Microwave Devices, Circuits and Subsystems for Communications Engineering - Hardcover

Hardcover

ISBN 10: 047189964X ISBN 13: 9780471899648

Publisher: Wiley, 2005

Synopsis

Microwave Devices, Circuits and Subsystems for Communications Engineering provides a detailed treatment of the common microwave elements found in modern microwave communications systems. The treatment is thorough without being unnecessarily mathematical. The emphasis is on acquiring a conceptual understanding of the techniques and technologies discussed and the practical design criteria required to apply these in real engineering situations.

Key topics addressed include:
* Microwave diode and transistor equivalent circuits
* Microwave transmission line technologies and microstrip design
* Network methods and s-parameter measurements
* Smith chart and related design techniques
* Broadband and low-noise amplifier design
* Mixer theory and design
* Microwave filter design
* Oscillators, synthesisers and phase locked loops

Each chapter is written by specialists in their field and the whole is edited by experience authors whose expertise spans the fields of communications systems engineering and microwave circuit design.

Microwave Devices, Circuits and Subsystems for Communications Engineering is suitable for senior electrical, electronic or telecommunications engineering undergraduate students, first year postgraduate students and experienced engineers seeking a conversion or refresher text.

* Includes a companion website featuring:
* Solutions to selected problems
* Electronic versions of the figures
* Sample chapter

About this title

About the Author

Dr. Ian A Glover is a Senior Lecturer. Research interests: radio science, microwave radio propagation, channel measure ments and modelling, and digital communications coding and modulation. He is co-author of the successful book Digital Communications.

Dr. Steve R. Pennock is a Senior Lecturer. Research interests: microwave engineering and communications, inset dielectric guide antennas and subsystems, monolithic microwave integrated circuits, flared slot antennas, discontinuities and non-uniformities in transmission lines and millimetre wave propagation effects.

Dr. Peter R. Shepherd is a Senior Lecturer and First Year Course Director. Research interests: microwave engineering and communications, inset dielectric guide antennas and subsystems, monolithic microwave integrated circuits, flared slot antennas, discontinuities and non-uniformities in transmission lines, millimetre wave propagation effects, and mixed signal integrated circuits

Excerpt. © Reprinted by permission. All rights reserved.

Microwave Devices, Circuits and Subsystems for Communications Engineering

John Wiley & Sons

Copyright © 2005 John Wiley & Sons, Ltd.
All right reserved.
ISBN: 978-0-471-89964-8

Chapter One

Overview

I. A. Glover, S. R. Pennock and P. R. Shepherd

1.1 Introduction

RF and microwave engineering has innumerable applications, from radar (e.g. for air traffic control and meteorology) through electro-heat applications (e.g. in paper manufacture and domestic microwave ovens), to radiometric remote sensing of the environment, continuous process measurements and non-destructive testing. The focus of the courses for which this text was written, however, is microwave communications and so, while much of the material that follows is entirely generic, the selection and presentation of material are conditioned by this application.

Figure 1.1 shows a block diagram of a typical microwave communications transceiver. The transmitter comprises an information source, a baseband signal processing unit, a modulator, some intermediate frequency (IF) filtering and amplification, a stage of up-conversion to the required radio frequency (RF) followed by further filtering, high power amplification (HPA) and an antenna. The baseband signal processing typically includes one, more, or all of the following: an antialising filter, an analogue-to-digital converter (ADC), a source coder, an encryption unit, an error controller, a multiplexer and a pulse shaper. The antialisaing filter and ADC are only required if the information source is analogue such as a speech signal, for example. The modulator impresses the (processed) baseband information onto the IF carrier. (An IF is used because modulation, filtering and amplification are technologically more difficult, and therefore more expensive, at the microwave RF.)

The receiver comprises an antenna, a low noise amplifier (LNA), microwave filtering, a down-converter, IF filtering and amplification, a demodulator/detector and a baseband processing unit. The demodulator may be coherent or incoherent. The signal processing will incorporate demultiplexing, error detection/correction, deciphering, source decoding, digital-to-analogue conversion (DAC), where appropriate, and audio/video amplification and filtering, again where appropriate. If detection is coherent, phase locked loops (PLLs) or their equivalent will feature in the detector design. Other control circuits, e.g. automatic gain control (AGC), may also be present in the receiver.

The various subsystems of Figure 1.1 (and the devices comprising them whether discrete or in microwave integrated circuit form) are typically connected together with transmission lines implemented using a variety of possible technologies (e.g. coaxial cable, microstrip, co-planar waveguide).

This text is principally concerned with the operating principles and design of the RF/ microwave subsystems of Figure 1.1, i.e. the amplifiers, filters, mixers, local oscillators and connecting transmission lines. It starts, however, by reviewing the solid-state devices (diodes, transistors, etc.) incorporated in most of these subsystems since, assuming good design, it is the fundamental physics of these devices that typically limits performance.

Sections 1.2-1.7 represent a brief overview of the material in each of the following chapters.

