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`n g i nfleeri n
`
`Herbert L. Kraus_$_
`and
`
`Charles W. Bostian
`
`Department of Electrical Engineering
`Virginia Polytechnic Institute and State University
`Blacksburg, Virginia
`
`Frederick H. Raab
`Consultant
`
`Green Mountain Radio Research Company
`Burlington, Vermont
`
`JOHN WILEY & SONS
`
`/
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`r
`
`‘3.
`
`".3
`
`New York - Chichester - Brisbane - Toronto - Singapore
`
`Momentum Dynamics Corporation
`Exhibit 1018
`Page 001
`
`Momentum Dynamics Corporation
`Exhibit 1018
`Page 001
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`

`

`KQBENHAVI’N‘S HEKNIKUM
`Bibliutekct
`
`Pb
`
`Copyright © 1980, by John Wiley & Sons, Inc.
`
`All rights reserved. Published simultaneously in Canada.
`
`Reproduction or translation of any part of
`this work beyond that permitted by Sections
`107 and 108 of the 1976 United States Copyright
`Act without the permission of the copyright
`owner is unlawful. Requests for permission
`or further information should be addressed to
`the Permissions Department, John Wiley & Sons.
`
`Library of Congress Cataloging in Publication Data
`
`Krauss, Herbert L.
`Solid state radio engineering.
`
`Includes indexes.
`1. Radio circuits. 2. Radio—Receivers and
`reception. 3. Radio-Transmitters and transmission.
`I. Bostian. Charles W., joint author. II. Raab,
`Frederick H., joint author. III. Title.
`
`TK6553.K73
`ISBN 0-471-03018-X
`
`621.3841‘2
`
`78-27797
`
`Printed in the United States of America
`
`10987654
`
`Momentum Dynamics Corporation
`Exhibit 1018
`Page 002
`
`Momentum Dynamics Corporation
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`Page 002
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`

`

`Preface
`
`This book is about the analysis and design of the radio-frequency electronic
`circuits that are the building blocks of radio transmitters and receivers. It
`reflects the developments of the past decade, which have initiated an unprec-
`edented growth in the use of analog radio systems for personal and business
`voice communications. Continuing advances in solid state technology have
`resulted in transmitters that are smaller, cheaper, and more reliable than ever
`before. Parallel developments have occurred in the home entertainment radio
`and television fields. As a result, radio engineers versed in the solid state art
`are in demand.
`
`Because of the rapid changes in radio technology, teachers of courses in
`radio circuits have often had to rely on material that is scattered through
`many difierent
`textbooks,
`technical
`journals, and application notes. This
`volume meets theaeed for a comprehensive book on radio electronics.
`Solid State Radio Engineering is unique because of its broad coverage of
`both receiver and transmitter circuits and its illustration of
`theoretical
`concepts with numerical examples from real circuits. Design that uses prac-
`tical
`circuit
`elements
`instead
`of
`idealized mathematical models
`is
`emphasized. The letter symbols used for semiconductor device currents and
`voltages conform for the most part with IEEE Standard notation.
`The last five chapters present for the first time in textbook form consider-
`able information on RF power amplifiers. Currently, power amplifier design is
`often accomplished by using cut-and-try techniques and rules of thumb. Often,
`theoretical explanations of power amplifier operation are too complicated or
`require too much time for a designer to use. This book brings these principles
`to the student or practicing design engineer in a way that not only makes them
`understandable but also makes them useful for design. The discussions in
`Chapters 12 to 16 include not only accepted state-of-the-art technology, based
`on bipolar junction transistors, but also VMOS RF power FETs, high-
`efiiciency techniques, envelope elimination and restoration, and other newly
`emerging technologies that are expected to play significant roles in radio
`engineering during the next decade.
`This book is intended to be both a reference for the working engineer and
`a textbook for senior-level students in electrical engineering and electrical
`technology. A knowledge of complex algebra, Fourier series, and Fourier
`transforms will enable its reader to handle the mathematics in the book. As an
`
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`

