Newnes Communications Technology Handbook

431 Pages·1994·10.355 MB·English

by  Geoff Lewis (Auth.)

Dedicated to my grandchildren, Amy, Jonathan,
Phillip and Simon, with the hope that their lives may
be enriched and enhanced by the rapidly developing
spread of communications technology.
Newnes
Communications Technology
Handbook
Geoff Lewis
BA, MSc MRTS, MIEIE
n N E W N ES
Newnes
An imprint of Butterworth-Heinemann Ltd
Linacre House, Jordan Hill, Oxford 0X2 8DP
6^ A member of the Reed Elsevier pic group
OXFORD LONDON BOSTON
MUNICH NEW DELHI SINGAPORE SYDNEY
TOKYO TORONTO WELLINGTON
First published 1994
© G.E. Lewis 1994
All rights reserved. No part of this publication may be reproduced in any
material form (including photocopying or storing in any medium by electronic
means and whether or not transiently or incidentally to some other use of this
publication) without the written permission of the copyright holder except in
accordance with the provisions of the Copyright, Designs and Patents Act 1988
or under the terms of a licence issued by the Copyright Licensing Agency Ltd,
90 Tottenham Court Road, London, England W1P 9HE. Applications for the
copyright holder's written permission to reproduce any part of this publication
should be addressed to the publishers
British Library Cataloguing in Publication Data
Lewis, Geoffrey E.
Newnes Communications Technology Handbook
I. Title
621.382
ISBN 0 7506 1729 2
Library of Cataloguing in Publication Data
Lewis, Geoffrey E.
Newnes communications technology handbook/Geoffrey E. Lewis
p. cm
Includes bibliographical references and index.
ISBN 0 7506 1729 2
1. Telecommunication-Handbooks, manuals, etc. I. Title.
TK5101.L483 93-45442
621.382-dc20 CIP
Typeset by TecSet Ltd, Wellington, Surrey
Printed and bound in Great Britain by Bath Press, Avon
Preface
The traditional boundaries between communications, Two distinct routes to the location of information
information processing, television and computing have been provided. The extensive contents listing gives
systems have disappeared. Today, the term telecommu- a fast access to general information about a particular
nications virtually represents an all-embracing technol- topic, while the conventional index provides more
ogy. The reasons for this convergence are related to the specific guidance. In addition, extensive cross-referen-
rapid development and expansion of electronic and cing has been used to provide a link between the
semiconductor technologies that have occurred over the converging topic areas.
recent past. Because a mathematical expression can often explain
This book owes its origins to the difficulties a system behaviour in a very concise manner, these
experienced in finding suitable reference information concepts have been included where it is felt to be
needed to understand the terminology and jargon of this necessary. However, in many cases, these expressions
expanding field. can be glossed over at first reading without losing too
The book is basically presented as a set of much of the context.
monographs organised in an alphabetical manner. Finally, the book is presented as an aid to all the
Each section provides an introduction to the topic, students, technicians, engineers, systems managers and
followed by more detailed descriptions and explana- those who work on the fringes of telecommunications,
tions. Many of the miscellaneous terms associated with that have a need to understand new areas of this rapidly
the subject are separately listed and each section ends expanding business.
with a listing of references suggested for further reading. Geoff Lewis
1 Analogue systems and
concepts
Analogue systems are characterised by electrical signals 1.1 Amplifiers
that vary continuously between two extreme amplitude
levels. There is thus an infinity of actual levels that such Amplifiers can be sub-divided into many different types,
signals can assume. Compared with other forms, each designed for some specific purpose. But depending
analogue signals usually occupy minimum bandwidth. upon the time period for which the output current flows
with respect to a changing input signal, they can be
assigned to one of the classifications shown in Fig 1.1.
χ
Figure 1.1 Amplifier classification by transfer characteristic.
2 Analogue systems and concepts
Class A the tuned circuit extracting the required harmonic
The output current flows for the whole or 360° of the frequency from the distortion components.
input sinusoidal signal. The amplifier is biased to the
mid-point of the straight part of its characteristic as Class D
shown, to ensure the minimum of distortion. As a large For audio and low frequency applications, the require-
signal or power amplifier, the efficiency as measured by ments of low distortion and high power output produce
the ratio: a conflict. The low efficiency of Class A is due to the
excessive power dissipation in the output stages.
