数字频率合成器的外文翻译资料Word格式.docx
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数字频率合成器的外文翻译资料@#@英文原文@#@ModulatingDirectDigitalSynthesizer@#@Inthepursuitofmorecomplexphasecontinuousmodulationtechniques,thecontroloftheoutputwaveformbecomesincreasinglymoredifficultwithanalogcircuitry.Inthesedesigns,usinganon-lineardigitaldesigneliminatestheneedforcircuitboardadjustmentsoveryieldandtemperature.AdigitaldesignthatmeetsthesegoalsisaDirectDigitalSynthesizerDDS.ADDSsystemsimplytakesaconstantreferenceclockinputanddividesitdownatoaspecifiedoutputfrequencydigitallyquantizedorsampledatthereferenceclockfrequency.ThisformoffrequencycontrolmakesDDSsystemsidealforsystemsthatrequireprecisefrequencysweepssuchasradarchirpsorfastfrequencyhoppers.Withcontrolofthefrequencyoutputderivedfromthedigitalinputword,DDSsystemscanbeusedasaPLLallowingprecisefrequencychangesphasecontinuously.Aswillbeshown,DDSsystemscanalsobedesignedtocontrolthephaseoftheoutputcarrierusingadigitalphasewordinput.Withdigitalcontroloverthecarrierphase,ahighspectraldensityphasemodulatedcarriercaneasilybegenerated.@#@ThisarticleisintendedtogivethereaderabasicunderstandingofaDDSdesign,andanunderstandingofthespuriousoutputresponse.Thisarticlewillalsopresentasampledesignrunningat45MHzinahighspeedfieldprogrammablegatearrayfromQuickLogic.@#@AbasicDDSsystemconsistsofanumericallycontrolledoscillator(NCO)usedtogeneratetheoutputcarrierwave,andadigitaltoanalogconverter(DAC)usedtotakethedigitalsinusoidalwordfromtheNCOandgenerateasampledanalogcarrier.SincetheDACoutputissampledatthereferenceclockfrequency,awaveformsmoothinglowpassfilteristypicallyusedtoeliminatealiascomponents.Figure1isabasicblockdiagramofatypicalDDSsystemdesign.ThegenerationoftheoutputcarrierfromthereferencesampleclockinputisperformedbytheNCO.ThebasiccomponentsoftheNCOareaphaseaccumulatorandasinusoidalROMlookuptable.AnoptionalphasemodulatorcanalsobeincludeintheNCOdesign.ThisphasemodulatorwilladdphaseoffsettotheoutputofthephaseaccumulatorjustbeforetheROMlookuptable.ThiswillenhancetheDDSsystemdesignbyaddingthecapabilitiestophasemodulatethecarrieroutputoftheNCO.Figure2isadetailedblockdiagramofatypicalNCOdesignshowingtheoptionalphasemodulator.@#@FIGURE1:
@#@TypicalDDSSystem.@#@FIGURE2:
@#@TypicalNCODesign.@#@TobetterunderstandthefunctionsoftheNCOdesign,firstconsiderthebasicNCOdesignwhichincludesonlyaphaseaccumulatorandasinusoidalROMlookuptable.ThefunctionofthesetwoblocksoftheNCOdesignarebestunderstoodwhencomparedtothegraphicalrepresentationofEuler’sformulaejwt=cos(wt)+jsin(wt).ThegraphicalrepresentationofEuler’sformula,asshowninFigure3,isaunitvectorrotatingaroundthecenteraxisoftherealandimaginaryplaneatavelocityofwrad/s.Plottingtheimaginarycomponentversustimeprojectsasinewavewhileplottingtherealcomponentversustimeprojectsacosinewave.ThephaseaccumulatoroftheNCOisanalogous,orcouldbeconsidered,thegeneratoroftheangularvelocitycomponentwrad/s.Thephaseaccumulatorisloaded,synchronoustothereferencesampleclock,withanNbitfrequencyword.@#@ThisfrequencywordiscontinuouslyaccumulatedwiththelastsampledphasevaluebyanNbitadder.TheoutputoftheadderissampledatthereferencesampleclockbyanNbitregister.WhentheaccumulatorreachestheNbitmaximumvalue,theaccumulatorrollsoverandcontinues.PlottingthesampledaccumulatorvaluesversustimeproducesasawtoothwaveformasshownbelowinFigure3.@#@FIGURE3Euler’sEquationRepresentedGraphically@#@ThesampledoutputofthephaseaccumulatoristhenusedtoaddressaROMlookuptableofsinusoidalmagnitudevalues.Thisconversionofthesampledphasetoasinusoidalmagnitudeisanalogoustotheprojectionoftherealorimaginarycomponentintime.Sincethenumberofbitsusedbythephaseaccumulatordeterminesthegranularityofthefrequencyadjustmentsteps,atypicalphaseaccumulatorsizeis24to32bits.SincethesizeofthesinusoidalROMtableisdirectlyproportionaltotheaddressingrange,notall24or32bitsofthephaseaccumulatorareusedtoaddresstheROMsinusoidaltable.OnlytheupperYbitsofthephaseaccumulatorareusedtoaddressthesinusoidalROMtable,whereY<@#@NbitsandYistypicallybutnotnecessarilyequaltoD,andDisthenumberofoutputmagnitudebitsfromthesinusoidalROMtable.@#@SinceanNCOoutputsacarrierbasedonadigitalrepresentationofthephaseandmagnitudeofthesinusoidalwaveform,designershavecompletecontroloverfrequency,phase,andevenamplitudeoftheoutputcarrier.Byaddingaphaseportandaphaseadderto