第三部分專業閱讀材料

ReadingMaterial閱讀材料

Chapter

1

TRANSISTORAMPLIFIERS

Theadventofelectronicsisreckonedfromthediscoverythatthecurrentinavacuumdiodecanbecontrolledbyintroducingathirdelectrode,whichmakesthetriodeanexceedinglyeffectiveamplifier.Thepossibilitiesinherentinelectroniccircuitsweregreatlyexpandedbytheinventionofthetransistorin1948.Sincethenawidevarietyofsemiconductordeviceshastoalargeextentsupplantedvacuumtubesinmostapplications.

Bothvacuumtubes,whicharediscussedinAppendixA,andtransistorsarenonlineardevices,andtheiroperationinanycircuitisdeterminedbygraphicalanalysisusingthedescriptionoftheirelectricalpropertiesgivenbycurrentvoltagecharacteristics.Theanalysisdiffersindetailforvoltagecontrolleddevicessuchasthetriodeandfieldeffecttransistorcomparedtocurrentcontrolleddevicessuchasthejunctiontransistor,butisnotdifferentinprinciple.Furthermore,ineithercaseitprovespossibletodevelopusefulequivalentcircuitrepresentationswhicharemostsatisfactoryforcircuitanalysis. 1-1

TheOperatingPoint

1-1.1

LoadLines

TheoperationofaFETasanamplifierisexaminedmosteasilywiththeaidoftheelementaryamplifiercircuitemployingapchannelFETshowninFig.1-1.InthiscircuitthedrainismaintainedatanegativepotentialwithrespecttothesourcebythebatteryVddwhilethegateisbiasedpositivelybythegatebiasbatteryVgg.VariationsingatevoltageresultingfromaninputsignalΔVgsproducechangesinthedraincurrentwhichareobservedasavoltagesignalΔVdsacrosstheloadresistorRL.

ThecircuitisanalyzedusingKirchhoff’srulesintheusualwayexceptthat,sincethecurrentvoltagepropertiesoftheFETarenonlinear,thesolutionmustbecarriedoutgraphically.ThevoltageequationaroundtheoutputcircuitisVdd-IdRL+Vds=0(1-1)whichmaybeputintheform(1-2)Equation(1-2)plotsasastraightlineonthedraincharacteristicsoftheFETandhasinterceptsatId=0,Vds=VddandVds=0,I

d=Vdd/RL.Itis,infact,thecurrentvoltagecharacteristicofRL

andiscalledtheloadline.

TheloadlinetogetherwiththedraincharacteristicsoftheFETrepresenttworelationsinthetwounknownsVdsandId.Byplotting

theloadlineonthedraincharacteristicsasinFig.1-2,thedraincurrentcorrespondingtoeachgatevoltageisgivenbytheintersectionoftheloadlinewiththecharacteristiccurveforthatgatevoltage.Figure1-1

Simplep－channelFETamplifier.Figure1-2

Operatingpointistheintersectionofloadlinewithgatebiascurve.

Theoperatingpointistheintersectionoftheloadlinewiththecharacteristiccurvecorrespondingtothegatebiasvoltage.Asthegatevoltagevariesinaccordancewithanappliedinputsignal,thedraincurrentexcursionsmovebackandforthalongtheloadlinesothatEq.(1-2)issatisfiedateveryinstant.InFig.1-2,forexample,aninputsignalof0.4Vcausesthedraincurrenttochangebyapproximately0.6mAandresultsinanoutputsignalofabout6V.Thatis,thecircuitamplifiestheinputsignalbyafactorof6/0.4=15.

Notethattheinputpowerisverysmallbecausethegatecurrentisnegligible.Incontrast,theoutputpower,whichisequaltotheproductofthechangeindraincurrenttimesthechangeindrainvoltage,maybequiteappreciable.Thispowerisderivedfromthedrainsupplybattery,Vdd,andiscontrolledbythevalvelikeactionofthegate.