`

`aid to self-study, practical design examples are included throughout. These
`are reinforced by homework problems that are keyed to the corresponding
`sections of the text.
`
`The material presented is appropriate for either a two—semester or three—
`quarter course sequence. For shorter course offerings, some chapters may be
`omitted. For example, if receiving systems are of primary interest, Chapters 1
`to ll can be used. For transmitters, Chapters 1 to 8 and 12 to 16 (with the
`possible omission of Sections 14-3 to 14-6) are recommended. If the students
`have an adequate background in noise and modulation theory, Chapters 2 and
`8 can be omitted. Prior knowledge of resonant impedance matching might
`permit skipping all of Chapter 3, with the exception of Section 3-6, which is
`used frequently in the following chapters.
`A brief introductory chapter considers the concept of modulation and the
`functions performed in a typical transmitter and receiver. It is followed by a
`discussion of electrical noise because of its importance in the design of RF
`amplifiers and mixers in receivers. Chapters 3 to 7 include the component
`parts of receiver systems, and Chapter 8 provides the modulation theory
`necessary for an understanding of the operation of AM, SSB, FM, and TV
`receivers.
`
`A thorough treatment of the design of narrowband, tapped resonant
`circuits for impedance matching, as well as the use of tapped mutual
`in-
`ductance circuits for both wideband and narrowband matching, is given in
`Chapter 3. The design of small-signal, tuned amplifiers for maximum gain with
`a specified degree of stability is considered next. This is followed by an
`analysis of sinusoidal oscillations in LC and crystal oscillator circuits; and a
`unique, laboratory-tested procedure is given for the design of a common-base
`Colpitts oscillator for specified output.
`A phase—locked loop will soon be included in nearly every radio receiver,
`transmitter, and piece of test equipment. Hence, the simpler aspects of loop
`operation are outlined in Chapter 6, along with the characteristics of the basic
`loop components and some applications to communication equipment. This is
`followed by analysis of diode, BIT, and FET mixer circuits.
`Chapters 9 to 11 are devoted to receivers. Because design techniques are
`changing constantly with the introduction of new integrated-circuit packages,
`the fundamental signal processing in each type of receiver is stressed without
`a detailed description of all possible circuits. Analysis of various types of AM
`and FM detectors is found in the corresponding chapters, along with in—
`formation on ceramic, crystal, and surface-acoustic-wave IF filters. The basic
`principles of color picture transmission are given in Chapter 11, along with a
`block-diagram explanation of the receiver circuitry. Although not directly
`related to the subject matter of the book,
`this is the one piece of radio
`
`vi
`
`l
`
`Preface
`
`Momentum Dynamics Corporation
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`

`

`it
`
`in-
`
`equipment that the student owns and uses every day. Furthermore,
`corporates most of the principles studied in preceding chapters.
`Chapters 12 to 16 are organized by practice rather than theory. Thus Class
`A and B amplification along with the broadband transformer and filter
`networks normally used in SSB transmitters are discussed first. Similarly,
`Class C and Class C mixed-mode power amplifiers and the discrete-element or
`transmission-line matching networks normally used with them, are presented
`together. Chapter 14 treats several types of high-efficiency power amplifiers
`(Classes D, E, F, and S). Chapter 15 includes CW, FM, and AM transmitters,
`since they have similar configurations. The last chapter examines single-
`sideband and multimode transmitters, envelope elimination and restoration,
`and other related techniques.
`
`Herbert L. Krauss
`Charles W. Bostian
`Frederick H. Raab
`
`Preface
`
`/
`
`vii
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`

`

`Acknowledgments
`
`This text originated with class notes written by H. L. Krauss; Chapters 1 to 11
`were completed with the collaboration of C. W. Bostian. Chapters 12 to 16
`were contributed by F. H. Raab.
`Assistance in manuscript preparation was provided by members of the
`secretarial staff of the Department of Electrical Engineering at Virginia
`Polytechnic Institute and State University. We are indebted to Professors C.
`A. Holt and E. A. Manus for their helpful advice on various technical points,
`and our special thanks go to Professors R. O. Claus and H. R. Skutt, who
`class-tested the material and provided many worthwhile suggestions.
`In the early stages of this project we received invaluable technical assis-
`tance from Roy C. Hejhall, of Motorola Semiconductor Products, and Ed
`Oxner, of Siliconix, Inc. For the material on power amplifiers, N. O. Sokal, of
`Design Automation, Inc., and A. D. Sokal, of the Physics Department,
`Princeton University, have been most helpful critics.
`Finally, we thank our wives, Anne Krauss, Frieda Bostian, and Becky
`Raab, for their patience, devotion, and encouragement during the writing of
`this book.
`
`H. L. K.
`C. W. B.
`F. H. R.
`
`ix
`
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`

