feedback cancause a public address system
to go unstable, turning the system into an oscillator.
2.4 RADIO TRANSMITTER OSCILLATOR 23
loudspeaker may be reflected from walls and other surfaces and reach the microphone.
If the reflected sound is louder than the original then it will in turn produce a
louder output at the loudspeaker which will in turn produce an even louder signal at
the microphone. It is fairly clear that this state of affairs cannot continue indefinitely;
the system reaches a limit and produces the characteristic loud whistle. Immediate
steps have to be taken to ensure that the sound level reaching the microphone is less
than that required to reach the self-sustained value. If, on the other hand, we are
interested in the generation of an oscillation, then the study of the characteristics of
the amplifying element, the conditions under which the feedback takes place, the
frequencies present in the signal and the optimization of the system to achieve
specified performance goals are in order.
The electronic oscillator is a particular example of a more general phenomenon of
systems which exhibit a periodic behavior. A mechanical example is the pendulum
which will perform simple harmonic motion at a frequency determined by its length
and the acceleration constant due to gravity, g, if the energy it loses per cycle is
replaced from an outside source. In the case of the pendulum used in clocks, the
source of energy may be a wound-up spring or a weight whose potential energy is
transferred to the pendulum. The solar system with planets performing cyclical
motion around the sun is another example of an oscillator, although this time there is
no periodic input of energy because the system is virtually lossless.
Three theoretical approaches to oscillator design are presented below. The first is
based on the idea of setting up a ‘‘lossless’’ system by canceling the losses in an LC
circuit due to the presence of (positive) resistance by using a negative resistance. The
second is based on feedback theory. The third is based on the concept of embedding
an active device and the optimization of the power output from the oscillator.
2.4.1 Negative Conductance Oscillator
Consider the circuit shown in Figure 2.7. The externally applied current and the
corresponding voltage are related to each other by
I ¼ G0 þ Gn þ sC þ
1

sLðVÞ ð2:4:1Þ
Figure 2.7. The negative conductance oscillator has a negative conductance generating signal
power which is dissipated in the (positive) conductance. The components L and C determine the
frequency of the signal. An alternate statement is that the negative conductance cancels all the
losses in the circuit. It then oscillates losslessly at a frequency determined by L and C.
24 AMPLITUDE MODULATED RADIO TRANSMITTER
where G0 is the load conductance, Gn is the negative conductance, I is current, V is
voltage, s is the complex frequency, C is capacitance, and L is inductance. If the
circuit is that of an oscillator, the external excitation current must be zero since an
oscillator does not require an excitation current. Hence
0 ¼ G0 þ Gn þ sC þ
1

sLðVÞ: ð2:4:2Þ
For a non-trivial solution, V is non-zero, therefore
G0 þ Gn þ sC þ
1
sL ¼ 0 ð2:4:3Þ
which gives the quadratic equation
s2CL þ sLðG0 þ GnÞ þ 1 ¼ 0: ð2:4:4Þ
The solution is then
s1; s2 ¼ ðG0 þ GnÞ
2C  ðffiffiGffiffiffiffi0ffiffiffiþffiffiffiffiGffiffiffiffinffiffiÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2
4C2 
1
LC s ð2:4:5Þ
when
jGnj ¼ G0 ð2:4:6Þ
that is, the system is lossless. Equation (2.4.5) becomes
s1; s2 ¼ ffiffiffiffiffiffiffiffiffiffiffi
1
LC r ¼ jo
1
ffiffiffiffiffiffiffi LC p ð2:4:7Þ
which is the resonant frequency for the tuned circuit. The circuit will continue to
oscillate at this frequency as if it were in perpetual motion.
A number of devices exhibit negative conductance under appropriate bias
conditions and may be used in the design of practical oscillators of this type.
These include tunnel diodes, pentodes (N-type negative conductance), uni-junction
transistors and silicon-controlled rectifier (S-type).
The voltage–current characteristics of N- and S-type negative conductances are
shown in Figures 2.8(a) and (b), respectively.