1-1.2

SourceBias

Aseparategatebiasbatteryiseconomicallyimpracticalinmostcircuits.Instead,gatebiascanbeobtainedbyinsertingaresistorinserieswiththesource,Fig.1-3.Figure1-3

PracticalnchannelFETamplifierusingsourcebias.Thegatepotentialissetbyavoltagedividerconnectedtothedrainvoltagesource.ThenthecurrentthroughthesourceresistorRsresultsinagatebiasoftheproperpolarityandmagnitudetomaintainthegatejunctionunderreversebias.Ausefulfeatureofthiscircuitisthatif,forexample,achangeinambienttemperatureincreasesthedraincurrent,thevoltagedropacrossthesourceresistorincreasesthegatebias,which

tendstoreturnthedraincurrenttotheoriginalvalue.Thatis,thedraincurrentandtheoperatingpointarestabilizedagainst

externalinfluences.

NotethatthesourceresistorisshuntedbyalargecapacitortopreventACsignalscausedbyacdraincurrentsinthesourceresistorfromappearinginthegatecircuit.Also,capacitorsareincludedattheinputandoutputtoisolatetheamplifierfrom

externaldcvoltagesthatcouldchangetheoperatingpoint.

Theoperatingpointisdeterminedbywritingtheequationfortheloadlinefromthedcvoltagedropsintheoutputcircuit,Vdd-IdRL-Vds-IdRs=0(1-3)Since,however,thegatebiasvoltageisnotknown,incontrasttothesimplercircuitpreviouslyanalyzed,anotherrelationisrequired.Thisisfoundfromthedcvoltagedrops

aroundthe

inputcircuit,(1-4)Bothequationsmaybeputinthesameform.Theloadlineis,fromEq.(1-3),(1-５)andthebiasline,fromEq.(1-4),is(1-6)NotethatEq.(1-5)issimilartoEq.(1-2),andthatEq.(1-6)hasthesameform.Theseexpressions,togetherwiththecurrentvoltagecharacteristicsoftheFET,representthreerelationsinthethreeunknowns,Vds,Id,andVgs,andaresolvedgraphically.

Thebiasline,Eq.(1-6),isastraightlineonthetransfercharacteristic,Fig.1-4,andtheintersectiongivesthedraincurrentandgatevoltageimmediately.Thedrainvoltageisthenfoundfromtheintersectionofthegatebiascurvewiththeloadlineplottedonthedraincharacteristics.Thustheoperatingpointiscompletelydetermined.Figure1-4Intersectionofbiaslinewithtransfercharacteristicyieldsoperatingpointofsourcebiasamplifier.

Aninputsignalappliedtothegateresultsincorrespondingchangesinthedraincurrent,asshownbythetransfercharacteristicinFig.1-4.Asinthepreviouscircuit,drainsignalcurrentsthroughtheloadresistorproduceanoutputsignal.ThisisillustratedgraphicallyinFig.1-4byplottingtheacloadlineonthedraincharacteristics.Theacloadlineissimplythedcloadline,Eq.(1-5),withthesourceresistorsetequaltozerotoaccountfortheshuntingactionofthesourcebypasscapacitoratsignalfrequencies.

Notethatasinusoidalinputsignalresultsinasinusoidaloutputsignaloflargeramplitude.Theoutputwaveformisnotanexactamplifiedreplicaoftheinputsignal,however,becauseofcurvatureinthetransfercharacteristic.Thisdistortionisminimizedbypropercircuitdesignandsuitablechoiceoftheoperatingpoint.NotealsoinFig.1-4thatanincreaseingatevoltageproducesadecreaseintheoutputvoltage.Thismeansthatthecircuitintroducesa180°phaseshiftbetweeninputandoutputsignals.

