模擬與數字通信導論課件4

模擬與數字通信導論-4SingleSidebandandVestigialSidebandAmplitudeModulationOverviewTheoriginalAMsignalthatwediscussediswastefulofbothbandwidthandpowerDSB-SCimprovespowerefficiencybyeliminatingtheunmodulatedcarrierWenowwishtoimprovethebandwidthefficiencyWecanreducethebandwidthrequirementsbyintroducingtwonewformsofAMSingleSideband(SSB)AMVesttigialSideband(VSB)AMReading3.6-3.7ObjectivesTheobjectivesofthislecturearetoDemonstratethatthebandwidthefficiencyofLC-AMandDSB-SCAMcanbeimprovedthroughtwotechniquesSSBandVSBDescribehowSSBandVSBsignalsarecreatedanddemodulatedSpectrumofDSB-SCAMSingleSideband(SSB)SingleSIdeband(SSB)AMisanAMmodulationschemewhereonlyoneofthetwosidebandsaretransmittedIfthelowersidebandistransmitted,wecallthisLowerSSBorLSSBIftheuppersidebandistransmitted,wecallthisUpperSSBorUSSBTodevelopSSB,letusfirstconsiderasinusoidalmessagesignal:ThetransmitsignalforaDSB-SCAMwaveisThus,thetransmitsignalhastwo"sidefrequencies"

OneatandoneatNowsupposethatwesuppress(somehow)thelower"side-frequency":ThiscanbewrittenasOntheotherhand,ifwewantedtosuppressthehigher"side-frequency",weobtainThus,wecansaythatingeneral,forasingleside-frequencywehavewherethenegativesignresultsintheupperside-frequencyandtheplussignresultsinthelowerside-frequencySingleSideband(SSB)Now,consideraperiodicsignalthatcanbewrittenintermsofcosinesToobtainonlyasingleside-band(i.e.,alloftheside-frequencies)wecancreatethesignalIfwedefineanewsignalWecanwriteFurther,weshouldnotethatissimplywitheachcosinetermshiftedby-90.CommentsLetusnowmakeonecomments:ThephaseshifterissimplytheHilbertTransformSingleSidebandAMForanymessagesignal(periodicoraperiodic),itcanbeshownwherethenegativesignproducestheuppersidebandandthepositivesignproducesthelowersidebandInotherwordsProofofSSBSpectrumTakingtheFourierTransformToevaluatetheuppersidebandwechoosethesignsonthetopToevaluatethelowersidebandwechoosethesignsonthebottomUpperSidebandLowerSidebandSpectrumofUSSBAMSpectrumofLSSBAMModulatorsforSSBFrequencydiscriminationmethodDirectimplementationofpreviousfrequencydomainequations(phasediscriminator)FrequencyDiscriminator-IssuesClearlythemostchallengingcomponentofthefrequencydiscriminatoristhebandpassfilterInordertotransmitoneofthetwosidebandsrequiresanimpracticallysteepfilterThisisonlypracticallyrealizableifthesignalhasanenergygapnearDCModulatorsforSSBPhasediscriminationmethodDirectimplementationofthetimedomainequationsPhaseDiscriminatorThemostchallengingaspectofthephasediscriminationmethodisthewidebandphaseshifterItmustphaseshiftallcomponentsofthesignalbyThetechniqueessentiallycreatesaphaseshiftedversionofthemessagesignaltocauseself-interferencewhichcancelseithertheupperorlowersidebandDemonstratestheusefulnessofphaseCoherentDetectionofSSBAcoherentproductdetectorwhichwasusedforDSBSCcanalsobeusedforSSBCoherentDetection-cont.AttheoutputoftheproductdemodulatorAttheoutputofthelowpassfilterFrequencyTranslationThebasicoperationinSSBmodulationanddemodulationisfrequencytranslationThisisalsocalledfrequencychanging,mixingorheterodyningFrequencyTranslation-cont.TherearetwobasicformsoffrequencytranslationUpconversionDownconversionUpconversionThetranslatedfrequencyishigherthantheincomingfrequencyTheoscilatorfrequencyDownconversionThetranslatedfrequencyislowerthantheincomingfrequencyTheoscillatorfrequencyExample-UpconversionOriginalSpectrumMixeroutput

VestigialSidebandAMSSBtransmissionhasseriouschallenges,particularlyifthemessagesignalhasenergynearDCFilterchallengesassociatedwithfrequencydiscriminationtechniqueWidebandphaseshifterassociatedwiththephasediscriminationtechniqueDSB-SCrequiresdoublethebandwidthofSSBbutiseasiertoimplementAcompromiseprovidesabandwidthbetweenSSBandDSB-SCbutismoreimplementableTermedVestigialSidebandVestigialSidebandAM-cont.VSBisdifferentfromSSBinthatInsteadofcompletelyeliminatingoneofthetwosidebands,aprotionorvestigeofthesidebandistransmittedInsteadoftransmittingtheentireportionoftheothersideband,mostoftheothersidebandistransmittedTotalbandwidthwhereVSBShapingfilter-cont.ThekeytoVSBamplitudemodulationistheshapingfilter.SimilartothefrequencydiscriminationmethodofproducingSSB,thisfilterisabandpassfilterwhichfiltersouteithertheupperorlowersideband.IncontrasttotheSSBcase,thisfilterdoesnotneedtocompletelyeliminatetheunwantedbandanddoesnotperfectlyretainthedesiredband.ThisisthekeytomakingthefilterpracticallyrealizableHowever,thekeytothefilterdesignisthatthetransmittedvestigemustcompensateforthedistortioncausedtothedesiredsideband.Thisismaintainedbythefollowingfilterrequirement:VSBShapingfilter-cont.ispositive-frequencyportionofthebandpasstransferfunctionshiftedtotheleftby.isnegative-frequencyportionofthebandpasstransferfunctionshiftedtotherightby.ThetransferfunctionoftheVSBshapingfilterexhibitsoddsymmetryaboutthecarrierfrequencyinthefrequencyregionoftheunwantedsideband.Therequirementabovemustholdonlyfortheintervalgiven.Thefiltercanhavearbitraryspecsoutsidethefrequencybandofthemessage.VSBShapingfilter-cont.PositivefrequencyportionoftheVSBshapingfilterExampleConsiderasinusoidalmessagesignalandthecarrierwaveWehavethefollowingposotiveportionofthespectrumassumingthatThetransmittedspectrumisthenTakingtheinverseFourierTransformwehaveNotethatif

K=0.5wehaveDSB-SCAM

K=0wehaveLSSBAM

K=1wehaveUSSBAM0<K<0.5wehaveVSBwheretheattenuateduppersidebanddefinesthelowersidebandvestige0.5<K<1wehaveVSBwheretheattenustedlowersidebanddefinestheuppersidebandvestigeCoherentDetectionofVSBAcoherentproductdetectorwhichwasusedforDSBSCcanalsobeusedforVSBCoherentDetectionofVSB-cont.Letusexaminecoherentdetection