2.4 RADIO TRANSMITTER OSCILLATOR 25
2.4.2 Classical Feedback Theory
Consider the system shown in Figure 2.9 where A is the gain of an amplifier and b
represents the transfer function of the feedback path. Es is the signal applied to the
input and Eo is the output of the system [1].
In the derivation that follows, it is necessary to make the following assumptions:
(1) the input impedances of both the amplifier and the feedback network are
infinite and their output impedances are zero,
(2) both A and b are complex quantities.
Figure 2.8. (a) Characteristics of an N-type negative conductance device. The device has a
negative conductance in the region where the slope of the curve is negative. Examples of
practical devices which have such characteristics are the tunnel diode and the tetrode. (b)
Characteristics of an S-type negative conductance device. The device has a negative conductance
in the region where the slope of the curve is negative. Examples of practical devices which
have such characteristics are the four-layer diode and the silicon controlled rectifier.
26 AMPLITUDE MODULATED RADIO TRANSMITTER
The gain of the amplifier alone is
A ¼
Eo
Eg
: ð2:4:8Þ
Application of Kirchhoff ’s Voltage Law (KVL) at the input gives
Eg ¼ Es þ bEo: ð2:4:9Þ
Substituting Equation (2.4.8) into Equation (2.4.9) gives
Eo ¼ AðEs þ bEoÞ ð2:4:10Þ
from which we obtain
Eo
Es ¼
A
1  bA
: ð2:4:11Þ
Since the Es and Eo are the input and output, respectively, of the system as a whole,
we can define this as A0 where
A0 ¼
Eo
Es ¼
A
ð1  bAÞ
: ð2:4:12Þ
Three separate conditions must be considered that depend on the value of the
denominator of Equation (2.4.12)
(1) Positive feedback. If the modulus of ð1  bAÞ is less than unity, then the gain
of the system A0 is greater then the gain of the amplifier A and therefore the
effect of the feedback is said to be positive.
(2) Negative feedback. If the modulus of ð1  bAÞ > 1, then A0 < A.
Figure 2.9. Classical feedback system with gain A and feedback factor b.
2.4 RADIO TRANSMITTER OSCILLATOR 27
(3) Oscillation. If the modulus of ð1  bAÞ ¼ 0 then the gain A0 is infinite
because with no input ðEs ¼ 0Þ there is still an output. In fact the system is
supplying its own input and
bA ¼ 1: ð2:4:13Þ
It must be noted that the waveform of the signal need not be sinusoidal and in fact it
can take any form so long as the waveform of the signal that is fed back, bEo, is
identical to the signal Eo. However, the object of this exercise is to generate a carrier
for a telecommunication system and therefore only sinusoidal signals are acceptable
– any other waveform will generate other carriers (harmonics of the fundamental)
and cause interference with transmissions of other stations.
2.4.3 Sinusoidal Oscillators
Since both A and b are complex quantities, condition (3) implies
jbAj ¼ 1: ð2:4:14Þ
Stated in words, the magnitude of the loop gain must equal unity, and
ffbA ¼ 0; 2p; 4p; etc. ð2:4:15Þ
Again, in words, the loop-gain phase shift must be zero or an integral multiple of 2p
radians. The condition given in Equation (2.4.13), which implies Equations (2.4.14)
and (2.4.15), is known as the Barkhausen Criterion.
These two conditions must exist simultaneously for sinusoidal oscillation to
occur.
2.4.4 General Form of the Oscillator
An oscillator circuit shown in Figure 2.10 [2] and an equivalent circuit is as shown in
Figure 2.11, where the amplifying element is replaced by a voltage-controlled
voltage source in series with a resistance Ro to simulate the output resistance of the
element. The amplifying element may be a tube, a transistor or an operational
amplifier.