AsimplerversionofthecircuitinFig.1-3eliminatesresistorR1.Quitesatisfactoryoperationisachieved,butbecausethesourceresistormustthenusuallybemadesmalltoobtainthecorrectgatebias,thecircuitisnotquitesostabilizedagainstshiftsinoperatingpointaccompanyingchangesin

ambient

temperatureorsupplypotential.Ineithercircuititoftenprovesmoreconvenienttofindtheoperatingpointbya“cutandtry”processratherthanbyresortingtothegraphicaltechnique.Thisisdonebyfirstchoosingareasonablevalueforthegatebias,Vgs,andestimatingIdfromthedraincharacteristics.ThisvalueisusedtocalculateVgsfromthebiascharacteristic,Eq.(1-6),tocomparewiththeoriginalestimate.Ifthetwoagree,theoperatingpointisdetermined;ifnot,arevisedestimateforVgsistakenandtheprocessrepeated.Usuallytwoorthreetrysaresufficienttoattainsatisfactoryagreement.

Itisinconvenienttomeasureelectrodevoltageswithrespecttothesourceincircuitsemployingsourcebias.Thisisparticularlytruewhenmorethanonetransistorisusedinacircuit.Rather,usualpracticeistoreferallpotentialstoacommonpointcalledground.Thegroundpointisconsideredtobeelectricallyneutralsothat,forexample,thegroundpointsoftwoseparatecircuitsmaybeconnectedwithnoinfluenceupontheoperationofeithercircuit.AtypicaluseofthecircuitsymbolforthegroundpointinFig.1-3isatthejunctionofthelowerinputandoutputterminalsandthebottomendofthesourceresistor.

1-1.3

SmallSignalParameters

VeryfrequentlythesignalamplitudesappliedtoanFETamplifieraresmallcomparedwiththefullrangeofvoltagescoveredbythedraincharacteristics.InthissituationgraphicalanalysisofFETperformanceisinaccuratebecausethedraincharacteristicsarenotgivenwithsufficientprecision.AmoresatisfactoryprocedureistoreplacetheFETwithanequivalentcircuitwhichcanbestudiedbyconventionalcircuitanalysis.Aftertheoperatingpointisdeterminedgraphically,smalldeparturesabouttheoperatingpointcausedbysmallsignalsaretreatedbyassumingthattheFETisalineardevice.Accordingtothedraincharacteristics,draincurrentdependsuponboththegatevoltageandthedrainvoltage.Therefore,asmallchangeindraincurrent,ΔId,awayfromtheoperatingpointmaybewrittenΔId=k1ΔVgs+k2ΔVds

(1-7)whereΔVgsandΔVdsaresmallchangesintheelectrodepotentialsandk1andk2areconstants.

TheratioΔId/ΔVdscanbeidentifiedasthereciprocalofanequivalentresistancecalledthedrainresistance,(1-8)Similarly,theratioΔId/ΔVgs

givesthechangeindraincurrentresultingfromachangeinvoltageinthegatecircuit.Ithasthedimensionsofconductanceandiscalledthemutualtransconductance,(1-9)Thetotalchangeindraincurrentis,fromEqs.(1-7),(1-8),and(1-9),(1-10)

ConsidernowthatthesmalldeparturesabouttheoperatingpointareACsignals,sothatEq.(1-10)becomes(1-11)Thisexpressionmaybeinterpretedasthecircuitequationforacurrentgeneratorinparallelwitharesistor,asinFig.1-5;summingthecurrentsattheuppernodeyieldsEq.(1-11)directly.Figure1-5istheacequivalentcircuitoftheFET.NotethatithastheconfigurationoftheNortonequivalentcircuitdiscussedinChap.1,whichisconsistantwiththeconstantcurrentpropertiesoftheFETdraincharacteristics.Figure1-5

FETACequivalentcircuitThemutualtransconductanceandthedrainresistancearecalledthesmallsignalparametersoftheFET.Themutualtransconductanceisjusttheslopeofthetransfercharacteristicattheoperatingpoint,andthedrainresistanceisthereciprocalofthedraincharacteristicattheoperatingpoint.TypicalvaluesofsmallsignalparametersgiveninTable1-1showthattheseveraldifferenttypesofFETsarecomparable.AratherwiderangeofparametersmaybeachievedbysuitablechoiceoftheFETdesign,however.Actually,sincetheFETisanonlineardevice,themagnitudesofthesmallsignalparametersdependuponthedcdraincurrentaswell,Fig.1-6.Forthisreason,itisusuallynecessarytoevaluategmandrdgraphicallyattheoperatingpoint.AccordingtoFig.1-6quiteappreciablevariationsinthesmallsignalparameterscanbeobtainedbyselectingtheoperatingpoint.ThisisimportantinthedesignofFETcircuitshavingspecificperformancerequirements.Table1-1