(ac signal power output/dc power input) χ 100% Operation has to remain within the safe working area
to protect the amplifier from thermal damage (see Fig.
is theoretically 50%. In a practical case however, an 1.2).
efficiency in excess of 35% is rarely achieved. Because of
the high degree of linearity, these amplifiers are
commonly used for audio or low frequency applications.
Class Β
The output current flows for only 180°, or half of the
sinusoidal input signal. The amplifier being biased to
Maximum power
cut-off causes such distortion that it effectively acts as a dissipation
half-wave rectifier. The theoretical efficiency is 78.5% ν curve (12 W)
(π/4 χ 100%), and in practice this rarely exceeds 65%.
In low frequency and audio applications, Class Β is used Safe
in push-pull, using two stages, one for the amplification working
area
of each half cycle. Even so, it is often necessary to take
care to minimise the effect of cross-over distortion. For
radio frequency applications, this class can be used with
0 2 4 6 8 10 12 V
a tuned circuit load, where the natural flywheel action N
replaces the missing half cycle.
Figure 1.2 Maximum power dissipation curve.
Class AB
The output current flows for more than 180° of the input If the amplifier can be operated in either the saturated
cycle as the amplifier is biased to the projected cut-off or cut-off state, then the power dissipation will be low,
point. This is defined as the bias point at which the because either the voltage across the active device or the
amplifier current would fall to zero if the characteristic current through it will be very low. Further, if the
had been linear. The theoretical efficiency is somewhat amplifier can be switched rapidly between these states,
less than that for Class B, but when used in a push-pull the power dissipation during the transition through the
circuit, the cross-over distortion is reduced. The class is active region will also be very low. The output power is
very commonly used for audio power amplification or as controlled by varying the time spent in these two
a radio frequency amplifier with a tuned circuit load. extreme states, this depending upon the instantaneous
amplitude of the signal to be amplified. Thus Class D
Class C operation is a form of pulse width or duration
These amplifiers are biased beyond the cut-off point so modulation, which can achieve a practical efficiency as
that output current flows for much less than 180° of the high as 97%.
input cycle. The typical angle of flow, which is defined Typically the switching frequency exceeds 50 kHz, but
by the ratio: significantly higher values may be used. This has the
advantages of smaller filters to remove the switching
Time for which output current flows frequency from the output, lower radiation of unwanted
χ 360°
Period of the input signal interference, reduced intermodulation distortion and a
wider bandwidth.
is less than 120°. Theoretically the efficiency approaches
100% as the angle of flow approaches zero. In practice, Class Ε
this value can be as high as 90%. Because of the severe When a switching mode is used for RF power amplifiers
distortion that Class C operation generates, this class and the transition periods between the On and Off states
can only be used with a tuned circuit load, i.e., as a radio become an appreciable part of the period of the signal
frequency amplifier, or as a harmonic generator, with being processed, a significant power loss arises. Class Ε
Analogue systems and concepts 3
overcomes this problem by including LC filters between A further feedback path provided by R3 operates over
the switcher and the load to delay the current transition the dumper stages.