The load seen by the amplifier is
ZL ¼
Z2ðZ1 þ Z3Þ
ðZ1 þ Z2 þ Z3Þ
: ð2:4:16Þ
28 AMPLITUDE MODULATED RADIO TRANSMITTER
The amplifier gain without feedback is
A ¼
Vo
V32 ¼ 
AvZL
ðRo þ ZLÞ ð2:4:17Þ
and the feedback constant is
b ¼
Z1
ðZ1 þ Z3Þ
: ð2:4:18Þ
The loop gain is
bA ¼ 
AvZ1ZL
ðRo þ ZLÞðZ1 þ Z3Þ
: ð2:4:19Þ
Figure 2.10. Circuit diagram for a more generalized form of the oscillator.
Figure 2.11. The equivalent circuit of the generalized form of the oscillator. Ro represents the
output resistance of the amplifier.
2.4 RADIO TRANSMITTER OSCILLATOR 29
Substituting for ZL as defined in Equation (2.4.16), Equation (2.4.19) becomes
bA ¼ 
AvZ1Z2
½RoðZ1 þ Z2 þ Z3Þ þ Z2ðZ1 þ Z3Þ
: ð2:4:20Þ
For simplicity, we may assume that the impedances are lossless; hence
Z1 ¼ jX1; Z2 ¼ jX2 and Z3 ¼ jX3 ð2:4:21Þ
Then Equation (2.4.20) becomes
bA ¼ 
AvX1X2
½jRoðX1 þ X2 þ X3Þ  X2ðX1 þ X3Þ
: ð2:4:22Þ
Recall that for oscillation to occur
1  bA ¼ 0: ð2:4:23Þ
This means that bA must be real and hence,
X1 þ X2 þ X3 ¼ 0 ð2:4:24Þ
that is,
X2 ¼ ðX1 þ X3Þ: ð2:4:25Þ
The expression for the loop gain becomes
bA ¼ Av
X1
X2
: ð2:4:26Þ
Since bA ¼ 1, it follows that X1 and X2 must have opposite signs; that is, if one of
them is inductive, the other must be capacitive and X3 can be capacitive or inductive,
depending on the sign of (X1 þ X2). The two possibilities are shown in Figures 2.12
and 2.13, respectively.
The circuit shown in Figure 2.12 is better known as a Colpitts oscillator. The
circuit is redrawn in Figure 2.12(b) to emphasize the symmetrical structure of the
circuit.
The circuit shown in Figure 2.13 is better known as a Hartley oscillator.
From the point of view of the structure of the circuits, it can be seen that they are
the same. It should be noted that the operational amplifier can be replaced by a tube
or a transistor.
30 AMPLITUDE MODULATED RADIO TRANSMITTER
2.4.5 Oscillator Design for Maximum Power Output
A major flaw in the two previous designs is that they do not anticipate the necessity
for the oscillator to supply power to a load. The theory of the design for maximum
power output from an oscillator [3] is based on the characterization of the amplifying
element (‘‘active device’’) as a two-port. A discussion of two-ports is beyond the
scope of this book but may be found in any standard text on circuit theory.
A two-port can be described in terms of its terminal voltages and currents by four
parameters: impedances, admittances, voltage ratios, and current ratios under
constraints of open or short-circuit. Without limiting the generality, assume that
the active device has been characterized in terms of the short-circuit admittance
Figure 2.12. (a) The generalized form of the oscillator with two of the impedances replaced by
capacitors and the third by an inductor to form a Colpitts oscillator. (b) The diagram in( a) has
beenredrawnto emphasize the symmetry of the circuit.
Figure 2.13. (a) The generalized form of the oscillator with two of the impedances replaced by
inductors and the third by a capacitor to form a Hartley oscillator. (b) The diagram in( a) has been
redrawnto emphasize the symmetry of the circuit.
2.4 RADIO TRANSMITTER OSCILLATOR 31
parameters, or Y parameters, for short. Figure 2.14 shows the two-port and its
terminal voltages and currents, which are assumed to be sinusoidal.