`

`Contents
`
`Chapter 1 Radio Communication Systems
`
`1—1
`
`Introduction
`
`1-2 Elements of a Radio System
`1-3 Modulation
`
`1-4 Frequency and Time Multiplexing
`1-5 Comparison of Modulation Systems
`
`Chapter 2 Electrical Noise
`
`2-1 Thermal Noise in Resistors and Networks
`2-2 Noise in Receiving Antennas
`2-3 Noise in Diodes, Transistors, and FETs
`2-4 Definitions of Noise Terms
`2-5 The Noise Figure
`2-6 Amplifier Noise Considerations
`Appendix 2-] Choice of RS to Minimize the Noise Figure
`
`Chapter 3 Resonant Circuits and Impedance Transformation
`
`3-1 Series Resonance
`3-2 Parallel Resonance
`33 Parallel Resonance with Series Load Resistance
`3-4 Effects of Source and Coil Resistances
`3-5 Parallel-to-Series Conversions for RC and RL Circuits
`
`3-6 Tapped Resonant Circuits
`3-7 Tapped Coil with Mutual Inductance
`3-8
`Single—Tuned Transformer
`3-9 Double-Tuned Transformers
`Appendix 3-1 Tables of Design Formulas
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`11
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`39
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`Chapter 4 Small-Signal High-Frequency Amplifiers
`
`4-1 Definition of a Small-Signal Amplifier
`4-2 Models for the Active Device
`
`4-3 Amplifier Stability
`4-4 Achieving Stability
`4—5 Amplifier Power Gain
`4-6 Design with Unconditionally Stable Device
`4-7 Design with Potentially Unstable Device
`4-8 Maximum Transducer Gain with Specified Stability
`4-9 Alignability
`4-10 Overall Design of the Tuned Amplifier Stage
`Appendix 4-1 Relations Between Two—Port Parameters
`Appendix 4-2 The y Parameters of the Hybrid-Pi Circuit
`Appendix 4—3 Corresponding Quantities in Various Parameter
`Systems
`Appendix 4—4 RF Transistor Data
`Appendix 4-5 Self-Resonance Frequencies for Several Types of
`Capacitors and RF Chokes
`
`Chapter 5 Slnewave Oscillators
`
`5-1 The Criteria for Oscillation
`
`5-2 Negative Resistance Oscillators
`5-3 Feedback Oscillators
`A
`5-4 Oscillator Design Techniques
`5—5 Colpitts Oscillator Analysis and Design
`5-6 Other Oscillator Circuits
`5-7 Maximum-Efficiency Oscillators
`
`5—8 Crystal—Controlled Oscillators
`5-9 Buffering
`5-10 Frequency Stability
`
`Chapter 6 Phase-Locked Loops
`
`Simplified Explanation of PLL Operation
`6-1
`6-2 Linear Analysis of the PLL
`
`xii
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`I Contents
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`83
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`83
`86
`89
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`8&88
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`101
`102
`102
`110
`114
`
`115
`117
`
`125
`
`128
`
`128
`129
`132
`134
`135
`146
`151
`151
`160
`16]
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`6-3
`6-4
`6-5
`6-6
`6-7
`
`Phase-Locked Loop Terminology
`The Loop Oscillator
`The Phase Detector
`
`Phase—Locked Loop Applications
`Example of PLL Design
`
`Chapter 7 Mixers
`
`7-1
`7—2
`7-3
`7-4
`7—5
`7—6
`7-7
`7—3
`
`Basic Mixer Theory and Spectral Analysis
`Mixer Terminology
`Balanced Diode Mixers
`FET and BJT Mixers
`
`Transistor (BJT) Mixers
`FET Mixers
`
`JFET Mixer Design
`MOSFET Mixer Design
`
`Chapter 8 Modulation
`
`8—1
`8—2
`8-3
`8-4
`8-5
`8—6
`8-7
`8—8
`8-9
`
`Amplitude Modulation
`Double-Sideband and Single-Sideband Systems
`Generation of Single-Sideband Signals
`@513
`Angle Modulation
`Spectra of Angle-Modulated Waves
`Phasor Diagrams of Angle-Modulated Waves
`Comparison of FM and PM
`Pulse Modulation
`
`Information Capacity of a Channel
`Appendix 8-1 Derivation of the Spectrum of Angle-Modulated
`Waves
`
`Chapter 9 Amplitude Modulation Receivers
`
`9—1
`9-2
`9-3
`94
`9-5
`
`Receiver Performance Specifications
`The RF Amplifier
`The Mixer
`The Local Oscillator
`
`The IF Amplifier
`
`171
`173
`175
`179
`184
`
`188
`
`188
`192
`196
`201
`202
`204
`211
`212
`
`221
`
`222
`228
`229
`234
`240
`245
`247
`250
`25 8
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`263
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`266
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`266
`268
`269
`270
`271
`
`Contents
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`/
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`9-6
`9—7
`9-8
`9-9
`
`Interstage Filters
`Diode Envelope Detectors
`Product Detectors
`Automatic Gain Control
`
`9-10 Squelch Circuit
`9—11 The AM Receiver
`
`Chapter 10 FM and PM Receivers
`
`10-1 IF Amplifier Systems
`10-2 FM Detector Characteristics
`10-3 Practical Detectors
`
`10-4 FM Stereo Reception
`10-5 A Note on Quadraphonic Sound
`10-6 Audio Considerations: Preemphasis and Deemphasis
`10—7 An Example of a Complete FM Receiver
`
`Chapter 11 Television Receivers
`
`11-1 Monochrome Television
`
