6CHAPTERl:IntroductiontoRadioCommunicationSystemsFIGURE1.3C2AmplifierSchematicdiagramofanand detectorearlyregenerativereceiver.frequency of the incident signals-the receiver's ability to discriminate betweensignals of different frequencies is called receiver selectivity Other functions of thereceiver are to detect (demodulate) the information contained in the signal and per-haps to reconstruct and amplify the original waveform.Receivers varyin kindfromtelephone, radio,television,radar, and navigation to satellite communicationsmodels. The complexity of each type varies with the complexity of the transmittedsignal, the frequency of operation, and the number and amplitude of the unwantedsignals in the same frequency band. All receivers have the common problems ofselectivity,rejecting input noise, and detecting the desired signal.The name“radio"originated around 1910 to distinguish receivers that receivedvoicetransmission from the earlier receivers which received only code (pulsetransmission).The first receivers did not have the capability of signal amplifica-tion, but this deficiency was soonovercome with theinvention of thevacuum tube(triode). The first vacuum-tube circuits did not provide much gain, but E. H. Arm-strong soon invented the "regenerative receiver,which used positive feedbackfrom the output to the input.A simplified schematic of a regenerative receiver isillustrated inFig.1.3.A vacuumtube in the circuit serves asboth an amplifier anda detector.Ademodulatorwhich recovers (detects)themodulating signal isknownas a detector. The ac output signal is fed back in phase with the input signal(regenerated),increasing the loop gain. The regenerative receiver was probablythe first application of electronic feedback; it quickly led to the invention of theelectronic oscillator, since the circuit was susceptible to oscillations.The electron-icoscillatorgreatlyimprovedtransmitterdesignThe regenerative receiver was soon replaced by the tuned radio-frequency(TRF)receiver.Ablock diagram of a typical TRFreceiver is illustrated inFig.1.4:itconsistsof threetunedRF amplifiers in cascadefollowed byadetectorandpowerAmplifierAmplifierAmplifierDetectorandandandandAudiofilterfilterfilteramplifierFIGURE1.4Blockdiagram of an earlytuned radio-frequencyreceiver
71.4Receiversamplifier.Thisreceiver washard to operatebecauseofthedifficulty of tuning all theRF amplifiers to the same frequency.The TRF receiver became obsolete with theinvention of the superheterodyne receiver by E.H.Armstrong.The"superhet"elim-inates the need for tuning all the RF amplifiers to the frequency of the input signalby shifting the input signal frequency to that of the fixed frequency of the receiverfilter.It is possibleto build fixed-frequency filters and amplifiers that arefar superi-or to variable-frequency filters. The superheterodyne principle is still used in virtu-ally all receivers.It consists of multiplying,or beating (heterodyneis from theGreek heteros,"other,”and dynamis,"force"),the input signal with a signal gener-ated in the local oscillator. If a sine wave of frequency w,is multiplied by a sinewave of frequency wz,the resultant signal consists of the sinusoids of frequencies±@.That is,cos(w-Wr)t-cos(we+w)tsinw,t sinwrt =2A simplified block diagram of a superheterodyne receiver is shown in Fig.1.5.Inthis typeof receiver,the incoming signal is converted to an intermediate frequencybythefirst local oscillator and then is reduced to alow-frequency signalbythe sec-ond mixer and low-pass filter.If the input signal consists ofa carrier f.and an audiocomponent fa and the first local oscillator frequency is fo, the output of the firstmixer consists of the two frequencies f+ fa + fand f.+ fa -fo. The localoscillatorfrequencyf。is selected so that one of these frequencies is equal to thecenter frequency of the intermediate-frequency (IF) filter (fir). Since fa is usuallymuchless thanfe,forall practicalpurposesfo= IfrF - falorfo=fir+fein order that themixer outputfrequencybeatthecenterof theIFfilterbandwidth.Oneadvantage of this form of detection is that the same high-quality filter canbe used for all inputfrequencies.The frequency selection is obtained by varying thelocal oscillatorfrequencyfo.TheIFfilteroutput fr+fais thenreducedtofainthesecond mixer, which mixes the IF output with the second oscillator frequency(which is fixed at fir).Aproblem with this form of detection occurs when there area large number of signals of differentfrequencies present at the input. Consider,forMixerAmplifierIFfilterDetectorBasebandandfilterLocaloscillatorFIGURE1.5A superheterodyne receiver
8CHAPTER1:Introduction to Radio Communication Systemsexample,the receiver designed to selectthe differencefrequencyat the outputof thefirst mixer. That is,fir = Ifo- feldownconversionorfiF=fo+ feupconversionThere exists another signal frequency fim,referred to as the image frequency, whichwhen mixed with the local oscillator frequency f。will produce a signal at the IF fre-quency.If fir =Ifo-fal, then fim =fo+ fir,orfM=fiF-f。=2frF十feExAMPLE1.4.Consider a receiver with the IF filter centered at 455kHz.If it is desiredto receive a 1-MHz input signal, the local oscillator is tuned to 1.455 MHz. Then animagefrequencyfim=fiF+foorfim=f+2fE=1.91MHzif present at the input, would also pass through the IF filter.There is no way to separate the desired signal from a signal at the image fre-quency after they have entered themixer.The image frequency signal must beremoved before it arrives at the mixer.This can be accomplished by adding animage suppression flter (called a preselector) before the mixer. For a receiverdesigned to covera band of frequencies,the preselector must be tunable.Tunablefilters tend to be complex and represent a significant portion of the cost of receiverconstruction.Themajority of receivers do include apreselector.In Example 1.4, the filter center intermediate frequency was lower than theinput signal frequency,and the input frequency was shifted down to the intermedi-ate frequency.The intermediate center frequency can also be selected above theinput signal frequency (up conversion). The following example illustrates the ad-vantages of up conversion.ExAMPLE 1.5. Consider again the receiver that is designed to cover the frequencyband of 1 to 30 MHz,but which uses an IF filter centered at 40 MHz.For an inputsignal frequency fs of 1 MHz, there are two local oscillator frequencies f。 (41 and39MHz)that will result in a mixer product at 40 MHz.If the local oscillator fre-quency is 41 MHz, the image frequency will be 81 MHz; and if the local oscillatorfrequency is 39 MHz,the image frequency will be 79 MHz.Table 1.1 lists the localoscillatorfrequency and corresponding image frequencyfor several inputfrequenciesspanning the frequency band to be covered. Either set of local oscillator frequenciesTABLE1.1Local oscillatorfrequencyand correspondingimage frequency, MHzfa.JJiMfiM141813979242823878501090307030701101050