The Y parameters are functions of frequency and bias conditions, and in general,
complex so that
Y11 ¼ g11 þ jb11: ð2:4:27Þ
The total power entering the two-port is
P ¼ V
 1 I1 þ V
 2 I2: ð2:4:28Þ
The Y parameters and the terminal voltages and currents are related by
I1 ¼ Y11V1 þ Y12V2 ð2:4:29Þ
and
I2 ¼ Y21V1 þ Y22V2: ð2:4:30Þ
Substituting for I1 and I2 in Equation (2.4.28) gives
P ¼ Y11jV1j2 þ Y22jV2j2 þ Y12V
 1 V2 þ Y21V1V
 2 : ð2:4:31Þ
The ratio of the output voltage, V2, to the input voltage, V1, can be defined as
V2
V1 ¼ A ¼ AR þ jAI : ð2:4:32Þ
The real power entering the two-port is
PR ¼ jV1j2½g11 þ g22ðA2
R þ A2
I Þ þ ðg12 þ g21ÞAR  ðb12 þ b21ÞAI : ð2:4:33Þ
Figure 2.14. A two-port representation of an active device to be used in the design of an
oscillator. Short-circuit admittance (Y) parameters are used in the design for convenience. Other
parameters could be used inthe description.
32 AMPLITUDE MODULATED RADIO TRANSMITTER
This can be rearranged as follows:
PR
g22jV1j2 ¼ AR þ ðg21 þ g12Þ
2g22  2
þ AI þ ðb21  b21Þ
2g22  2
þ
4g11g22  ðg21 þ g12Þ2  ðb21  b12Þ2
4g2
22
: ð2:4:34Þ
This equation is of the form:
z ¼ ðx  aÞ2 þ ðy  bÞ2 þ c ð2:4:35Þ
and therefore it is that of a paraboloid in space with axes PRðg22=V1=2Þ, AR and AI as
shown in Figure 2.15.
It was assumed that real, positive power was supplied and dissipated in the twoport;
therefore, it follows that negative values of power, as shown in Figure 2.15,
must represent power generated by the two-port and dissipated in the surrounding or
embedding circuit; that is, above the A plane, real power is supplied to the two-port,
and below it the device supplies real power to the embedding circuit. Because the
object of the exercise is to generate and supply real power to an external circuit, the
most interesting part of Figure 2.15 is the part below the A plane. It is clear that
movement towards the apex of the paraboloid represents increasing levels of power
supplied by the ‘‘active’’ two-port and that the maximum power supplied occurs at
the apex. We shall return to this remark when we consider the optimization of the
power output.
The most general embedding circuit for the two-port is as shown in Figure 2.16
with each branch made up of a conductance in parallel with a susceptance. The
susceptances can be considered as the tuned circuit which will determine the
Figure 2.15. Three-dimensional representation of the output power of the oscillator as a
function of the complex parameter A.
2.4 RADIO TRANSMITTER OSCILLATOR 33
frequency of oscillation and the conductances as the destination of the power
generated by the active two-port.
The embedding network can also be described in terms of a two-port as follows:
I01
¼ ðY2 þ Y3ÞV1  Y3V2 ð2:4:36Þ
I02
¼ Y3V1 þ ðY1 þ Y3ÞV2: ð2:4:37Þ
When the active device and the embedding are connected as shown in Figure 2.17,
the composite circuit can be described by the two-port equations which are
[Equations (2.4.29) þ (2.4.36) and Equations (2.4.30) þ (2.4.37)]:
I1 þ I01
¼ ðY11 þ Y2 þ Y3ÞV1 þ ðY12  Y3ÞV2 ð2:4:38Þ
I2 þ I02
¼ ðY21  Y3ÞV1 þ ðY1 þ Y3 þ Y22ÞV2: ð2:4:39Þ
Figure 2.16. The general passive embedding circuit for a two-port.
Figure 2.17. The active two-port is shown with the passive embedding connected.