1.2 RF Devices

Chapter 2 begins with a review of semiconductors, their fundamental properties and the features that distinguish them from conductors and insulators. The role of electrons and holes as charge carriers in intrinsic (pure) semiconductors is described and the related concepts of carrier mobility, drift velocity and drift current are presented. Carrier concentration gradients, the diffusion current that results from them and the definition of the diffusion coefficient are also examined and the doping of semiconductors with impurities to increase the concentration of electrons or holes is described. A discussion of the semiconductor energy-band model, which underlies an understanding of semiconductor behaviour, is presented and the important concept of the Fermi energy level is defined. This introductory but fundamental review of semiconductor properties finishes with the definition of mean carrier lifetime and an outline derivation of the carrier continuity equation, which plays a central role in device physics.

Each of the next six major sections deals with a particular type of semiconductor diode. In order of treatment these are (i) simple P-N junctions; (ii) Schottky diodes; (iii) PIN diodes; (iv) step-recovery diodes; (v) Gunn diodes; and (vi) IMPATT diodes. (The use of the term diode in the context of Gunn devices is questionable but almost universal and so we choose here to follow convention.) The treatment of the first three diode types follows the same pattern. The device is first described in thermal equilibrium (i.e. with no externally applied voltage), then under conditions of reverse bias (the P-material being made negative with respect to the N-material), and finally under conditions of forward bias (the P-material being made positive with respect to the N-material). Following discussion of the device's physics under these different conditions, an equivalent circuit model is presented that, to an acceptable engineering approximation, emulates the device's terminal behaviour. It is a device's equivalent circuit model that is used in the design of circuits and subsystems. There is a strong modern trend towards computer-aided design in which case the equivalent circuit models (although of perhaps greater sophistication and accuracy than those presented here) are incorporated in the circuit analysis software. The discussion of each device ends with some comments about its manufacture and a description of some typical applications.

The treatment of the following diode types is less uniform. Step-recovery diodes, being a variation on the basic PIN diode, are described only briefly. The Gunn diode is discussed in some detail since its operating principles are quite different from those of the previous devices. Its important negative resistance property, resulting in its principal application in oscillators and amplifiers, is explained and the relative advantages of its different operating modes are reviewed. Finally, IMPATT diodes are described, that, like Gunn devices, exhibit negative resistance and are used in high power (high frequency) amplifiers and oscillators, their applications being somewhat restricted, however, by their relatively poor noise characteristics. The doping profiles and operating principles of the IMPATT diode are described and the important device equations are presented. The discussion of IMPATT diodes concludes with their equivalent circuit.

Probably the most important solid-state device of all in modern-day electronic engineering is the transistor and it is this device, in several of its high frequency variations, that is addressed next. The treatment of transistors starts with some introductory and general remarks about transistor modelling, in particular, pointing out the difference between small and large signal models. After these introductory remarks three transistor types are addressed in turn, all suitable for RF/microwave applications (to a greater or lesser extent). These are (i) the gallium arsnide metal semiconductor field effect transistor (GaAs MESFET); (ii) the high electron mobility transistor (HEMT); and (iii) the heterojunction bipolar transistor (HBT). In each case the treatment is essentially the same: a short description followed by presentations of the current-voltage characteristic, capacitance-voltage characteristic, the small signal equivalent circuit and the large signal equivalent circuit.

1.3 Signal Transmission and Network Methods

Chapter 3 starts with a survey of practical transmission line structures including those without conductors (dielectric waveguides), those with a single conductor (e.g. conventional waveguide), and those with two conductors (e.g. microstrip). With one exception, all the two-conductor transmission line structures are identified as supporting a quasi-TEM (transverse electromagnetic) mode of propagation - important because this type of propagation can be modelled using classical distributed-circuit transmission line theory. A thorough treatment of this theory is given, starting with the fundamental differential equations containing voltage, current and distributed inductance (L), conductance (G), resistance (R) and capacitance (C), and deriving the resulting line's attenuation constant, phase constant and characteristic impedance. Physical interpretations of the solution of the transmission line equations are given in terms of forward and backward travelling waves and the concepts of loss, dispersion, group velocity and phase velocity are introduced. The frequency-dependent behaviour of a transmission line due to the frequency dependence of its L, G, R and C (due in part to the skin effect) is examined and the special properties of a lossless line (with R = G = 0) are derived.

Following the distributed-circuit description of transmission lines, the more rigorous field theory approach to their analysis is outlined. A short revision of fundamental electromagnetic theory is given before this theory is applied to the simplest (TEM) types of transmission line with a uniform dielectric and perfect conductors. The relationship between the time-varying field on the TEM line and the static field solution to Maxwell's equations is discussed and the validity of the solutions derived from this relationship is confirmed. The special characteristics of the TEM propagation mode are examined in some detail. The discussion of basic transmission line theory ends with a physical interpretation of the field solutions and a visualisation of the field distribution in a coaxial line.