91.4Receiverscould be selected,but normallythe first set,fo,would be used, since theratio ofthehighest to the lowestfrequency,70:41,is lower than that used forfo,which is 39:10.In thedesign and construction of variable-frequency oscillators,the ratio of highest tolowestfrequencies is an importantfactor (the lowerthe ratio,the simplerthe design)One importantfeature of the up conversion technique is that all image frequen-cies lie above the frequency band to be covered. This implies that all image fre-quencies can be suppressedbyadding a low-pass filterto theinput (30-MHz band-width); a tunable bandpass filter is not needed with up conversion. Until recently itwas notpossible to useup conversion in this frequency range because high-qualitybandpassfilters werenotreadilyavailableinthe 30-to 50-MHz region;however,recent improvements in the manufacturing technology now provide for high-quality crystal filters in this frequency range.Another advantage of up conversion is that the oscillator-tuning ratio fmax/fminis less than thatfordown conversion.Ifadownconversionreceiverwitha 455-kHzIFis used to cover the same frequencyband,thelocal oscillatorfrequencywill needto vary from fmin =1.455MHz to fmax =30.455MHz, a tuning range of 20.93:1.AModern Communications ReceiverA block diagram of the high-frequency section of a modern radio receiver is essen-tially the same as that of the superheterodyne receiver illustrated in Fig.1.5, but thecircuits differ from the earlier models.One difference is that up conversion is oftenused in high-qualityreceivers so that the input filter cen remain a relatively simplelow-pass filter that need not be tuned.Whether an amplifier is required before themixerdepends on the particularapplication and thereceiver specifications.It willbe shown inChap.3that the absenceof this amplifiercan actually improvereceiv-er performance in many applications.Modem receivers differ fromolderreceivers inmany ways.Amain differenceis the frequency synthesizer used to generate the frequencies needed from thevariable-frequency oscillator.The frequency synthesizer is capable of generating alargenumber of relatively precisefrequencies from a single reference frequency.Although the receiver does contain at leasttwo oscillators,it has less frequency driftand noisethan theolder,conventional variable-frequency oscillators.Todaythe out-put frequency of frequency synthesizers can be precisely controlled using digitalcircuitry.This makes possible the microprocessor control of radio receivers andspectrum analyzers. Frequency synthesizers are described in detail in Chap. 10.Since most synthesizers now incorporate a phase-locked loop,a thorough under-standing of phase-locked loop characteristics is important for frequency synthesiz-er design.This information is presented in Chaps.8 and9.Direct ConversionReceiversAn immediate extension of the superheterodyne receiver is the direct conversionreceiver in which the IF section is eliminated by converting the input signal directly
10CHAPTER l:Introduction to Radio Communication SystemsMixerLPFDetectorPreamplifierLocaloscillatorFIGURE1.6Aphase-coherentdirectconversionreceiver.todirectcurrent baseband.Such a receiver is shown inFig.1.6.Thelocal oscillatoris set at the same frequency as the input carrier frequency.The mixer output thencontains the baseband signal and a signal attwice the carrier frequency.The higher-frequency signal is then removed with a low-pass filter.An advantage of the directconversionreceiver isthat it ismucheasiertobuildalow-pass filterthananarrow.band intermediate-frequencyfilter.NarrowbandIF filtersrequiremore componentsandconsumemorepowerthandolow-passfilters.Theeliminationoftheinterme-diate frequency results in the direct conversion receiver's frequentlybeing referredto as a zero IF or ZIFF receiver.One of the problems limiting the application ofdirect conversion receivers is local oscillator drift.And dc offsets are another prob-lem.Aproblem for directconversion receivers is local oscillator leakage back intothe input stage from which it again mixes with the local oscillator output, creatingan erroneous dc output.These problems are being reduced with improvements inmodern components,and the direct conversion receiver is finding application inmanybattery-operated systems.Amajor problem with the direct conversion method is created by the phaseuncertainty between the received signal and the local oscillator.(Thephasedif-ferenceisknown in coherentdetection schemes,butthisrequires somemeans ofalerting thereceiverof this phasedifference.)Ifa constant-amplitudesignal is sent,the dc component at themixer outputwillbeK cos ,where is the phasediffer-ence.Thisphase differencecreates an error in theestimation ofthe input waveformamplitude.If the local oscillator is in phase synchronization with the input signal,the receiverisknown-as a homodynereceiverFigure 1.7 shows a modern direct conversion receiver used for the more com-mon application in which the local oscillator is notphase-synchronized to theinput signal. The phase uncertainty problem is solved by using an in-phase and aquadrature channel created by using the local oscillator signal and the local oscil-lator signal shifted by 90°.The outputs of the two quadrature channels are thencombined to recover the input signal (another problem).This topology is rathereasily implemented in integrated circuits, and the ZIFF receiver is being used inmanybattery-operated applications,particularlythose using somevariantofdigi-tal modulation such as QPSK.The integrated-circuit realization minimizes the