34 AMPLITUDE MODULATED RADIO TRANSMITTER
For an oscillator, no external signal current is supplied at port 1 and therefore
I1 þ I01
¼ 0. Similarly I2 þ I02
¼ 0. From Equation (2.4.32) we have
V2 ¼ V1ðAR þ jAIÞ ð2:4:40Þ
From Equation (2.4.38) we have
V1½Y11 þ Y2 þ Y3 þ ðAR þ jAI ÞðY12  Y3Þ ¼ 0 ð2:4:41Þ
and from Equation (2.4.39) we have
V1½Y21  Y3 þ ðAR þ jAI ÞðY1 þ Y3 þ Y22Þ ¼ 0: ð2:4:42Þ
For non-trivial values of V1, real and imaginary values of Equations (2.4.41) and
(2.4.42) are separately equal to zero; that is,
g11 þ G2 þ G3 þ ARðg12  G3Þ  AI ðb12  B3Þ ¼ 0 ð2:4:43Þ
b11 þ B2 þ B3 þ ARðb12  B3Þ þ AI ðg12  G3Þ ¼ 0 ð2:4:44Þ
g21  G3 þ ARðG1 þ G3 þ g22Þ  AI ðB1 þ B3 þ b22Þ ¼ 0 ð2:4:45Þ
and
b21  B3 þ ARðB1 þ B3 þ b22Þ þ AI ðG1 þ G3 þ g22Þ ¼ 0: ð2:4:46Þ
Equations (2.4.43) to (2.4.46) can be written in the form of a matrix as follows:
AR 0 ðAR  1Þ AI 0 AI
AI 0 AI AR 0 ðAR  1Þ 0 1 ð1  ARÞ 0 0 AI
0 0 AI 0 1 ð1  ARÞ
2664
3775
G1
G2
G3
B1
B2
B3
26666664
37777775
¼
g21 ReðAy22Þ
b21 ImðAy22Þ
g11 ReðAy12Þ
b11 ImðAy12Þ
2664
3775
ð2:4:47Þ
All the terms in the matrix are known except G1, G2, G3, B1, B2 and B3; that is, there
are six unknowns but only four equations so a unique solution cannot be found
unless arbitrary values are chosen for at least two of the unknowns. Fortunately, an
oscillator normally has only one conductive load and therefore two of the three
conductances can be set to zero. The matrix equation can then be solved for one
conductance and three susceptances.
2.4.6 Crystal-Controlled Oscillator
The oscillator used in a transmitter has to have a very tight tolerance on the stability
of its frequency. This is necessary if interference between radio stations is to be
2.4 RADIO TRANSMITTER OSCILLATOR 35
avoided. The drift of the frequency of an ordinary LC oscillator, for example, makes
it unsuitable for this purpose. Greater frequency stability can be achieved by using a
crystal as a part of the oscillator circuit [4]. In Section 2.3.1, the behavior of the
crystal when it is excited by an ac signal was discussed. It is evident that, since the
crystal reacts to electrical excitation, it must be possible to devise an electrical circuit
made up of inductors, resistors and capacitors whose frequency characteristics are
approximately those of the crystal. Such a circuit is shown in Figure 2.18. The
approximate circuit is reasonably accurate at frequencies close to the resonant
frequency. Over a larger frequency range a more complicated equivalent circuit has
to be used.
Typical values of the components of the equivalent circuit are C ¼ 0:0154 pF,
R ¼ 8O, L ¼ 0:0165 H, Co ¼ 4:55 pF. The capacitance Co is due largely to the
electrodes which are attached to the crystal. The crystal will therefore resonate in the
series mode at a frequency os where
o2s
¼
1
LC ð2:4:48Þ
which gives fs ¼ 9:984  106 Hz. It will resonate in the parallel mode at an angular
frequency given approximately by
o2p
¼
1
L
CCo
ðC þ CoÞ ð2:4:49Þ
which gives a resonant frequency, fp ¼ 10:001  106 Hz – a change of less than
0.2%. The corresponding quality factor of the crystal is then Qo ¼ 130;000.
Figure 2.18. (a) The equivalent circuit of the crystal and its package. (b) The electrical symbol
for the crystal.
36 AMPLITUDE MODULATED RADIO TRANSMITTER
Figure 2.19 shows the reactance of the crystal plotted against frequency. It should
be noted that the reactance of the crystal is inductive over a narrow band of
frequency and also that both reactance and frequency are not to scale.