FETsmallsignalparametersType gm/10-6mhord/Ω2N5484

n-channelFET 2000 50,0002N5268

p-channelFET 1700 15,0002N3797

n-channelMOSFET2300 40,000

(depletion)[BH]3N157

p-channelMOSFET2000

17,000

(enhancement)Figure1-6Variationofsmallsignalparametersoftype2N5268pchannelFETwithdraincurrent.ItisillustrativetoanalyzethesimpleFETamplifierinFig.1-3bymeansoftheacequivalentcircuittechnique.AccordingtoFig.1-5,theequivalentcircuitoftheamplifierisobtainedbyreplacingtheFETwithaconstantcurrentgeneratorinparallelwitharesistor,Fig.1-7(a).Notethatthegateterminalisnotconnectedtotheremainderoftheequivalentcircuit.Thisisaconsequenceoftheveryhighinputresistanceofthereversebiasedgatejunction.Thiscircuitignoresthecapacitanceofthegatejunction,however,andisonlyaccurateatfrequencieslowenoughthatitscapacitivereactancecanbeneglected.ThismatteristreatedmoreextensivelyinChap.7.Figure1-7

(a)acequivalentcircuitofFETamplifierinFig.1-3,and

(b)simplifiedversionassumingcapacitivereactancesarenegligible.

Theoutputsignalmaybewrittendownimmediatelyasthevoltagedropacrosstheparallelcombinationoftheloadresistanceandthedrainresistance,(1-12)

Theratiooftheoutputsignaltotheinputsignaliscalledthegainoftheamplifier.Sincethegatevoltageisequaltotheinputsignal,thegainis(1-13)MostoftenthedrainresistanceismuchgreaterthantheloadresistorsothatEq.(1-13)reducestoa=-gmRL

(1-14)

AccordingtoTable1-1,againofabout7000×2000×10-6=14canbeanticipated.Amoreprecisevalueiscalculatedafterdeterminingtheactualmutualtransconductanceattheoperatingpoint.Also,theminussigninEq.(1-14)representsthe180°phaseshiftbetweeninputandoutputsignals,asdiscussedpreviously.AnimportantfeatureoftheFETamplifieristhelargeinputimpedancewhichisessentiallyequaltoReqinFig.1-7(b).Alsoofinterestistheoutputimpedance,whichisequaltotheloadresistorsolongasthedrainresistanceislarge,asassumedinarrivingatEq.(1-14). 1-2

JunctionTransistorCircuits

1-2.1

CommonEmitter

ThecompletecircuitofapracticalcommonemitteramplifierusingannpntransistorisshowninFig.1-8.TheinputandoutputcouplingcapacitorsC1andC2passacsignalvoltagesandassurethatthedcoperatingpointofthetransistorisindependentofthesourceandloadconditions.TheemitterbypasscapacitorCEshortsouttheemitterbiasresistorREforacsignals.Figure1-8Practicalcommonemitteramplifierusingnpntype2N930transistor.Assumingthatthereactanceofallthreecapacitorsisnegligible,thehybridequivalentcircuitappropriateforFig.1-8isshowninFig.1-9.Notethatithasthesameformasforthegroundedbaseconfiguration,Fig.1-6.Therelationbetweengrounded-emitterandgroundedbasehparametersisdevelopedbyfirstwritingthecircuitequationspertainingtoFig.1-9,ic=hfeib+hoevcevbe=hieib+hrevce