Most traditional transmission lines (wire pair, coaxial cable, waveguide) are purchased as standard components and cut to length. Microstrip, and similarly fabricated line technologies, however, are typically more integrated with the active and passive components that they connect and require designing for each particular circuit application. A detailed description of microstrip is therefore given along with the design equations required to obtain the physical dimensions that achieve the desired electrical characteristics, given constraints such as substrate permittivity and thickness that are fixed once a (commercial) substrate has been selected. The limitations of microstrip including dispersion and loss are discussed and methods of evaluating them are presented. The problem of discontinuities is addressed and models for the foreshortened open end (an approximate open circuit termination), vias (an approximate short circuit termination), mitred bends (for reducing reflections at microstrip corners) and T-junctions are described.

In addition to a simple transmission line technology for interconnecting active and passive devices, microstrip can be used as a passive device technology in its own right. Microstrip implementation of low-pass filters is described and illustrated with a specific example. The general theory of coupled microstrip lines, useful for generalised filter and coupler design, is presented and the concepts of odd- and even-modes explained. Equations and design curves for obtaining the physical microstrip dimensions to realise a particular electrical design objective are presented. The directivity of parallel microstrip couplers is discussed and simplified expressions for its calculation are presented. Methods of improving coupler performance by capacitor compensation are described. The discussion of practical microstrip design methods concludes with a brief survey of other microstrip coupler configurations including Lange couplers, branch-line couplers and hybrid rings.

Network methods represent a fundamental way of describing the effect of a device or subsystem inserted between a source and load (which may be the Thvenin/Norton equivalent circuits of a complicated existing system). From a systems engineering perspective, the network parameters of the device or subsystem describe its properties completely - knowledge of the detailed composition of the device/subsystem (i.e. the circuit configuration or values of its component resistors, capacitors, inductors, diodes, transistors, transformers, etc.) being unnecessary. The network parameters may be expressed in a number of different ways, e.g. impedance (z), admittance (y), hybrid (h), transmission line (ABCD) and scattering (s) parameters, but all forms give identical (and complete) information and all forms can be readily transformed into any of the others. Despite being equivalent, there are certain practical advantages and disadvantages associated with each particular parameter set and at RF and microwave frequencies these weigh heavily in favour of using s-parameters. A brief review of all commonly used parameter sets is therefore followed by a more detailed definition and interpretation of s-parameters for both one- and two-port (two- and four-terminal) networks.

The reflection and transmission coefficients at the impedance discontinuities of a device's input and output are described explicitly by the device's s-parameters. One of the central problems in RF and microwave design is impedance matching the input and output of a device or subsystem with respect to its source and load impedances. (This problem may be addressed in the context of a variety of objectives such as minimum reflection, maximum gain or minimum noise figure.) The chapter therefore continues with an account of the most widely used aids to impedance matching, namely the Smith chart and its derivatives (admittance and immitance charts). These aids not only accelerate routine (manual) design calculations but also present a geometrical interpretation of relative impedance that can lead to analytical insights and creative design approaches. Both lumped and distributed element techniques are described including the classic transmission line cases of single and double stub matching. The treatment of matching ends with a discussion of broadband matching, its relationship to quality factor (Q-) circles that can be plotted on the Smith chart, and microstrip line transformers.

Chapter 3 closes with a description of network analysers - arguably the most important single instrument at the disposal of the microwave design engineer. The operating principles of this instrument, which can measure the frequency dependent s-parameters of a device, circuit or subsystem, are described and the sources of measurement error are examined. The critical requirement for good calibration of the instrument is explained and the normal calibration procedures, including the technologies used to make measurements on naked (unpackaged) devices, are presented.

1.4 Amplifiers

Virtually all systems need amplifiers to increase the amplitude and power of a signal. Many people are first introduced to amplifiers by means of low frequency transistor and operational amplifier circuits. At microwave frequencies amplifier design often revolves around terms such as available power, unilateral transducer gain, constant gain and constant noise figure circles, and biasing the transistor through a circuit board track that simply changes its width in order to provide a high isolation connection. This chapter aims to explain these terms and why they are used in the design of microwave amplifiers.

Chapter 4 starts by carefully considering how we define all the power and gain quantities. Microwave frequency amplifiers are often designed using the s-parameters supplied by the device manufacturer, so following the basic definitions of gain, the chapter derives expressions for gain working in terms of s-parameters. These expressions give rise to graphical representations in terms of circles, and the idea of gain circles and their use is discussed.

If we are to realise an amplifier, we want to avoid it becoming an oscillator. Likewise, if we are to make an oscillator, we do not want the circuit to be an amplifier. The stability of a circuit needs to be assessed and proper stability needs to be a design criterion. We look at some basic ideas of stability, and again the resulting conditions have a graphical interpretation as stability circles.

(Continues...)

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Edition details

• Publisher: Wiley
• Publication date: 2005
• Language: English
• ISBN 10: 047189964X
• ISBN 13: 9780471899648
• Binding: Hardcover
• Edition number: 1
• Number of pages: 560
• Editor: Glover Ian A., Pennock Steve, Shepherd Peter