Figure 2.20 shows a typical crystal-controlled oscillator. The crystal is substituted
for one of the inductors in what would otherwise be classified as a Hartley oscillator.
This type of crystal-controlled oscillator is called a Pierce oscillator. Similarly, the
crystal-controlled oscillator corresponding to the Colpitts variety is called a Miller
oscillator. In the circuit shown in Figure 2.20, the active element is a field-effect
transistor whose gate-to-drain capacitance plus stray capacitance constitute C3.
The very high Qo of the crystal ensures that the oscillator has an extremely
limited range of frequencies in which it can continue to oscillate. Various other
measures may be taken to improve the frequency stability, such as placing the crystal
in a temperature-controlled environment and the Q factor can be enhanced by
evacuating the glass envelope which protects it.
High precision oscillators are invariably connected to their load through a buffer
amplifier. This ensures that variations in the load do not affect the operation of the
oscillator.
2.5 FREQUENCY MULTIPLIER
The purpose of the frequency multiplier is to raise the frequency generated by the
crystal-controlled oscillator to the value required for the transmitter carrier. As
explained earlier, it is not possible to obtain physically robust crystals at high
Figure 2.19. The reactance characteristics of the crystal. Note that this is not to scale.
2.5 FREQUENCY MULTIPLIER 37
frequency since their physical size gets smaller as the frequency of oscillation gets
higher. The standard technique is therefore to use a crystal to generate a signal at a
frequency which is a subharmonic of the required carrier frequency and then to raise
the frequency up to the required value using a cascade of frequency multipliers.
A useful analog of a frequency multiplier is a child’s swing. With a child on the
swing, the adult must give it a push to get the swing into operation. Subsequent to
that, further supplies of energy must take place at a frequency determined by the
length of the swing and the gravitational constant of acceleration, g. It is also
necessary to supply the energy at a point in time when it enhances the swinging
action rather than oppose it; on average, the adult will have to supply energy equal to
that lost during the cycle to maintain a constant amplitude. If the energy supplied per
cycle is less than the energy lost, the amplitude of the swing will decrease to a
smaller value so as to restore the energy balance. If the energy supplied per cycle
is greater than that lost per cycle, the amplitude will grow to a new steady-state
value. The motion of the child will be very nearly a simple harmonic one if the total
energy stored in the system is large compared to the energy supplied by the adult,
that is, the system Q has to be large if the child is to execute a near-sinusoidal
motion. The most important point of this analog is that the energy does not have to
be supplied at the same frequency as the swing: it can be supplied at a subharmonic
frequency, that is, the push can be given every other cycle of the swing or every third
cycle or higher so long as enough energy is supplied to maintain the energy balance.
When the push occurs every other cycle, it is clear that the output of the system is at
twice the frequency of the input – this is a frequency multiplier with a multiplication
Figure 2.20. (a) A Hartley oscillator with one of the inductors replaced by a crystal. This circuit
is called a Pierce oscillator. The field-effect transistor may be replaced by any other suitable
active device. (b) The equivalent circuit of the Pierce oscillator demonstrating its symmetrical
structure.
38 AMPLITUDE MODULATED RADIO TRANSMITTER
factor of two. When the energy is supplied every third cycle, the multiplication
factor of three is obtained, and so on. Evidently, there is a limit on how high the
multiplication factor can be and it is determined by the amount of variation in the
amplitude of the swing which can be tolerated. Table 2.1 shows a comparison of the
swing and the frequency multiplier.