(1-15)(1-16)Figure1-9

HybridequivalentcircuitofamplifierinFig.1-8.wherethehparametersubscriptsaretakenfromTable1-2.Thecorrespondingrelationsforthegroundedbasecase,Eqs.(1-15)and(1-16),canbeputintothisformwiththeaidofthefollowinggeneralrelations:vbe+vec+vcb=0ib+ie+ic=0(1-17)(1-18)Whichapplytoallconfigurations.UsingEqs.(1-17)and(1-18)toeliminateieandvcb,Eqs.(1-15)and(1-16)maybeputintheform(1-19)(1-20)TheapproximationthathrbandhobaresmallhasbeenintroducedinarrivingatEqs.(1-19)and(1-20).ComparingtheseequationswithEqs.(1-15)and(1-16)establishesthevalidityoftheequivalentcircuitinFig.1-9andalsogivestherelationsbetweenthecommonemitterhparametersandthecommonbasehparameters.TheserelationshipsarealsosummarizedinTable1-3.

Theperformanceofthegroundedemitteramplifierisexaminedbyanalyzingtheequivalentcircuitasfollows.Theoutputvoltagecanbewrittendirectlyasthecurrentthroughtheparallelcombinationof1/hoeandRL

(1-21)Kirchhoff’sruleappliedtotheinputcircuityieldsvi=hieib+hrevo

(1-22)Equation(1-22)issolvedforibandthisissubstitutedintoEq.(1-21).Theresultisarrangedtogivethevoltagegain(1-23)ByreferringtoTable1-3tocomparethehparameterswiththeTequivalentparameters,itmaybeseenthathreissmallandthathoeRLmaybeneglectedwithrespecttounity.ThereforeEq.(1-23)hastheapproximateform(1-24)AccordingtoEq.(1-24),thevoltagegainisapproximatelyequaltotheforwardcurrentgainofthetransistortimestheratiooftheloadresistancetotheinputresistance.Thevoltagegainofthecommonemitteramplifierisappreciablesincebothfactorsarelarge.Theminussignsignifiesthattheinputandoutputsignalsare180°outofphase.

Thetransistorisacurrentcontrolleddevice,andthecurrentgain,whichistheratiooftheoutputcurrenttotheinputcurrent,isalsoimportant.TheoutputcurrentisdeterminedfromEq.(1-21)(1-25)Forsimplicity,theeffectofReqinthebiasnetworkisassumednegligible,sothecurrentgainis(1-26)Hereagain,hoeRL<<1,sotheapproximatecurrentgainisjusthfe.

Theinputresistanceistheratiooftheinputvoltagetotheinputcurrent,orUsingEq.(1-21),(1-27)InarrivingatEq.(1-27),Eq.(1-22)hasbeenusedandtheeffectofReqhasagainbeenneglected.Ifnecessary,ReqmaybeincludedbycalculatingtheparallelcombinationofReqandRi.InEq.(1-27),thesecondtermisoftennegligible,sotheinputresistanceisapproximatelyhie.Note,however,thattheexactinputresistancedependsuponthevalueoftheloadresistanceRL.Thisillustratesthecouplingbetweeninputandoutputterminalsinherentintransistors.

Theoutputresistanceoftheamplifierincludestheinternalresistanceoftheinputsignalsourcebecauseofthecouplingbetweeninputandoutputterminals.TheeffectiveinternalresistanceoftheamplifierasviewedfromtheoutputterminalsisdeterminedusingtheTheveninequivalentcircuit.TheequivalentinternalresistanceintheThevenincircuitistheratiooftheopencircuit(RL=∞)voltagetotheshortcircuit(RL=0)current.Thus(1-28)ThenumeratorusesEq.(1-21)withRL=∞,andthedenominatorcomesfromtheoutputloopofFig.1-9.Kirchhoff’sruleappliedtotheinputloopyields

wherevgandRgarethevoltageandinternalresistanceofthesignalsource,respectively.Equation(1-29)maybesolvedforibandusedtoevaluate(ib)ocbyintroducingvofromEq.(1-21).Similarly,(ib)scresultsfromsettingvo=0inEq.(1-29).SubstitutingthesevaluesintoEq.(1-28)givesthefollowingexpressionfortheoutputresistanceoftheamplifier.

