111.4ReceiversMixereselectoLPFPreamplifierLocaloscillatorMixerZIFF ReceiverFIGURE1.7A modern direct conversion receiver.problems associatedwith amplitude andphase imbalancesbetween thetworeceiver sections. It is relatively easy to build such a phase shifter at a particular fre-quency,but most receivers must cover a band of frequencies. It is difficult to real-ize a90°phase shifter over a widebandwidth.An oscillatorthat does generatethein-phase and quadrature components of the oscillator signal is referred to as aquadrature oscillator.Quadrature oscillators are an important topic in contempo-raryreceiverresearchanddevelopment,Anycomponentsof theZiFFreceiver,particularly the sections after the mixers, are easily implemented using a digitalsignal processor.A digital receiver usually refers not to a receiverof digital signals,but to one in which a significantpart of the reception and signal extraction isdonedigitally.There is no image frequency in the direct conversion receiver, and this is per-haps the major advantage of this receivertopology.AMreceivers have an interme-diatefrequencyof 455kHz;broadcastFM uses a frequencyof 10.7MHz.Thisintermediate frequency is selected so that the image frequencies lie outside thebroadcastband;NTSC television uses a43.5-MHz intermediate frequency,andanalog cellular telephones use a 90-MHz intermediatefrequency.AnIntegrated-CircuitFMReceiverIntegrated-circuittechnology has made it possible to design aradio with aminimumof external components.Figure 1.8 contains a simplifiedblock diagram of the Sie-mans 5469,a high-frequency integrated circuit with a high level of system integration for FM communication.This bipolar device contains a complete FM receiverfor input frequencies up to 50 MHz.The device includes (1)an RF input amplifier
12CHAPTER1:IntroductiontoRadioCommunication SystemsMixerIFamplifierandRFamplifierDemodulatorlimiterFampifOscillatorFIGURE1.8Block diagram of an integrated-circuit FMreceiver.(2)an oscillator,(3)amixer,(4)an intermediate-frequency (IF)amplifierand lim.iter,(5)acoincidencedemodulator,and ()audio-frequency(AF)amplifiers withmuteand volumecontrol.AcompleteFM systemcan beconstructedwiththis one integrated circuitplusa crystal, a minimum of two inductors,and several resistors and capacitors used forfrequency trimming and gain adjustment.The external resistors and capacitors provide the circuit with additional flexibility. Although the resistors and capacitorscould beincluded in theIC,an inductor-capacitor tuned circuitcannotbefabricat-edinanICchip;soacomplete,self-containedICreceiyerortransceiver(combinedtransceiver andreceiver)is notpossiblewiththecurrentIC technology1.5TRANSMITTERSThetransmitter modulates theinformationto be communicated onto acarrier,amplifies the waveform tothe desired powerlevel, and delivers itto thetransmit-ting antenna.Theelements of a transmitter are illustrated inFig.1.9.Thetransmit-ter includes a radio-frequency oscillator that is modulated by the message signal.The modulated signal is then multiplied in frequencyupto thedesired transmittingfrequency and is ampifiedtothedesired powerlevelin thepoweramplifier.Thefirstradiotransmitters worked by chargingtwo electrodes separated by a gap.Whenthe charge became sufficiently large, a spark was created across the gap and electricenergy was radiated.However, the spark-gap transmitterwas slow,and it was diffi-culttoaccurately control thewaveform andfrequency of oscillation.The spark-gaptransmitterbecameobsoletewiththedevelopment of theelectronic oscillator andvacuumtubesthatcouldhandlelargeamounts ofpower.Thetransmitter topology illustrated inFig.1.9 is only oneof manytypes.Themodulation can actuallytake place in the power amplifier.Transmitter topologydepends on thetype ofmodulationused and thenecessary power level.Narrowbandtransmitters usuallyemploypulse,amplitude,orfrequencymodulation.Wideband
