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<<WorkingWithAssistants Browser>>

Spectrum

TheSpectrumassistantisusedtoplotandcalculatetheFastFourierTransform(FFT)ofaperiodic

waveformanditsdifferentmeasurements--Signal-to-Noise-and-DistortionRatio(SINAD),SpuriousFree

DynamicRange(SFDR),EffectiveNumberofBits(ENOB),andSignal-to-NoiseRatio(SNRwithout

distortion)ENOB,SINAD,SNR,SFDR,THD,sigpower,thddb,totalharmpower,peakharmpower,snb,snrh,

dcpower--foragiveninputsignal.ThespectrummeasureisusedforcharacterizingA-to-Dconvertersandis

typicallysupportedfortransientsimulationdata.

Note:InVirtuosoVisualizationandAnalysisXL,DiscreteFourierTransform(DFT)andFastFourier

Transform(FFT)arethesame.

ToopentheSpectrumassistant,selectasignalinthewindowanddooneofthefollowing:

 ChooseWindow-Assistants-Spectrum.

 ChooseMeasurements-Spectrum.

TheSpectrumassistantappearsontherightinthewindowbydefault.

FFTInputMethod--SelecttheFastFourierTransform(FFT)inputmethodfromthedrop-down

listbox.

IfyouselectCalculateSampleFrequency,whichisthedefaultoption,youneedto

specifythestartandstoptime.TheSampleCountandFrequencyfieldsdisplaythe

valuescalculatedbasedonthespecifiedstartandstoptime.

StartTime=Starttimeofthewaveform

EndTime=Endtimeofthewaveform

SampleFrequency=SampleCount/(StopTime-StartTime)

IfyouselectCalculateStartTime,youneedtospecifythestoptime,samplecount,

andfrequency.TheStartTimefielddisplaysthevaluecalculatedbasedonthe

specifiedstoptime,samplecount,andfrequency.

StartTime=StopTime-SampleCount/SampleFreq

IfyouselectCalculateStopTime,youneedtospecifythestarttime,samplecount,

andfrequency.TheStopTimefielddisplaysthevaluethatiscalculatedbasedon

starttime,samplecount,andfrequencythatyouspecify.

  Start/StopTime--Specifythestartandstoptimeoftheinputtimedomainperiodicwaveform.

  SampleCount/Freq--Specifythesamplecountthatisusedtodeterminethenumberoffrequency

binsintheFFTwaveform.Also,intheFreqfield,specifythesamplingfrequencythatdetermines

thesizeofthefrequencybinoftheFFTwaveform.

TheSampleCountvaluemustbegreaterthanzero.Bydefault,thisfielddisplays

thenumberofdatapointsintheselectedsignal.

SampleFrequency=SampleCount/(StopTime-StartTime)

StartTime=StopTime-(SampleCount/SampleFreq)

StopTime=Startime+(SampleCount/SampleFreq)

ThenumberofinputperiodsintheperiodoftheFouriertransformneedstobean

integerandaprimenumber,wheretheperiodoftheFouriertransformis1/the

fundamentalfrequencyoftheFouriertransform.

  WindowType--Specifythewindowfunctionthatyoucanapplytothewaveform,suchasHanning

orKaiser.Windowfunctionisamathematicalfunctionwithzero-valuesoutsideofagiven

interval.Thefunctioniscalledrectangularwindowfunctionwhenitisconstantinsidethegiven

intervalandzeroelsewhere,whichalsodescribestheshapeofitsgraphicalrepresentation.When

anotherfunctionorwaveformismultipliedbyawindowfunction,thentheresultisalsozero-

valuedoutsidethegiveninterval.

Bydefault,thisfielddisplaysRectangular.Someofthesewindowtypesincrease

theamplitudeofthesignal.Thishelpsindistinguishingthenoiseinthesignal.

YoucanuseRectangularwindowtypewhenyouhaveanintegernumberof

cycles.However,ifyouaresimulatingasigma-deltamodulator,youcanuse

HanninginsteadofRectangularevenwhenanintegernumberofcyclesoccur

betweenStartTimeandEndTime.

ForNyquistRateconverters,useRectangularwindow.ForDelta-Sigma

converters,useCosine2(Hanning)windowinsteadofRectangularevenwhenan

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