`11—2 Bandwidth of the Composite Video Signal
`11-3 Vestigial-Sideband Transmission
`11-4 The Monochrome Receiver
`11-5 Color Television
`11—6 Transmission of Chrominance Information
`11-7 The Color Receiver
`
`Chapter 12 Linear Power Amplifiers
`
`12-1
`12—2
`12—3
`12-4
`12-5
`12-6
`12-7
`12-8
`12-9
`
`Class A Amplification
`Class B Amplification
`Practical Considerations
`Intermodulation Distortion and Bias
`Drive and RF Feedback
`Wideband Transformers
`
`Power Combiners and Splitters
`Output Filters
`Heatsink Design
`Appendix 12-1 RF Power Transistors
`
`xiv
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`272
`280
`284
`286
`288
`289
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`295
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`295
`297
`301
`318
`322
`322
`325
`
`329
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`329
`332
`333
`334
`334
`340
`342
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`348
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`352
`355
`359
`361
`366
`371
`379
`382
`384
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`Chapter 13 Tuned Power Amplifiers
`
`13-1 Current-Source Class C Amplifiers
`13-2 Saturating Class C Amplifiers
`13-3 Solid State Class C Mixed-Mode Amplifiers
`13-4 Drive and Bias
`
`13-5 Amplitude-Modulation Characteristics
`13-6 Frequency Multipliers
`13-7 Impedance Matching
`
`Chapter 14 High-Efficiency Power Amplifiers
`
`14-1 Class D Amplification—Idealized Operation
`14-2 Class D Amplification—Practical Considerations
`14-3 Class E Amplification
`14—4 Class F Amplification
`14-5 Class S Amplification
`14—6 Other High-Efficiency Amplifiers
`Appendix 14-1 Tabulation of PA Characteristics
`Appendix 14-2 Performance of Class E PA
`Appendix 14-3 Distortion in Pulse—Width Modulation
`
`Chapter 15 cw, FM, and AM Transmitters
`
`15-1 CW Transmitters
`15-2 FM Transmitters
`
`15-3 Amplitude-Modulated Transmitters
`
`Chapter 16 Single-Sideband Transmitters
`
`16-1 Transmitter Organization
`16-2 Linear Amplifier Chains
`16-3 Peak and Average Power
`16-4 AGC and VSWR Protection
`
`16-5 Envelope Elimination and Restoration
`
`Index
`
`394
`
`394
`399
`402
`408
`410
`412
`417
`
`432
`
`433
`441
`448
`454
`458
`468
`472
`474
`476
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`477
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`482
`490
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`500
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`525
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`Contents
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`1 Radio
`
`Communication
`Systems
`
`1-1
`
`lntroductlon
`
`This book is devoted to the analysis and design of the electronic circuitry
`used in radio communication systems. It presumes that the reader is familiar
`with audio-frequency amplifiers; hence the primary emphasis will be on radio
`transmitter and receiver circuits. The transmitting and receiving antennas and
`the propagation path between them are important parts of an overall system,
`but a discussion of these elements is left to other texts.
`Communication systems transmit information in the form of electrical
`signals that represent speech, music, television pictures, scientific and busi-
`ness data, and so forth. The waveforms of these signals are complex and
`continually changing, but the frequency spectrum of the signals is usually
`limited to a specified bandwidth either by the nature of the signal source or by
`filters in the transmitting equipment. Since many of these signals occupy a
`frequency band that extends downward to a few hertz,
`they cannot be
`transmitted in their original form over a common transmission path because it
`would not be possible to separate them at the receiving end. A separate
`transmission line or separate radio path for each signal would not be feasible
`from either an economic or a practical standpoint. Hence the overall com-
`munication system must provide a means for simultaneous transmission of a
`number of signals either by shifting them into different parts of the frequency
`spectrum or by sending samples of the signals on a time-shared basis.
`The wavelength (A) in meters of a radio wave is given by c/f, in which 0 is
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`the velocity of light (3X10‘ meters per second), and f is in hertz. (For RF
`calculations it is convenient to remember that f in megahertz X A in meters =
`300.) A radio antenna should have a physical size of one-half wavelength or
`more for reasonable efficiency. Hence, as the transmission frequency is
`increased, the physical size and cost of the antenna are reduced and its
`efliciency increases.
`
`1-2 Elements of a radlo system
`
