王春莲 外文资料.docx

王春莲 外文资料.docx

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王春莲外文资料

外文资料

MODESOFOPERATION

TheAD9857hasthreeoperatingmodes:

•Quadraturemodulationmode（default）

•Single-tonemode

•InterpolatingDACmode

Modeselectionisaccomplishedbyprogrammingacontrolregisterviatheserialport.TheinverseSINCfilterandoutputscalemultiplierareavailableinallthreemodes.

QUADRATUREMODULATIONMODE

Inquadraturemodulationmode,boththeIandQdatapathsareactive.AblockdiagramoftheAD9857operatinginthequadraturemodulationmodeisshowninFigure18.

Inquadraturemodulationmode,thePDCLK/FUDpinisanoutputandfunctionsastheparalleldataclock（PDCLK）,whichservestosynchronizetheinputofdatatotheAD9857.Inthismode,theinputdatamustbesynchronizedwiththerisingedgeofPDCLK.ThePDCLKoperatesattwicetherateofeithertheIorQdatapath.ThisisduetothefactthattheIandQdatamustbepresentedtotheparallelportastwo14-bitwordsmultiplexedintime.OneIwordandoneQwordtogethercompriseoneinternalsample.Eachsampleispropagatedalongtheinternaldatapathwayinparallelfashion.

TheDDScoreprovidesaquadrature（sinandcos）localoscillatorsignaltothequadraturemodulator,wheretheIandQdataaremultipliedbytherespectivephaseofthecarrierandsummedtogether,toproduceaquadrature-modulateddatastream.

Allofthisoccursinthedigitaldomain,andonlythenisthedigitaldatastreamappliedtothe14-bitDACtobecomethequadrature-modulatedanalogoutputsignal.

SINGLE-TONEMODE

AblockdiagramoftheAD9857operatinginthesingle-tonemodeisshowninFigure19.Inthesingle-tonemode,boththeIandQdatapathsaredisabledfromthe14-bitparalleldataportuptoandincludingthemodulator.ThePDCLK/FUDpinisaninputandfunctionsasafrequencyupdate（FUD）controlsignal.Thisisnecessarybecausethefrequencytuningwordisprogrammedviatheasynchronousserialport.TheFUDsignalcausesthenewfrequencytuningwordtobecomeactive.

Insingle-tonemode,thecosineportionoftheDDSservesasthesignalsource.Theoutputsignalconsistsofasinglefrequencyasdeterminedbythetuningwordstoredintheappropriatecontrolregister,pereachprofile.

Inthesingle-tonemode,no14-bitparalleldataisappliedtotheAD9857.TheinternalDDScoreisusedtoproduceasinglefrequencysignalaccordingtothetuningword.Thesingle-tonesignalthenmovestowardtheoutput,wheretheinverseSINCfilterandtheoutputscalingcanbeapplied.Finally,thedigitalsingle-tonesignalisconvertedtotheanalogdomainbythe14-bitDAC.

INTERPOLATINGDACMODE

AblockdiagramoftheAD9857operatingintheinterpolatingDACmodeisshowninFigure20.Inthismode,theDDSandmodulatorarebothdisabledandonlytheIdatapathisactive.TheQdatapathisdisabledfromthe14-bitparalleldataportuptoandincludingthemodulator.

Asinthequadraturemodulationmode,thePDCLKpinisanoutputandfunctionsasaclockwhichservestosynchronizetheinputofdatatotheAD9857.Unlikethequadraturemodulationmode,however,thePDCLKoperatesattherateoftheIdatapath.ThisisbecauseonlyIdataisbeingpresentedtotheparallelportasopposedtotheinterleavedI/Qformatofthequadraturemodulationmode.

IntheInterpolatingDACmode,thebasebanddatasuppliedattheparallelportremainsatbasebandattheoutput;thatis,nomodulationtakesplace.However,asamplerateconversiontakesplacebasedontheprogrammedinterpolationrate.Theinterpolationhardwareperformsthenecessarysignalprocessingrequiredtoeliminatethealiasedimagesatbasebandthatwouldotherwiseresultfromasamplerateconversion.TheinterpolatingDACfunctioniseffectivelyanoversamplingoperationwiththeoriginalinputspectrumintactbutsampledatahigherrate.

SIGNALPROCESSINGPATH

TobetterunderstandtheoperationoftheAD9857itishelpfultofollowthesignalpathfrominput,throughthedevice,totheoutput,examiningthefunctionofeachblock（refertoFigure1）.TheinputtotheAD9857isa14-bitparalleldatapath.ThisassumesthattheuserissupplyingthedataasinterleavedIandQvalues.Anyencoding,interpolation,andpulseshapingofthedatastreamshouldoccurbeforethedataispresentedtotheAD9857forupsampling.

TheAD9857demultiplexestheinterleavedIandQdataintotwoseparatedatapathsinsidethepart.Thismeansthattheinputsamplerate（fDATA）,therateatwhich14-bitwordsarepresentedtotheAD9857,mustbe2×theinternalI/QSampleRate（fIQ）,therateatwhichtheI/Qpairsareprocessed.Inotherwords,fDATA=2×fIQ.

Fromtheinputdemultiplexertothequadraturemodulator,thedatapathoftheAD9857isadualI/Qpath.

