Communications Technology Handbook

Geoff Lewis

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781136025938
• Edition: 2
• Topic: Languages & Linguistics
• Subtopic: Communication Studies
• Index: Languages & Linguistics

This is the first point of reference for the communications industries. It offers an introduction to a wide range of topics and concepts encountered in the field of communications technology. Whether you are looking for a simple explanation, or need to go into a subject in more depth, the Communications Technology Handbook provides all the information you need in one single volume.This second edition has been updated to include the latest technology including:
Video on Demand
Wire-less Distribution systems
High speed data transmission over telephone lines
Smart cards and batteries
Global positioning Systems
The contents are ordered initially by communications systems. This is followed by an introduction to each topic and goes on to provide more detailed information in alphabetical order. Every section contains an explanation of common terminology, and further references are provided. This approach offers flexible access to information for a variety of readers. Those who know little about communications professionals, the book constitutes a handy reference source and a way of finding out about related technologies. The book addresses an international audience by referring to all systems and standards throughout.This book has been revised to include new sections on:
* Video on demand
* Wire-less distribution systems
* High speed data transmission over telephone lines
* Smart cards
* Global positioning systems * provides a basic understanding of a wide range of topics
* offers a flexible approach for beginners and specialists alike
* addresses an international audience by referring to all systems and standards throughout

1 Analogue systems and concepts

Analogue systems are characterised by electrical signals that vary continuously between two extreme amplitude levels. There is thus an infinity of actual levels that such signals can assume. Compared with other forms, analogue signals usually occupy minimum bandwidth.

1.1 Amplifiers

Amplifiers can be sub-divided into many different types, each designed for some specific purpose. But depending 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.

Figure 1.1 Amplifier classification by transfer characteristic.

Class A

The output current flows for the whole or 360° of the input sinusoidal signal. The amplifier is biased to the mid-point of the straight part of its characteristic as shown, to ensure the minimum of distortion. As a large signal or power amplifier, the efficiency as measured by the ratio:

(ac signal power output/dc power input) x 100%

is theoretically 50%. In a practical case however, an 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 B

The output current flows for only 180°, or half of the sinusoidal input signal. The amplifier being biased to cut-off causes such distortion that it effectively acts as a half-wave rectifier. The theoretical efficiency is 78.5% (π/4 x 100%), and in practice this rarely exceeds 65%. In low frequency and audio applications, Class B is used in push-pull, using two stages, one for the amplification 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 a tuned circuit load, where the natural flywheel action replaces the missing half cycle.

Class AB

The output current flows for more than 180° of the input cycle as the amplifier is biased to the projected cut-off point. This is defined as the bias point at which the amplifier current would fall to zero if the characteristic had been linear. The theoretical efficiency is somewhat less than that for Class B, but when used in a push-pull circuit, the cross-over distortion is reduced. The class is very commonly used for audio power amplification or as a radio frequency amplifier with a tuned circuit load.

Class C

These amplifiers are biased beyond the cut-off point so that output current flows for much less than 180° of the input cycle. The typical angle of flow, which is defined by the ratio:

is less than 120°. Theoretically the efficiency approaches 100% as the angle of flow approaches zero. In practice, this value can be as high as 90%. Because of the severe distortion that Class C operation generates, this class can only be used with a tuned circuit load, i.e., as a radio frequency amplifier, or as a harmonic generator, with the tuned circuit extracting the required harmonic frequency from the distortion components.

Class D

For audio and low frequency applications, the requirements of low distortion and high power output produce a conflict. The low efficiency of Class A is due to the excessive power dissipation in the output stages. Operation has to remain within the safe working area to protect the amplifier from thermal damage (see Fig. 1.2).

Figure 1.2 Maximum power dissipation curve.

If the amplifier can be operated in either the saturated or cut-off state, then the power dissipation will be low, because either the voltage across the active device or the current through it will be very low. Further, if the amplifier can be switched rapidly between these states, the power dissipation during the transition through the active region will also be very low. The output power is controlled by varying the time spent in these two extreme states, this depending upon the instantaneous amplitude of the signal to be amplified. Thus Class D operation is a form of pulse width or duration modulation, which can achieve a practical efficiency as high as 97%.

