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1、BasebandandBandpassSignalUp&downconversionVSsamplingWhattolearn?Alwaysrememberforrealsignals:TheshapeofthespectrumofanyrealsignalissymmetricalabouttheyaxisThebasebandsignalspectrumshapeissymmetricalabouttheyaxisThebandpasssignalspectrumshapeissymmetricalabouttheyaxisThoughbandpasssignalisupconverte
2、dfrombaseband,itsshapeisnotnecessarilysymmetricalaboutthecarrierfrequencyaxis,actually,itisasymmetricalaboutthecarrierfrequencyforsignalssuchasquadraturemodulatedones(containingphaseorfrequencymodulatedinformation),ifthemodulationisamplitudeonly,thesignalspectrumshapewillalsobesymmetricalabouttheca
3、rrierfrequencyWhattolearn?Samplingwillnotchangetheshapeofthesignalspectrum,itjustduplicatestheoriginalspectrumexactlybyintegershiftsofthesamplingfrequencySamplingofbasebandsignalleadstoduplicatesofbasebandsignalwhoseshapeissymmetricalabouttheaxis(includingDCzerofrequency,isequivalenttozerofrequency
4、insamplingsystems),andsodoesthebandpasssignalUpconversionanddownconversionwillpossiblychangethespectrumshapeforsignals,forexample,quadraturemodulatedones(Actually,itcannotbecalledachange,itjustmakesthedifferenceoftheupperandlowerfrequencypartsshowsup,whentheyareinbaseband,theupperandlowerpartareima
5、getoeachotherandcannotbediscriminated)Forquadraturesignals,thebandpasssignalisthesumoftheIpathandtheQpath,thephaseinformationiscontainedinthebandpassspectrumandtheupperpartandlowerpartaboutthecarrierfrequencyaxisisdifferent,thisisbecausetheimagesareseparatedbyupconversionForheterodyne(IF)receiverar
6、chitecture,theimageandthewantedsignalismirroredaboutthecarrierfrequency,theimagerejectingbyHilbertfiltering(complexfilter)intheanalogdomainisstraitforwardtocanceltheunwantedsignal,however,non-idealitiesmainlyduetoI/Qimbalancewillleadtoimageresidue,thereforeimagerejectionratioIRRisdefinedWhattolearn
7、?Fordirectconversionorhomodyneorzero-IFreceiverarchitecture,theimageisthesignalitself,asweknow,forsignalssuchasthequadraturemodulatedones,theIpathandtheQpatharebothnecessarytorecovertheoriginalphaseinformat