AlltimingwithintheAD9857isprovidedbytheinternalsystemclock（SYSCLK）signal.Theexternallyprovidedreferenceclocksignalmaybeusedasis（1×）,ormultipliedbytheinternalclockmultiplier（4×−20×）togeneratetheSYSCLK.AllotherinternalclocksandtimingarederivedfromtheSYSCLK.

INPUTDATAASSEMBLER

InthequadraturemodulationorinterpolatingDACmodes,thedeviceaccepts14-bit,twoscomplementdataatitsparalleldataport.ThetimingofthedatasuppliedtotheparallelportmaybeeasilyfacilitatedwiththePDCLK/FUDpinoftheAD9857,whichisanoutputinthequadraturemodulationmodeandtheinterpolatingDACmode.Inthesingle-tonemode,thesamepinbecomesaninputtothedeviceandservesasafrequencyupdate（FUD）strobe.

FrequencycontrolwordsareprogrammedintotheAD9857viatheserialport（seetheControlRegisterdescription）.Becausetheserialportisanasynchronousinterface,whenprogrammingnewfrequencytuningwordsintotheon-chipprofileregisters,theAD9857’sinternalfrequencysynthesizermustbesynchronizedwithexternalevents.ThepurposeoftheFUDinputpinistosynchronizethestartofthefrequencysynthesizertotheexternaltimingrequirementsoftheuser.TherisingedgeoftheFUDsignalcausesthefrequencytuningwordoftheselectedprofile（seetheProfilesection）tobetransferredtotheaccumulatoroftheDDS,thusstartingthefrequencysynthesisprocess.

Afterloadingthefrequencytuningwordtoaprofile,aFUDsignalisnotneededwhenswitchingbetweenprofilesusingthetwoprofileselectpins（PS0,PS1）.Whenswitchingbetweenprofiles,thefrequencytuningwordintheprofileregisterbecomeseffective.

Inthequadraturemodulationmode,thePDCLKrateistwicetherateoftheI（orQ）datarate.TheAD9857expectsinterleavedIandQdatawordsattheparallelportwithonewordperPDCLKrisingedge.OneIwordandoneQwordtogethercompriseoneinternalsample.Eachsampleispropagatedalongtheinternaldatapathwayinparallel.

IntheinterpolatingDACmode,however,thePDCLKrateisthesameastheIdataratebecausetheQdatapathisinactive.Inthismode,eachPDCLKrisingedgelatchesadatawordintotheIdatapath.

ThePDCLKisprovidedasacontinuousclock（i.e.,alwaysactive）.However,theassertionofPDCLKmaybeoptionallyqualifiedinternallybythePLLlockindicatoriftheuserelectstosetthePLLlockcontrolbitintheappropriatecontrolregister.Datasuppliedbytheusertothe14-bitparallelportislatchedintothedevicecoincidentwiththerisingedgeofthePDCLK.

Inthequadraturemodulationmode,therisingedgeoftheTxENABLEsignalisusedtosynchronizethedevice.WhileTxENABLEisintheLogic0state,thedeviceignoresthe14-bitdataappliedtotheparallelportandallowstheinternaldatapathtobeflushedbyforcing0sdowntheIandQdatapathway.OntherisingedgeofTxENABLE,thedeviceisreadyforthefirstIword.ThefirstIwordislatchedintothedevicecoincidentwiththerisingedgeofPDCLK.ThenextrisingedgeofPDCLKlatchesinaQword,etc.,untilTxENABLEissettoaLogic0statebytheuser.

Wheninthequadraturemodulationmode,itisimportantthattheuserensurethatanevennumberofPDCLKintervalsareobservedduringanygivenTxENABLEperiod.ThisisbecausethedevicemustcapturebothanIandaQvaluebeforethedatacanbeprocessedalongtheinternaldatapathway.

ThetimingrelationshipbetweenTxENABLE,PDCLK,andDATAisshowninFigure21andFigure22.

INVERSECICFILTER

Theinversecascadedintegratorcomb（CIC）filterprecompen-satesthedatatooffsettheslightattenuationgradientimposedbytheCICfilter.SeetheProgrammable（2×to63×）CICInterpolatingFiltersection.TheI（orQ）dataenteringthefirsthalf-bandfilteroccupiesamaximumbandwidthofone-halffDATAasdefinedbyNyquist（wherefDATAisthesamplerateattheinputofthefirsthalf-bandfilter）.ThisisshowngraphicallyinFigure23.

IftheCICfilterisemployed,theinbandattenuationgradientcouldposeaproblemforthoseapplicationsrequiringanextremelyflatpassband.Forexample,ifthespectrumofthedataassuppliedtotheAD9857IorQpathoccupiesasignificantportionoftheone-halffDATAregion,thehigherfrequenciesofthedataspectrumreceivesslightlymoreattenuationthanthelowerfrequencies（theworst-caseoveralldroopfromf=0toone-halffDATAis<0.8dB）.Thismaynotbeacceptableincertainapplications.TheinverseCICfilterhasaresponsecharacteristicthatistheinverseoftheCICfilterresponseovertheone-halffDATAregion.

Thenetresultisthattheproductofthetworesponsesyieldsinanextremelyflatpassband,therebyeliminatingtheinbandattenuationgradientintroducedbytheCICfilter.Thepricetobepaidisaslightattenuationoftheinputsignalofapprox-imately0.5dBforaCICinterpolationrateof2and0.8dBforinterpolationratesof3to63.

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