relative to the voltage swing. This ensures that either the
voltage or the current is simultaneously zero during the
switching period. 1.2 Distortions (see also under
Theoretically the efficiency for Class Ε can be 100% Measurements, p. 152)
with practical values in excess of 90% at VHF
frequencies being achieved. The information carried by an analogue signal is
Class Ε is equally applicable to thermionic or contained within the signal's instantaneous amplitude,
semiconductor RF power devices and has a low frequency and phase. Any phenomenon that disturbs
sensitivity to component tolerances this relationship, distorts the waveform and reduces the
intelligibility of the signal. Since this is an unwanted
feature of the signal processing, distortion has a noise-
Current dumping amplifiers like effect. This effect can be quantified either by
This system was devised to provide audio frequency measurement or analysis and can be expressed as a
power amplification with very low distortion. Operation kind of signal to noise ratio, usually in the form:
is based on the premise that a system that provides a
distortionless no load voltage and an output impedance Magnitude of distortion component
χ 100%
that is independent of the load, must be distortionless. Magnitude of undistorted signal
The basic circuit is shown in Fig. 1.3, where Aj is a small
Class A amplifier that is capable of providing the total
output voltage swing, but with a very limited current Amplitude or non-linear distortion
capability. The emitter follower transistors Tr\, and Tri In a practical amplifier, the dynamic transfer character-
are designed to dump current into the load under the istic is always to some extent non-linear. That is, equal
control of Ai. In this way, the two sections combine to incremental inputs do not produce equal incremental
provide the total output power. The very low level of outputs. This is particularly noticeable in large signal
distortion is achieved by the use of the feedback power amplifiers, where a saturation condition arises. A
networks. Capacitor C\ ensures that the integrator useful measure of a system's performance in this respect
circuit which has a response that extends to well is shown by the 1 dB compression point as indicated in
beyond the maximum operating frequency, provides a Fig. 1.4. This point represents the output power level
gain that falls with rising frequency. Ri provides a where the signal is 1 dB below the value that would
feedback voltage that depends upon the impedance of L\ obtain if the system were linear. In the case shown, 1 dB
and thus has an opposite effect to that produced by C\. compression occurs at an input level of 7 dBm.
Figure 1.3 Basic current dumping amplifier circuit.
4 Analogue systems and concepts
Attenuation distortion
I Intercept
In any transmission system, some frequencies will be
attenuated more than others. If the transmitted signal
contains frequency components that are so attenuated,
then the wave shape of the original signal will become
distorted.
Cross-over distortion
When push-pull amplifiers are biased to operate in Class
B, the non-linearity near to the origin causes distortion
during the zero crossing period as shown in Fig. 1.5, the
distortion component being predominantly the third
harmonic.
Group delay distortion
Any non-linearity in the phase versus frequency
response of a circuit or system, results in group delay,
which is defined by the slope of this characteristic and
-30 -20 -10 0 710 15 20
given by: group delay = δβ/δω, where β is the phase
Power input (dBm)
angle in radians and ω is the angular velocity = 2π/. The
Figure 1.4 Measurement of intermodulation and non- same relationship for / in Hz and phase in degrees is
3
linearity. given by T = Αφ/Afx 2.778 χ 10~ .
D
Figure 1.5 Class Β push-pull operation and cross-over distortion.
Analogue systems and concepts 5
The effect of group delay on a modulated wave is to with increasing input, a point is reached at which
generate envelope distortion. The spectrum of a saturation occurs due to over driving. The output
modulated wave has components, f and f ± nf (see signal now contains fundamental and harmonic
c c m
p. 186), where η is an integer representing the harmonics components. If the signal level is now slowly reduced,
of / . Any non-linear phase shift will thus distort the the harmonic content falls faster than the fundamental.
m
modulation envelope. In the case of audio signals, this The 3rd harmonic falls at 3 dB/dB as opposed to the
is not of great importance, because the ear is fairly 1 dB/dB of the fundamental. The two slopes intersect at
tolerant of this form of distortion. However, for video the intercept point. If this point is known for a given
signals, the phase shift represents a time shift of the fundamental output power, the level of the third-order
component frequencies so that the result becomes term can be evaluated. In the example shown, the
obvious to the eye. Differential group delay is defined intercept occurs at an input of 15dBm.
as the group delay variation over the total bandwidth
being considered. Phase distortion
When the signal propagation time through a system is
Harmonic distortion frequency dependent, the different components of a
Due to the non-linearity of an amplifier's transfer complex wave will arrive at a load at relatively
characteristic, frequency components that were not different times. This will result in change of wave
present at the input will appear at the output. The shape that in audio systems is not very important as
magnitude of each unwanted component normally the ear is not offended by such phase distortion.
diminishes as the order of the harmonic, in most cases However, in the case of video signals, this can lead to
the second- and third-order components being most obvious picture impairments. For zero phase distor-
troublesome. tion, the phase and group velocities of the complex
wave should be constant over the range of frequencies
Intermodulation distortion of interest.