Figure 2.21 shows a typical frequency multiplier. Energy is fed into it by applying
a suitable positive pulse to the base of the transistor. This causes the transistor to
conduct momentarily, that is, current flows from the direct current (dc) power supply
through the inductor and a finite amount of energy is stored in the inductor. The
current flow is shut off when the input pulse ends and the transistor is essentially an
open-circuit. The energy stored in the magnetic field of the inductor is transformed
into energy stored in the electric field of the capacitor. The transformation of energy
from one form to another and back again would continue indefinitely in a sinusoidal
form if the system were lossless and this would take place at a frequency determined
by the values of the inductance and capacitance. The resistance RL represents the
losses in the system – the amplitude of the sinusoid will decay with time. The
steady-state amplitude of the voltage or current will be determined by the equaliza-
TABLE 2.1 Swing Analogy for a Frequency Multiplier
Swing, Including Child Frequency Multiplier
Adult (timing) Input signal source
Adult (energy transferred to child) DC power supply
Length of swing, l , and gravitational constant, g Inductance, L, and capacitance, C
Air resistance and bearing friction Energy loss in R
Frequency f ¼
1
2p ffiffiffi 1
rg
Frequency f ¼
1
2p ffiffiffiffiffiffiffi pLC
Amplitude of swing Amplitude of voltage or current in tank circuit
Figure 2.21. A class-C amplifier to be used, with minor modifications, as a frequency
multiplier.
2.5 FREQUENCY MULTIPLIER 39
tion of the energy input and the energy output (loss). Subsequent to the initial input
pulse, all input pulses must be timed to enhance rather than oppose the stored energy
of the system. It is, of course, not necessary to have an input pulse for every cycle of
the output; the input pulse can be supplied once for every two, three or greater
number of cycles. When the input frequency is the same as the output, the frequency
multiplier (multiplication factor¼1) is simply a class-C amplifier. Since a class-C
amplifier represents the simplest frequency multiplier, the design of a class-C
amplifier will be discussed next.
2.5.1 Class-C Amplifier
In a class-C amplifier, the current in the active device flows for a period much less
than p radians of the output waveform. The active device current waveform is
therefore highly non-sinusoidal. In a class-A or -B amplifier this would give a
correspondingly non-sinusoidal output. However, in the class-C amplifier the
collector load consists of a parallel LC circuit which is tuned to the frequency of
the input signal. The tuned circuit (sometimes referred to as a tank circuit) presents a
very high impedance at the resonant frequency to the collector of the transistor and
hence a high gain is obtained at this frequency. Other frequencies, such as harmonics
of the input frequency, are attenuated. Therefore, the output is very nearly sinusoidal.
In a practical class-C amplifier, the base of the transistor is held at a voltage such
that the transistor is in the off state. The input signal brings the transistor into
conduction at its positive peaks and causes enough current to flow in the inductor to
store energy equal to that dissipated in the load resistor RL and at other sites in the
circuit. When the input signal drops below the threshold of the transistor, it switches
off and the LC tank oscillates freely with a sinusoidal waveform at the resonant
frequency of the tank circuit. Because of the dissipation in the circuit, the waveform
is actually an exponentially damped sinusoid as shown in Figure 2.22.
The collector voltage, the base voltage and collector current waveforms are
shown in Figure 2.23. Note that the quiescent value of the collector voltage
waveform is Vcc and that its maximum amplitude is 2Vcc.
The conversion efficiency of the amplifier is given by:
Z ¼ (ac power output)=(dc power input):
A class-C amplifier can have a relatively high conversion efficiency, usually about
85%, because the dc current flows for a very short part of the cycle and this happens
when the collector voltage is at its lowest value. Thus the power lost in the transistor
is minimal.
2.5.2 Converting the Class-C Amplifier into a Frequency Multiplier
To convert a class-C amplifier into a frequency multiplier with a multiplication factor
of 2, the L and C of the tank circuit are chosen to resonate at 2oo when the input
signal frequency is oo. Successful operation of the system demands that the Q factor
40 AMPLITUDE MODULATED RADIO TRANSMITTER
Figure 2.22. The output waveform of a class-C amplifier after a single pulse excitation. Note
the sinusoidal waveform and the exponential decay of the envelope.
Figure 2.23. The collector voltage (vc), base voltage (vbe) and collector current (ic) of the class-
C amplifier. Note that the base voltage need not be sinusoidal for the collector voltage to be
sinusoidal.