(1-29)(1-30)Tothesamedegreeofapproximationintroducedpreviously,theoutputresistanceisjust1/hoe,whichisafairlyhighvalue.

Insummary,thecommonemittertransistoramplifieryieldsbothvoltageandcurrentgain.Ithasamodestlyhighinputresistanceandlargeoutputresistance.Becauseofthefavorablevaluesofthesefourquantities,itisthemostcommonlyusedtransistoramplifiercircuit.Table1-4illustratesthemagnitudeoftheseimportantparameters,calculatedfromthehparametersofFig.1-17atanemittercurrentof2mA.Thevariousapproximationsintroducedabovehavebeenusedinpreparingthistable.1-2.2

GroundedBase

AtypicalcommonbaseamplifiercircuitusingannpntransistorisshowninFig.1-20.Carefulcomparisonofthiscircuitwiththecommonemitterconfiguration,Fig.1-18,revealsthatthebiasarrangementsareidentical.Therefore,biasconsiderationsandthetechniquesfordeterminingtheoperatingpointpreviouslydescribedforthecommonemitteramplifierapplytothegroundedbasecircuitaswell.Figure1-20Commonbasetransistoramplifier.Notebiascircuitisidenticaltocommonemittercase.CapacitorCBbypassesthebaseresistorR2,andR1isshortedoutforacsignalsbythelowimpedanceofthebattery.Consequently,intheappropriatehybridequivalentcircuit,Fig.1-16,theseresistorsareabsent.ThepropertiesofthiscircuitcanbefounddirectlybycomparingFig.1-16withthehybridequivalentofthecommonemittercase,Fig.1-19.Thetwoareidenticalinformsothatthepreviousresultsforthevoltageandcurrentgain,Eqs.(1-23)and(1-26),andfortheinputandoutputresistance,Eqs.(1-27)and(1-30),applydirectlyuponsubstitutingtheapplicablehparameters.NotealsothatREreplacesReqandiereplacesibinconvertingtheequationstothecommonbaseconfiguration.

Theapproximatevoltagegaininthiscaseis-hfbRL/hib,whichcanbemadelarge,butonlyifRLisverygreat.Thisissobecausethecurrentgainisonly-hfb,whichisapproximatelyequaltounity.Notethathfbisinherentlyanegativequantity(Table1-3),sothattheinputandoutputvoltagesignalsareinphase.Theapproximateinputresistancehibisverylowbecauseitisessentiallytheresistanceoftheforwardbiasedemitterjunction.Conversely,theoutputresistance1/hobistheresistanceofthereversebiasedcollectorjunctionandisthereforeverylarge.Thiswidedisparitybetweeninputandoutputresistancemakesthegroundedbasecircuitlesspopularthanthecommonemitteramplifier,exceptforspecialapplications.AsummaryoftypicalcommonbasecircuitpropertiesispresentedinTable1-4.

1-2.3

EmitterFollower

Thecommoncollectoramplifier,Fig.1-21,ismoreoftentermedanemitterfollowerbecauseofsimilaritytothesourcefollowercircuit.Hereagain,biasconsiderationsareidenticalwiththosepreviouslydiscussed,andthehybridequivalentcircuitalsotakesthesameformastheoneforthecommonemitterandthegroundedbasecircuits.Relationsbetweenthecommoncollectorhparametersandthoseoftheotherconfigurationsarefoundbynetworkanalysisidenticaltothatusedabove.TherelationshipsaresummarizedinTable1-3.Figure1-21Commoncollectortransistoramplifier.Thiscircuitisalsocalledemitterfollower.