`The process whereby the original message is converted into a new form
`suitable for radio transmission is called modulation. The modulation process
`causes some property—such as the amplitude, frequency, or phase—of a
`high—frequency carrier‘ wave to be deviated from its unmodulated value by an
`amount proportional to the instantaneous value of the modulating (message)
`signal. Thus the content of the original message is shifted to a portion of the
`frequency spectrum in the vicinity of the carrier frequency. In the receiver
`this process is reversed in a detector that recovers the original signal.
`Figure 1-1 shows a simplified block diagram of a radio transmitter and
`receiver in order to illustrate the signal processing that takes place. The
`function of each block is explained below.
`
`1. The source of the message signal may be a microphone, phono pickup,
`television camera, or other device that transforms the desired information
`into an electrical signal.
`2. The signal is amplified and often passed through a low-pass filter to
`limit the bandwidth.
`3. The RF oscillator establishes the carrier frequency or some submultiple
`of it. Since good frequency stability is required to keep the transmitter on its
`assigned frequency, the oscillator is often controlled by a quartz crystal.
`4. One or more amplifier stages increase the power level of the signal from
`the oscillator to that needed for input to the modulator. Class C operation is
`used wherever possible to obtain high efiiciency. Tuning of the output circuits
`to a harmonic of the input frequency results in “frequency multiplication" so
`that the final carrier frequency can be a multiple of the oscillator frequency.
`5. The modulator combines the signal and carrier frequency components
`to produce one of the varieties of modulated waves discussed in Section 1-3.
`In the simplified system shown in Fig. 1-1 the output signal spectrum lies in
`the vicinity of the desired RF carrier frequency. In many transmitters a
`second oscillator and mixer (similar to blocks 10 and 11) are inserted between
`
`
`‘The “carrier" may be a sinusoidal wave or a train of pulses.
`
`2
`
`/ Radio Communication Systems
`
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`blocks 5 and 6 in order to shift the modulated wave to a higher-frequency
`range.
`6. Additional amplification may be required after modulation to bring the
`power level of the signal to the desired value for input to the antenna.
`7. The transmitting antenna converts the RF energy into an electromag-
`netic wave of the desired polarization. If a single (fixed) receiver is to be
`reached, the antenna is designed to direct as much of the radiated energy as
`possible toward the receiving antenna.
`8. The receiving antenna may be omnidirectional for general service or
`highly directional for point-t0 point communication. The wave propagated
`from the transmitter induces a small voltage in the receiving antenna. The
`range of amplitudes of the induced antenna voltage may be from tens of
`millivolts to less than 1 microvolt, depending upon a wide variety of con-
`ditions.
`9. The RF amplifier stage increases the signal power to a level suitable for
`input to the mixer and it helps to isolate the local oscillator from the antenna.
`This stage does not have a high degree of frequency selectivity but does serve
`to reject signals at frequencies far removed from the desired channel. The
`increase in signal power level prior to mixing is desirable because of the noise
`that is inevitably introduced in the mixer stage.
`10. The local oscillator in the receiver is tuned to produce a frequency fw
`that differs from the incoming signal frequency flu: by the intermediate
`frequency flp; that is, fLo can be equal to fRF+ in: or fu— fly.
`1]. The mixer is a nonlinear device that shifts the received signal at flu: to
`the intermediate frequency fIF. Modulation on the received carrier is also
`transformed to the intermediate frequency.
`12. The IF amplifier increases the signal to a level suitable for detection
`and provides most of the frequency selectivity necessary to “pass” the
`desired signal and filter out the undesired signals that are found in the mixer
`output. Because the tuned circuits in blocks 11 and 12 always operate at a
`fixed frequency (flp),
`they can be designed to provide good selectivity.
`Ceramic or crystal filters are often used.
`13. The detector recovers the original message signal from the modulated
`IF input.
`14. The audio or video amplifier increases the power level of the detector
`output to a value suitable for driving a loudspeaker, a television tube, or other
`output device.
`15. The output device converts the signal information back to its original
