Netflix 1030 - Page 1
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`_
` ’ Electronic
`Communication
` Systems
`
`
`
`Fourth Edition
`
`GEORGE KENNEDY
`
`Supervising Engineer,
`Overseas Telecommunications Commission,
`Australia
`
`BERNARD DAVIS
`Electronics Instructor
`
`Dade County Public Schools
`
`GU.-'lVL'0E
`
`Macmillan/McGraw-Hill
`
`New York, New York
`Columbus, Ohio
`Mission Hills, California
`Peoria, Illinois
`
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`Netflix 1030 - Page 2
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`This textbook has been prepared with the assistance of Publishing Advisory Service.
`
`Cover photo: Copyright H. Mark Weidman
`
`Library of Congress Cataloging-in-Publication Data
`
`Kennedy, George, date
`Electronic Communication systems / George Kennedy, Bernard Davis,
`—4th ed.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN 0-O2-800592-9 (hard cover)
`1. Telecommunication.
`I. Davis, Bernard, date.
`TK5101.K39
`1993
`'
`621 .382—dc2O
`
`II. Title.
`
`92-3905
`CIP
`
`Electronic Communication Systems, Fourth Edition
`
`Copyright © 1993 by the Glencoe Division of Macmillan/McGraw-Hill School Publishing
`Company. All rights reserved. Copyright © 1985, 1977, 1970 by McGraw-Hill, Inc. All
`rights reserved. Except as permitted under the United States Copyright Act, no part of
`this publication may be reproduced or distributed in any form or by any means, or stored
`in a database or retrieval system, without prior written permission of the publisher.
`
`Send all inquiries to:
`GLENCOE DIVISION
`Macmillan/McGraw-Hill
`936 Eastwind Drive
`Westerville, OH 43081
`ISBN 0-O2-800592-9
`
`Printed in the United States of America.
`
`23456789
`
`RFID-C
`
`9998979695949392
`
`
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`2 ELECTRONIC COMMUNICATION SYSTEMS
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`use of satellites andfiber optics has made communications even more widespread, with
`an increasing emphasis on computer and other data communications.
`A modern communications system is first concerned with the sorting, process-
`ing and sometimes storing of information before its transmission. The actual transmis-
`sion then follows, with further processing and the filtering of noise. Finally we have
`reception, which may include processing steps such as decoding, storage and interpre-
`tation. In this context, forms of communications include radio telephony and telegra-
`phy, broadcasting, point—to—point and mobile communications (commercial or mili-
`tary), computer communications, radar, radiotelemetry and radio aids to navigation.
`All these are treated in turn, in following chapters.
`In order to become familiar with these systems, it is necessary first to know
`about amplifiers and oscillators, the building blocks of all electronic processes and
`equipment. With these as a background, the everyday communications concepts of
`noise, modulation and information theory, as well as the various systems themselves,
`may be approached. Any logical order may be used, but the one adopted here is, basic
`systems, communications processes and circuits, and more complex systems, is con-
`sidered most suitable. It is also important to consider the human factors influencing a
`particular system, since they must always affect its design, planning and use.
`
`l "2
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`COMMUNICATIONS SYSTEMS
`Before investigating individual systems, we have to define and discuss important terms
`such as information, message and signal, channel (see Section 1-2.3), noise and dis-
`tortion, modulation and demodulation, and finally encoding and decoding. To corre-
`late these concepts, a block diagram of a general communications system is shown in
`Figure 1-1.
`
`1-2.1 Information
`The communications system exists to convey a message. This message comes from the
`information source, which originates it, in the sense of selecting one message from a
`group of messages. Although this applies more to telegraphy than to entertainment
`broadcasting, for example,
`it may nevertheless be shown to apply to all forms of
`communications. The set, or total number of messages, consists of individual mes-
`
`Encoding
`modulation
`(distortion)
`
`(distortion)
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`Decoding
`demodulation
`(distortion)
`
`
`
` Destination
`
`Noise
`source
`
`
`
`Information
`source
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`FIGURE 1-1 Block diagram of communications system.
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`INTRODUCTION TO COMMUNICATIONS SYSTEMS 3
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`sages which may be distinguished from one another. These may be words, groups of
`words, code symbols or any other prearranged units.
`Information itself is that which is conveyed. The amount of information con-
`tained in any given message can be measured in bits or in dits, which are dealt with in
`Chapter 13, and depends on the number of choices that must be made. The greater the
`total number of possible selections, the larger the amount of information conveyed. To
`indicate the position of a word on this page, it may be sufficient to say that it is on the
`top or bottom, left or right side, i.e.,
`two consecutive choices of one out of two
`possibilities. If this word may appear in any one of two pages, it is now necessary to
`say which one, and more information must be given. The meaning (or lack of meaning)
`of the information does not matter, from this point of view, only the quantity is
`important. It must be realized that no real information is conveyed by a redundant (i.e. ,
`totally predictable) message. Redundancy is not wasteful under all conditions. Apart
`from its obvious use in entertainment, teaching and any appeal to the emotions, it also
`helps a message to remain intelligible under difficult or noisy conditions.
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`1-2.2 Transmitter
`
`Unless the message arriving from the information source is electrical in nature, it will
`be unsuitable for immediate transmission. Even then, a lot of work must be done to
`make such a message suitable. This may be demonstrated in single-sideband modula-
`tion (see Chapter 4), where it is necessary to convert the incoming sound signals into
`electrical variations, to restrict the range of the audio frequencies and then to compress
`their amplitude range. All this is done before any modulation. In wire telephony no
`processing may be required, but in long-distance communications, a transmitter is
`required to process, and possibly encode, the incoming information so as to make it
`suitable for transmission and subsequent reception.
`is
`Eventually,
`in a transmitter, the information modulates the carrier, i.e.,
`superimposed on a high—frequency sine wave. The actual method of modulation varies
`from one system to another. Modulation may be high level or low level, and the system
`itself may be amplitude modulation, frequency modulation, pulse modulation or any
`variation or combination of these, depending on the requirements. Figure 1-2 shows a
`high-level amplitude—modulated broadcast transmitter of a type that will be discussed
`in detail in Chapter 6.
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`.
`
`El
`
`l E
`
`E
`
`
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`::dVS§v::
`amplifiers
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`RF buffer
`amplifier
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`
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`oscillator
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`
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`RF output
`power amplifier
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`
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`
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`Modulation
`Modulation
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`power
`voltage
`amplifiers
`amplifiers
`
`
`
`
`Modulation
`.
`processing
`
`
`
`
`Modulation in
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`FIGURE 1-2 Block diagram of typical radio transmitter.
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