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1、外文文獻(xiàn)原文：外文文獻(xiàn)原文：AskTheApplicationEngineer—33AllAboutDirectDigitalSynthesisByEvaMurphy(eva.murphy@)ColmSlattery(colm.slattery@)ByAnalogDialogueVolume38Number3ofADIWhatisDirectDigitalSynthesisDirectdigitalsynthesis(DDS)isame

2、thodofproducingananalogwavefm—usuallyasinewave—bygeneratingatimevaryingsignalindigitalfmthenperfmingadigitaltoanalogconversion.BecauseoperationswithinaDDSdeviceareprimarilydigitalitcanofferfastswitchingbetweenoutputfrequ

3、enciesfinefrequencyresolutionoperationoverabroadspectrumoffrequencies.Withadvancesindesignprocesstechnologytoday’sDDSdevicesareverycompactdrawlittlepower.Whywouldoneuseadirectdigitalsynthesizer(DDS)Aren’tthereothermethod

4、sfeasilygeneratingfrequenciesTheabilitytoaccuratelyproducecontrolwavefmsofvariousfrequenciesprofileshasbecomeakeyrequirementcommontoanumberofindustries.Whetherprovidingagilesourcesoflowphasenoisevariablefrequencieswithgo

5、odspuriousperfmancefcommunicationssimplygeneratingafrequencystimulusinindustrialbiomedicaltestequipmentapplicationsconveniencecompactnesslowcostareimptantdesignconsiderations.Manypossibilitiesffrequencygenerationareopent

6、oadesignerrangingfromphaselockedloop(PLL)basedtechniquesfveryhighfrequencysynthesistodynamicprogrammingofdigitaltoanalogconverter(DAC)outputstogeneratearbitrarywavefmsatlowerfrequencies.ButtheDDStechniqueisrapidlygaining

7、acceptancefsolvingfrequency(wavefm)generationrequirementsinbothcommunicationsindustrialapplicationsbecausesinglechipICdevicescangenerateprogrammableanalogoutputwavefmssimplywithhighresolutionaccuracy.Furthermethecontinua

8、limprovementsinbothprocesstechnologydesignhaveresultedincostpowerconsumptionlevelsthatwerepreviouslyunthinkablylow.FexampletheAD9833aDDSbasedprogrammablewavefmgenerat(Figure1)operatingat5.5Vwitha25MHzclockconsumesamaximu

9、mpowerof30milliwatts.Figure1.TheAD9833—aonechipwavefmgenerat.ADDSproducesasinewaveatagivenfrequency.Thefrequencydependsontwovariablesthereferenceclockfrequencythebinarynumberprogrammedintothefrequencyregister(tuningwd).T

10、hebinarynumberinthefrequencyregisterprovidesthemaininputtothephaseaccumulat.Ifasinelookuptableisusedthephaseaccumulatcomputesaphase(angle)addressfthelookuptablewhichoutputsthedigitalvalueofamplitude—crespondingtothesineo

11、fthatphaseangle—totheDAC.TheDACinturnconvertsthatnumbertoacrespondingvalueofanalogvoltagecurrent.Togenerateafixedfrequencysinewaveaconstantvalue(thephaseincrement—whichisdeterminedbythebinarynumber)isaddedtothephaseaccum

12、ulatwitheachclockcycle.Ifthephaseincrementislargethephaseaccumulatwillstepquicklythroughthesinelookuptablethusgenerateahighfrequencysinewave.Ifthephaseincrementissmallthephaseaccumulatwilltakemanymestepsaccdinglygenerati

13、ngaslowerwavefm.Let’stalksomemeaboutthephaseaccumulat.HowdoesitwkContinuoustimesinusoidalsignalshavearepetitiveangularphaserangeof0to2π.Thedigitalimplementationisnodifferent.Thecounter’scarryfunctionallowsthephaseaccumul

14、attoactasaphasewheelintheDDSimplementation.Tounderstthisbasicfunctionvisualizethesinewaveoscillationasavectrotatingaroundaphasecircle(seeFigure4).Eachdesignatedpointonthephasewheelcrespondstotheequivalentpointonacycleofa

15、sinewave.Asthevectrotatesaroundthewheelvisualizethatthesineoftheanglegeneratesacrespondingoutputsinewave.Onerevolutionofthevectaroundthephasewheelataconstantspeedresultsinonecompletecycleoftheoutputsinewave.Thephaseaccum

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