`form (sound waves, picture, etc.).
`
`In addition to the desired signal that is processed by the receiver, electrical
`noise is added in the transmission path, and is generated within the RF
`
`4
`
`1 Radio Communication Systems
`
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`amplifier. local oscillator, mixer, and so forth. The block diagrams shown in
`Fig. 1-1 are for illustrative purposes only. In practice, so many variations in
`transmitter and receiver systems are encountered that no single block diagram
`could even be considered typical. The general layout of receivers or trans-
`mitters for particular applications will be discussed in detail in later chapters.
`
`1-3 Modulation
`
`To extend the concept of modulation introduced in the previous section, the
`basic definitions of commonly used types of modulation will be given here.
`Let the voltage of an unmodulated carrier wave be given by
`
`v(t) = Vc sin (wet + 4;) = Vc sin 6(t)
`
`(l—l)
`
`is the carrier (radian) frequency, Vt the amplitude, and ()5 an
`in which a),
`arbitrary phase angle.
`
`Amplitude Modulation
`
`In an amplitude—modulated (AM) wave, the deviation of the amplitude V,
`from its unmodulated value is proportional to the instantaneous value of the
`modulating wave. In other words, if the modulating signal is F(t), the carrier
`amplitude must vary in time according to the expression
`
`Vc(t) = Vc[1+ maF(t)]
`
`(1-2)
`
`in which ma is the modulation factor. The value of maF(t) cannot exceed
`unity without
`introducing distortion. Figure 1-2a illustrates a modulating
`signal F(t), and the corresponding amplitude-modulated wave is shown in Fig.
`l-2b. Note that the envelope of the AM wave [given by (1-2)] has the same
`shape as the modulating signal.
`
`Angle Modulation
`
`In angle modulation, the angle 0(t) in (1-1), rather than the amplitude, is
`varied from its unmodulated value by the modulating signal. Phase and
`frequency modulation are particular forms of angle modulation. In phase
`modulation (PM), the angle 0(t) in (1-1) is made to vary proportionately to the
`modulating signal F(t). In frequency modulation (FM) the instantaneous
`frequency,
`
`Modulation
`
`/
`
`5
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`g. 1-2
`
`(a) A triangular medulailng slgnal; and the resulting (b) AM, (c), FM,
`
`
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`is caused to vary from its unmodulated value in a manner proportional to
`F(t). In both cases the amplitude of the wave remains constant. Figure l-2
`illustrates AM, FM, and PM waves resulting from a triangular modulating
`signal.
`
`Pulse Modulation
`
`In pulse modulation systems, the “carrier” consists of a train of pulses that
`can be modulated in amplitude, repetition frequency, or spacing in a manner
`completely analagous to the AM, FM, and PM waves previously discussed.
`The sampling theorem shows that the continuous transmission of a message
`signal is unnecessary—it can be reconstructed completely from samples taken
`at a rate at least twice the highest frequency present in the signal. Thus, for a
`signal that is bandlimited to 4kHz, a modulated pulse train with a mean
`repetition frequency of 8 kHz is sufficient and the pulses can be of arbitrarily
`short duration.
`
`In pulse code modulation, each sample is represented by a set of seven or
`more pulses (or spaces) that represent a binary code for the sample amplitude.
`This system has greater immunity to noise' at the expense of higher pulse
`repetition rates and greater bandwidth.
`
`1-4 Frequency and Time Multiplexing
`
`The modulated pulse train produced by any of the foregoing methods is still
`of relatively low frequency. For radio transmission, this pulse train forms the
`modulating signal that in turn modulates (by AM, FM, etc.) a high-frequency
`carrier.
`
`In frequency-division multiplexing (FDM) the modulating process shifts
`the signal to a portion of the frequency spectrum in the vicinity of the carrier
`frequency. Since the portion of the spectrum to be utilized is determined by
`the carrier frequency, different signals can modulate carriers of different
`frequencies and all of them can be transmitted simultaneously. The receiver
`can choose the desired signal band by means of selective filters. This is done,
`for example, in AM, FM, and TV broadcasting, and in long-distance carrier-
`telephone systems.
`Time-division multiplexing (TDM) is an alternate process in which a
`number of signals are transmitted over a common facility by letting the
`signals occupy the same frequency band on a time—sharing basis. This method
`is used with pulse modulation systems. The individual “samples” of a given