131.6ProblemsVMPowerFrequencyModulatormultipliersamplifierOscillatorFIGURE1.9Block diagram of a transmitter.transmitters use single-sided or multimode modulation and are used for long-rangemilitary,marine,aircraft,andamateurcommunications.Many transmitters andreceiver circuits are similar;both require low-noise amplifiers and oscillators.Receivers are designed for the minimum detectable signal, while output power isofprime importanceinthetransmission of signals.TheRFpoweramplifierisdescribed in Chap.11.1.6PROBLEMS1.1 Determine the transfer function V(s)/1(s) of the circuit shown in Fig.P1.1.FIGUREP1.1Abandpass filter.1.2 Calculate the poles and zeros of. the circuit in Fig.P1.1 for L =1 H, C =15 F, andR=12.1.3A receiver is tobe designed to cover the frequencies of 20 to 40 MHz using an IF filtercentered at 10o MHz. Specify two different local oscillator frequencies for each inputfrequency,and determinethecorresponding imagefrequencyforeach inputfrequency1.4 Select thelocal oscillatorfrequencies and specify the preselectorfrequency responseforan up conversion receiver covering the2-to 30-MHz frequency range.The center fre-quency of the IF filter is 50 MHz.1.5 Figure P1.5 illustrates a direct conversion receiver in which the input signal is con-verted directly to an audio signal. Specify the local oscillator frequencies and the
14CHAPTERl:IntroductiontoRadioCommunication Systemscorresponding image frequencies for a direct conversion receiver covering the 2-to30-MHz range.AmplifierandTobasebandfilterLocal oscillatorFIGUREP1.5Adirectconversionreceiver.1.6 The double-conversion receiver in Fig.P1.6 employs two IF filters.Specify therequired local oscillatorfrequencies(fiand f2)for areceivercovering the 2-to30-MHz range. The center frequency of the first IF filter is 50 MHz, and that of thesecond IF filter is 10 MHz.SecondAmplifierIF filterIFfiterR1andandDetectorBasebandandflteramplifieramplifierLocaloscillatorLocaloscillatorFIGUREP1.61.7 NTSC television uses a 41.25-MHz intermediate frequency for the sound carrier and45.75 MHz for the picture carrier.The local oscillator operates at 45.75 MHz abovethe desired incoming picture frequency.The VHF band has channels 2 to 6 covering54 to88MHz and channels 7to13 covering 174 to 216MHz.Determine thelocal-dscillator frequency and image frequencyfor each VHF channel.The bandwidth ofeachchannelis6MHz.*1.8 For the circuit shown in Fig.1.1 plot the input impedance and voltage transfer func-tion as afunction offrequencyfor L=10mH,C=0.01μF,and R=1k2.Whatis the circuit's -3-dB bandwidth? Determine the circuit's rise time and peak over-shoottoa unit step input ofvoltage.*Problems with an asterisk are intended for solution using computer simulation (SPICE or PSPICE)
151.7Additional Reading1.7ADDITIONALREADINGAbidi, A.A.:"Direct-Conversion Radio Transceivers for Digital Communications,"1995IEEEInternational SolidStateCircuitsConference,1995,p.186.Armstrong,F.H.:ANewSystemofShort-WaveAmplification,Proc.RadioEng.9:3-27,1919.Brannon, B., Wide-Dynamic-Range A/D Converters Pave the Way for Wideband Digital-RadioReceivers,EDN,November7,1966,pp.187-205.Fessenden, R.H.: Wireless Telephony,Am. Inst. Elect. Eng., 27:553-629, 1908.Lessing,L.:Man ofHigh Fidelity:Edwin Howard Armstrong,Lippincott,Philadelphia,1956.Rohde, U. L., and T. T. N. Bucher: Communication Receivers: Principles and Design,McGraw-Hill,NewYork,1988.Teman,F.F:Electronic and RadioEngineering,4th ed.,McGraw-Hill,NewYork,1955Tucker, D. G.: The History of the Homodyne and Synchrodyne, J. British Inst. of RadioEngineers, 14(4):143-154,1954