When a signal that contains two or more components
passes through a non-linear system, a kind of modula- Total harmonic distortion (THD)
tion effect results in the production of sum and This defines the distortion factor for a system. If D , £3,
2
difference frequencies plus various beat notes that are Z>4, etc., represent the distortion components at 2nd,
not harmonically related to the original signal. In the 3rd, 4th harmonic, etc., then the total harmonic
case of audio systems, these produce discordant sounds. distortion D, is given by:
The spectrum of second- (2IM) and third-order (3IM)
intermodulation products are shown in Fig. 1.6. Except D = yf(D\ + D\ + D\ + ...)
in the case of very wide band systems, the 2nd-order
terms (fi - f\,fa + f\) will fall well outside the pass band
and only the 3rd-order terms (2/i - f and 2f -f\) will
2 2
be troublesome.
Figure 1.4 uses the third-order intercept point to 1.3 Feedback amplifiers
equate 3IM with the linear response of a power
amplifier that has a nominal gain of 10 dB. Over the Signal feedback from output to input is a technique used
linear part of the curve, the gain slope is 1 dB/dB, but to modify the performance of an amplifier. For example,
for any given amplifier, there is a gain χ bandwidth
product that is a constant and by using feedback, gain
can be traded for bandwidth and vice versa. This effect is
shown in Fig. 1.8. The magnitude of the sample or
fraction obtained from the output signal is given by
Q) b = Vfb/vo
Ό
D I
α where vn, and v are feedback and output voltages,
Ε CM 0
< respectively (see Fig. 1.7).
νπ, can be used in one of two ways. As (regenerative)
positive feedback (PFB), or (degenerative) negative
feedback (NFB), where it either aids (in phase) or
opposes (anti-phase) the original input signal ν,·. The
Figure 1.6 Frequency spectrum of 2nd- and 3rd-order
intermodulation products. former leads to increased gain with reduced bandwidth
6 Analogue systems and concepts
100
_ 80
CO
* A
•£ 60
CO
CJ)
α
η ο
f -2 40
c
ω
α
Ο
20
(a)
1 3 4 5 6 7
10 10 10 10 10 10 10
Frequency (Hz)
JL
Figure 1.8 The gain Χ bandwidth product and frequency
response of an operational amplifier.
amplifier output resistance R is the same for both PFB
0
and NFB.
(0 (d) Effects of feedback
Neglecting any loading effect transmitted through the
Figure 1.7 Application and derivation of feedback signal vn>. feedback loop, to a first approximation, the effect on
both R and R\ is as follows:
0
and ultimately to oscillations, while the latter has the
for voltage derived feedback, R becomes R/D
opposite effect. Q 0
for current derived feedback R becomes R.D
If the amplifier open-loop gain (gain before feedback) 0 0
for series applied feedback R\ becomes R.O
is A, then the closed-loop gain is given by: {
for shunt applied feedback R becomes R\/D.
{
A' = A/(\±ßA) where D = 1 + β A, and is sometimes known as the
densensitising factor.
plus for NFB and minus for PFB. In a practical Negative feedback improves the amplifier perfor-
situation, both A and β may be complex quantities. As mance by reducing the sensitivity to drift in component
the gain bandwidth product is a constant, the bandwidth tolerances due to heat and time, the level of distortion
will be modified in the same proportion as the change of generated, the response to noise, the degree of phase
gain. shift that it generates, and increases its bandwidth.
Feedforward amplifiers
Application of feedback signal
This is a technique that can be used to minimise the
Figure 1.7 shows how the feedback signal can be applied
distortion produced in a power amplifier. The output
and derived. (At (a) and (c) the signal is applied either in
signal is sampled and this is then compared, after
series or in shunt (parallel) with Vj the input signal. No
making due allowance for any phase shift, with the input
matter whether the feedback is positive or negative, the
signal. Any difference between the two must be
resulting effect on the input resistance Ri will be the
representative of the distortion that is occurring and
same.
this is then used to generate a correcting signal that is fed
forward to be added to the output signal to correct the
Generation of feedback signal distortion.
At (b) and (d), yn, is either proportional to the load voltage
or the load current. Either = v χ Ri/{R\ + Ri) or Gyrators
0
Vfb = /'l Χ ^i · In either case, the values of R\ and Ri are This is a circuit arrangement that can be used to
chosen to be high enough or low enough so as not to synthesise an inductor, without using a physical coil. An
unduly load the amplifier output. Again the effect on the ideal gyrator is a non-reciprocal two-port device whose