Chapter

2

INTRODUCTIONTOCOMMUNICATIONSSYSTEMS

Thischapterservestointroducethesubjectof“CommunicationsSystems,”andalsothisbookasawhole.Havingstudiedit,youwillhavebeenintroducedtoinformation,abasiccommunicationssystem,transmitters,receiversandnoise.Importantly,“modulation”willhavebeenintroduced,andtheabsoluteneedtouseitinconveyinginformationwillhavebeenmadeclear.Thefinalsectionbrieflydiscussesbandwidthrequirements,andshowsthatthebandwidthrequiredtotransmitsomewaveformsisagreatdealmorethanmighthavebeensuspected. 2-1

COMMUNICATIONS

Initsbasicelectricalsense,thetermcommunicationsreferstothesending,receptionandprocessingofinformationbyelectricalmeans.Assuch,thesubjecthasitsoriginsinthebeginningsofwiretelegraphyintheeighteenforties,telephonysomedecadeslaterandradioatthebeginningofthiscentury.Radiocommunication,madepossiblebytheinventionofthetriode,wasgreatlystimulatedbytheworkdoneduringWorldWarII.Ithassubsequentlybecomeevenmorewidespreadandrefinedthroughtheinventionanduseofthetransistor,integratedcircuitsandothersemiconductordevices.

Amoderncommunicationssystemisfirstconcernedwiththecollation,processingandstorageofinformationbeforeitstransmission.Theactualtransmissionthenfollows,withfurtherprocessingandthecombatingofnoise.Finallywehavereception,whichmayincludeprocessingstepssuchasdecoding,storageandinterpretation.Inthiscontext,formsofcommunicationsincluderadiotelephonyandtelegraphy,broadcasting,pointtopointandmobilecommunications(commercialormilitary),computercommunications,radar,radiotelemetryandradioaidstonavigation.Allthesearetreatedinturn,infollowingchapters.

Inordertobecomefamiliarwiththesesystems,itisnecessaryfirsttoknowaboutamplifiersandoscillators,thebuildingblocksofallelectronicprocessesandequipment.Withtheseasabackground,theeverydaycommunicationsconceptsofnoise,modulationandinformationtheory,aswellasthevarioussystemsthemselves,maybeapproached.Anylogicalordermaybeused,butitisbelievedthattheoneadoptedhere,thatis,basicsystems,communicationsprocessesandcircuits,andmorecomplexsystems,isthemostsuitable.Fromtimetotimeitisalsowelltoconsiderthehumanfactorsinfluencingaparticularsystem,astheymustalwaysaffectitsdesign,planninganduse.2-2

COMMUNICATIONSSYSTEMS

Beforeinvestigatingindividualsystems,itisnecessarytodefineanddiscussimportanttermssuchasinformation,messageandsignal,channel,noiseanddistortion,modulationanddemodulation,andencodinganddecoding.Tocorrelatetheseconcepts,ablockdiagramofageneralcommunicationssystemisshowninFig.2-1.Figure2-1

Blockdiagramofcommunicationssystem.2-2.1

Information

Thecommunicationssystemexiststocommunicateamessage.Thismessagecomesfromtheinformationsource,whichoriginatesit,inthesenseofselectingonemessagefromafiniteset.Althoughthisappliesmoretotelegraphythantoentertainmentbroadcasting,forexample,itmayneverthelessbeshowntoapplytoallformsofcommunications.Theset,ortotalnumberofmessages,consistsofindividualmessageswhichmaybedistinguishedfromoneanother.Thesemaybewords,groupsofwords,codesymbolsoranyotherprearrangedunits.

Informationitselfisthatwhichisconveyed.Theamountofinformationcontainedinanygivenmessageismeasuredinbitsorindits,whicharedealtwithinChap.15,anddependsonthenumberofchoicesthatmustbemade.Thegreaterthetotalnumberofpossibleselections,thelargertheamountofinformationconveyed.Forexample,toindicatethepositionofawordonthispage,itmaybesufficienttosaythatitisonthetoporbottom,leftorrightside,i.e.,twoconsecutivechoicesofoneoutoftwopossibilities.Ifthiswordmayappearinanyoneoftwopages,anotherchoiceofasimilarnaturemustbeindicated,andhencemoreinformationmustbegiven.Themeaning(orlackofmeaning)oftheinformationisquiteimmaterial,fromthispointofview;onlythequantityisimportant.However,itmustberealizedthatnorealinformationisconveyedbyaredundant(i.e.,totallypredictable)message.Redundancyis