`signal are of sufficiently short duration that the time between samples can be
`used to transmit other signals. In such systems the transmitter is switched or
`
`Frequency and Time Multiplexing
`
`/
`
`7
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`commutated to each signal channel sequentially. The receiving system must
`then be switched in synchronism with the transmitter to separate the various
`signals prior to final demodulation.
`
`1-5 Comparison of Modulation Systems
`
`Each of these modulation systems has its good and bad features. Amplitude
`modulation utilizes the simplest detectors and requires the least bandwidth
`(particularly if only one sideband is transmitted). However, it has the lowest
`immunity to noise and both transmitter and receiver circuitry becomes more
`complicated if single-sideband transmission is used.
`Wideband frequency modulation employs simpler transmitter circuitry and
`provides much better rejection of noise and interfering signals than AM. It
`requires a bandwidth approximately five times that of a comparable AM
`signal, however. Pulse code modulation (PCM) provides even better trans-
`mission in the presence of noise but requires a further increase in circuit
`complexity and bandwidth. The choice of a particular technique depends
`upon the communication system requirements.
`
`a
`
`/ Radio Communication Systems
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`2 Electrical
`
`Noise
`
`Anyone who has tried to watch snowy images from a weak television signal
`or who has listened to a distant radio station against a background of static
`crashes has met electrical noise. Noise is always present in communications
`systems, but under normal operating conditions it goes unnoticed because the
`signal
`levels are much higher than the noise levels. A weak signal ac-
`companied by noise can be amplified if the associated noise level
`is low
`relative to the signal level. However, if the noise level is close to the signal
`level, amplification will be useless because any amplifier will amplify both the
`incoming signal and the incoming noise as well as adding more noise of its
`own. This process is evident in any receiver when an incoming signal fades
`into the noise or the external noise level rises to the point that it drowns out
`the signal.
`To simplify the mathematical detail in this and the following chapters, the
`desired signal
`is assumed to be a sinusoid or a group of sinusoids that
`comprise the transmitted information. This may be called a deterministic
`signal. Noise is defined as any extraneous electrical disturbance tending to
`interfere with the normal reception of the transmitted signal. Noise can
`consist of deterministic signals from unwanted sources plus random fluctua-
`tions of voltages and currents caused by physical phenomena. Various types
`of random noise are thermal noise, shot noise, Johnson noise, and flicker
`noise.
`
`One of the goals of communications system design is to keep the ratio of
`average (or peak) signal power to average noise power so large that the noise
`has no harmful effects on system performance. Techniques for doing this
`include (1) using powerful transmitters and high-gain antennas to develop
`
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`

`large signals at the receiver, (2) designing amplifier and mixer circuits so that
`they introduce a minimum amount of additional noise when processing
`signals, and (3) using modulation or coding schemes that facilitate the separa-
`tion of signals from noise. In the case of man-made noise sources (e.g.,
`automobile ignitions) there exists the fourth option of suppressing the noise at
`its source by filtering, bypassing, or redesign. Attention is usually given to
`each of these four options, and the mix selected is determined by such factors
`as cost, weight, and efficiency.
`It is desirable to characterize electrical noise as accurately as possible. A
`common characteristic of most
`types of noise, however,
`is its nondeter-
`ministic nature; that is, the exact waveform of the noise cannot be predicted.
`A measure of the amount of electrical noise can be obtained by connecting a
`meter across a noise source to measure the average, peak, rectified-average,
`or true-rms voltage (or current). Relationships between these quantities are
`different for different types of noise, that is, the average value may be zero
`whereas the others are not. The true-rms voltage (or current) can be used to
`calculate the average noise power delivered to a resistive load. As will be
`shown, the measured value depends upon the spectrum of the noise source
`and the frequency response of the measuring instrument.
`The frequency-domain characterization of