2.5 FREQUENCY MULTIPLIER 41
of the circuit is sufficiently high so that the amount of damping at the end of the
second cycle is negligible. Higher multiplication factors are possible with the
damping problem progressively getting worse as the multiplication factor increases.
Figure 2.24 shows the input current and output voltage waveforms of a frequency
multiplier with a multiplication factor of 2. Note the effect of damping on the
amplitude of the output voltage.
Example 2.5.1 Frequency Multiplier. A frequency multiplier with a multiplication
factor of 2 is driven by an input current whose waveform can be assumed to be a
half-sinusoid with a peak value of 200 mA at a frequency of 2=pMHz. The input
current flows for a period corresponding to 5 of one cycle of the output waveform
and the average input impedance over this period is 750O resistive. The following
data applies to the circuit:
(1) multiplier load¼50O resistive
(2) transformer turns ratio¼15 : 1
(3) transformer coupling, k ¼ 1
(4) dc voltage supply¼15V
(5) current gain of the transistor¼100
(6) loaded Q factor of the transformer primary¼50 (min).
Figure 2.24. The output voltage waveform of a times-3 frequency multiplier and its driving
current. The decay is exaggerated to show the effect of low Q factor.
42 AMPLITUDE MODULATED RADIO TRANSMITTER
Calculate the following:
(a) the value of the primary inductance
(b) the value of the tuning capacitance
(c) the impedance of the collector load at the output frequency
(d) the ac power dissipated in the load
(e) the power gain of the multiplier.
Solution. A frequency multiplier is essentially a class-C amplifier whose input is
driven at a frequency which is a subharmonic of the output frequency. A suitable
circuit is shown in Figure 2.25
Input frequency ¼ 2pf ¼ 4  106 rad=s:
Output frequency ¼ 8  106 rad=s:
The input current and the output voltage waveforms are shown in Figure 2.26.
When the load is transferred to the primary the collector circuit is as shown in
Figure 2.27 with the equivalent resistor having a value n2RL ¼ ð15Þ2  50 ¼ 11:25 kO.
(a) Assuming that the tank circuit is lossless, we can assign the loss in n2RL to
the inductor and determine the series equivalent RL circuit (see Figure 2.28).
Equating the impedances,
joLpRp
Rp þ joLp ¼ Ro þ joLo: ð2:5:1Þ
Figure 2.25. Circuit diagram for the frequency multiplier example.
2.5 FREQUENCY MULTIPLIER 43
Rationalizing the left-hand side and equating real and imaginary parts gives
Ro ¼
o2L2p
Rp
R2p
þ o2L2p
ð2:5:2Þ
and
Lo ¼
LpR2p
R2p
þ o2L2p
: ð2:5:3Þ
Figure 2.26. Collector and base current waveforms of the frequency multiplier example.
Figure 2.27. The frequency multiplier with the transformer load transferred to the primary.
44 AMPLITUDE MODULATED RADIO TRANSMITTER
The loaded Q factor of the tank circuit has a minimum value of 50; therefore
Qo ¼ 50 ¼
oLo
Ro ¼
Rp
oLp ¼
n2RL
oLp
: ð2:5:4Þ
The primary inductance is
Lp ¼
n2RL
50o ¼
11:25  103
50  8  106 ¼ 28:13 mH: ð2:5:5Þ
(b) For the tank circuit at resonance, then
o2 ¼
1
LpC ð2:5:6Þ
and
C ¼
1
o2Lp ¼ 555 pF ð2:5:7Þ
(c) Because the parallel LC tank circuit is at resonance, it is actually an opencircuit.
Therefore the load seen by the collector is n2RL ¼ 11:25 kO.
(d) The base and collector currents are related by
ic ¼ bib ¼ 20mA peak: ð2:5:8Þ
Current in the secondary¼ n  20mA peak. Since the system is in steady
state, the energy dissipation associated with the current n  20mA peak
flowing in the load can be averaged over one cycle of the input signal (2
cycles of the output) and the ac output power calculated o