Typically the switching frequency exceeds 50 kHz, but significantly higher values may be used. This has the advantages of smaller filters to remove the switching frequency from the output, lower radiation of unwanted interference, reduced intermodulation distortion and a wider bandwidth.

Class E

When a switching mode is used for RF power amplifiers and the transition periods between the On and Off states become an appreciable part of the period of the signal being processed, a significant power loss arises. Class E overcomes this problem by including LC filters between the switcher and the load to delay the current transition relative to the voltage swing. This ensures that either the voltage or the current is simultaneously zero during the switching period.

Theoretically the efficiency for Class E can be 100% with practical values in excess of 90% at VHF frequencies being achieved.

Class E is equally applicable to thermionic or semiconductor RF power devices and has a low sensitivity to component tolerances

Current dumping amplifiers

This system was devised to provide audio frequency power amplification with very low distortion. Operation is based on the premise that a system that provides a distortionless no load voltage and an output impedance that is independent of the load, must be distortionless. The basic circuit is shown in Fig. 1.3, where A₁ is a small Class A amplifier that is capable of providing the total output voltage swing, but with a very limited current capability. The emitter follower transistors Tr₁, and Tr₂ are designed to dump current into the load under the control of A₁. In this way, the two sections combine to provide the total output power. The very low level of distortion is achieved by the use of the feedback networks. Capacitor C₁ ensures that the integrator circuit which has a response that extends to well beyond the maximum operating frequency, provides a gain that falls with rising frequency. R₂ provides a feedback voltage that depends upon the impedance of L₁ and thus has an opposite effect to that produced by C₁. A further feedback path provided by R₃ operates over the dumper stages.

1.2 Distortions (see also under Measurements, p. 159)

The information carried by an analogue signal is contained within the signal’s instantaneous amplitude, frequency and phase. Any phenomenon that disturbs 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 noiselike effect. This effect can be quantified either by measurement or analysis and can be expressed as a kind of signal to noise ratio, usually in the form:

Amplitude or non-linear distortion

In a practical amplifier, the dynamic transfer characteristic is always to some extent non-linear. That is, equal incremental inputs do not produce equal incremental outputs. This is particularly noticeable in large signal power amplifiers, where a saturation condition arises. A useful measure of a system’s performance in this respect is shown by the 1 dB compression point as indicated in Fig. 1.4. This point represents the output power level where the signal is 1 dB below the value that would obtain if the system were linear. In the case shown, 1 dB compression occurs ...

Table of contents

• Cover
• Halftitle
• Dedication
• Title
• Copyright
• Contents
• Preface to the Second Edition
• Preface to the First Edition
• 1 Analogue systems and concepts
• 2 Antennas or aerials
• 3 Audio signal processing
• 4 Bar code and data matrix technology
• 5 Codes and coding formats
• 6 Computers in communications
• 7 Digital communication systems
• 8 Digital pulse code modulation
• 9 Electromagnetic compatibility/interference (EMC/EMI)
• 10 Encryption and decryption
• 11 Error control
• 12 Facsimile (Fax) systems
• 13 Filters
• 14 Frequency ranges in use
• 15 Image processing
• 16 Information theory
• 17 Logic
• 18 Measurement of system parameters
• 19 Memories
• 20 Microwave devices
• 21 Mixer signal processing
• 22 Modulation and demodulation
• 23 Networks
• 24 Noise
• 25 Optical communications, devices and systems
• 26 Power supplies
• 27 Propagation
• 28 Quality and reliability/quality assurance (QA)
• 29 Radar, navigation and tracking systems
• 30 Radio frequency receivers
• 31 Satellite systems
• 32 Semiconductor devices and technology
• 33 Signals
• 34 Spread spectrum techniques
• 35 Standards organisations and associated bodies
• 36 Telephony and associated systems
• 37 Television systems and signals
• 38 Transmission lines and waveguides
• 39 Video on demand systems
• 40 Videotex(t)
• 41 Wire-less distribution systems
• Appendix 1. CCITT recommendations
• Appendix 2. Abbreviations and acronyms
• Index