Introduction to RadioCommunication Systems1.1INTRODUCTIONCommunication systems transmit information from one place to another by meansofelectricenergy.Thefrequencyusedfortheinformationtransmissionvaries fromthevery low frequencies used in direct telephone communication to optical fre-quencies,also used fortelephone communication.This book describes the analysisand design of electronic circuits used in radio-frequency communication systemscovering the frequency range up through several hundred megahertz. The actualfrequency limitdepends upon whether the circuit is realized with discrete compo-nents or as an integrated circuit.The material is directly applicable to many othersystems, including television and spectrum analyzers where the design of low-noise, high frequency receivers is of paramount importance.For very high fre-quency circuits,different circuitmodels,particularly distributed-parametercircuits,are more accurate. Low-frequency circuitry is discussed, but the emphasis here ison the radio-frequency circuits.The following chapters treat the analysis and design of fundamental circuits ofcommunication receivers and transmitters. Integrated circuits have simplified thesystem design, but the communication system designer still needs to be familiarwith many circuit techniques and the simplifying approximations which apply inthis frequency ranige. The designer is often faced with a choice between using anintegrated circuit (IC)or a discrete component versionof the samecircuit.The deci-sionis based onmanyfactors,including cost, size,power consumption,noise,anddistortion. Chapter 3 presents quantitative criteria for evaluating a circuit's noiseand distortion performance.The application of integrated circuits in a communica-tion system requires aknowledge of electronic circuit theoryto properly interfacethe IC with the rest of the system. We will study the electronic circuits of the vari-ous subsystems, including oscillators, amplifiers, transformers, modulators, anddemodulators,which make upa communication system.The mathematical analysisof the many modulation methods is not considered, as it is well described in themanygoodtextsoncommunicationstheory.1
2CHAPTER 1:Introduction to Radio Communication Systems1.2NETWORKTHEORYThis section briefly reviews the concepts of network theory that are applied in thefollowing chapters.The usual variables in an electronic circuit are the voltages andcurrents measured at various points in the circuit. The excitation and response canbe described in the time domain, but determining the response in the time domaininvolves the solution of integrodifferential equations and rarely provides insightinto the design process.For linear time-invariant systems, it is usually easier toobtain the system response using the Laplace transform.The Laplace transform ofthetimevariablev(t)isv(t)e-srdtV(s) :(1.1)where s has the dimensions of frequency and is known as the complex-frequencyvariable.AlinearsystemtransferfunctionH(s)isdefinedasR(s)H(s) =(1.2)V(s)whereR(s)is the Laplacetransformoftheresponseto an excitation V(s).Linearcircuittransfer functions can easilybe obtained by interpreting an inductor as hav-inga compleximpedance sLanda capacitoras havinga complex impedance(sC)-1.Themethod is illustrated bythefollowing example.EXAMPLE1.1. Determine the transfer function V(s)/V,(s) of the circuit shown inFig. 1.1.Solution. In this circuit the inductor has been modeled as a complex impedance sL andthe capacitor as a complex impedance (sC)-1.By using the voltage-divider rule of cir-cuit analysis, We find that the transfer function H(s) isRV,(s)R/(RsC+1)H(s) =(1.3)V;(s)R/(RsC +1)+sL =$?RLC+sL +RFor simplicity,the excitation and response are often written as V。and V,respectively,when there is no possibility of confusion.Another advantage of describing the system response by the linear transferfunction H(s) is that the frequency response of the network can be obtained by set-ting s- jw. That is, if the linear system is stable and time-invariant and the excitation is a sinusoid, the steady-state response (after the transients have decayed to aSLFIGURE 1.15000CAlow-pass filterV1/sC
31.1 Introductionnegligible value) will also be a sinusoid of the same frequency.Ifv;(t)=Vsinwtthe steady-state response isr(t)=Rsin(ot +Φ)VwhereandargH(jw)=Φ[H(jw)| =REXAMPLE1.2.Determine the frequency response ofthe network shown inFig.1.1forL=0.5H,C=2FandR=12Solution.The frequency response is obtained by substituting s =jo in the transferfunction.Inthiscase,Eg.(1.3)becomesH(jw) = [(jw)2+0.5 j +1]-The magnitude of the response is the frequency-dependent function[a-+(g)[H(j)I=and the phase shift is also frequency-dependentn-10.50)argH(jo)=-tan1-02A linear transfer function without ideal delay elements will have the formA(s)H(s) =(1.4)B(s)where A(s) and B(s) arepolynomials in s.The zeros of A(s) are referred to aszeros of thetransfer function, and the zeros of B(s) arereferred to as poles of thetransferfunction.Thepoles are thevaluesofsforwhich themagnitude of the trans-fer function is infinite. In order for the transfer function to be stable, all the polesmust lie in the left half of thesplane (the real partof the pole mustbe negative).Thestabilityproblemis considered in detail in Chap.9.EXAMPLE 1.3. Calculate the poles and zeros of the transfer function given inBxample 1.2:H(s) = (s2 + 0.5s + 1)-1Solution.The transfer function has no finite zeros. Since the order of the denominatorpolynomial is 2 higher than that of the numerator, the transfer function has two zeros atinfinity. The poles,(3.75)1/2S1,52=0.25±2are located in the left half-plane. The real part of each pole is -0.25, and the imaginaryparts are ±j(3.75)//2/2
4CHAPTER 1:Introduction toRadio Communication Systems1.3MODULATIONFor a signal to contain information, some feature of the signal must be varied inaccordancewith the information to be transmitted.Early radio communicationsconveyed information by thepresence,or absence, of the signal.This method wassoon surpassedbyamplitudemodulationoftheradiowaveby anaudio signal.Theamplitudemodulationprocessprovidesameansof transmittingvoicecommunica-tions,and its development led to the rapidestablishmentof theradiobroadcastingindustry.Angle modulation is another method of transmitting information widely used inhigh-frequency communication systems.An angle-modulated signal is described bythe equationS(t)=A sin(oot+Φ)The amplitude remains constant,and the angle@ is varied in responseto the modulating signal.Both phase and frequency modulation can be used.One function ofthe receiver is to recover (demodulate) the original from the modulated signal. Thecircuitryfor implementing thevarious types of modulation and demodulation isdescribed in greater detail in Chap. 12. Today digital modulation techniques arebeing more frequently employed, particularly in satellite and telephone communi-cation systems. Digital modulation implies that a parameter of the signal is variedin response to a digital signal.Amplitude, phase, or frequency modulation can bevaried in response to a digital signal. That is, digital-modulation is an extension ofone or more of the conventional amplitude or angle modulation methods.Figure 1.2 illustrates a simplified block diagram of a digital single-channel-percarrier(SCPC)satellitecommunicationschannel.Eachvoicechannel is sampledand then converted to a digitally encoded signal that modulates a low-frequency(baseband) carrier signal. The modulated signal uses a different carrier frequencysufficiently separated so that the signals can be combined without frequency over-lay.This process is known as frequency multiplexing.The frequency-multiplexedsignal is shiftedup in frequencybymixing with a local oscillator signal,then ampli-fied and transmitted. The transmission channel contains several voice channels.This illustration is only one of many possible methods of simultaneously transmit-ting several data channels. It is the function of the receiver to recover the originalvoicechannelsfromthereceived signal,1.4RECEIVERSThe modulated signal is transmitted to a receiver where the signal is amplified andthe information extracted.It is usually the case that many different signals aresimultaneously present at the receiver input, and it is necessary for the receiverto be able to select the desired signal. This selection is made on the basis of the
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