Rigol DS6104 Administrator Guide

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Oscilloscope
SignalAnalysis
UsingFFTs
Application Notes
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FAQs
How-To Guides
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877-4-RIGOL-1 How-To Guide
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FastFourierTransformswithRigolOscilloscopes
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FFT(FastFourierTransform)isaprocesswhichconvertsatime‐domain
signalintoitsfrequencydomainequivalent.Commonapplicationsforusage
ofFFTwaveformsare:
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zMeasuringharmoniccontentanddistortioninsystems
zCharacterizingnoiseinDCpowersupplies
zAnalyzingvibration
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Howtoobtainawaveform'sFFT?
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InthisexampleanFFTwillbegeneratedforthe1KHzsquare
waveacquiredonChannel1asseenbelow.Thissignalcanbe
duplicatedonanyRigolscopebyattachingtheprobetothesquare
wavegeneratorportonthefrontoftheinstrument.Onceyoudo
that,pushAUTOthenyouwillseethis:
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Pressthe“MATH”button,amenuwillappearontheside,thenselectFFT
andsetthesourcetoChannel1:
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AFastFourierTransformofthecurrentsignalonChannel1willappearinthe
Purplecolorbelowthesquarewave.
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TheFFTcanbedisplayedfullscreenwiththechannelaswellbyadjusting
theWINDOWsetting.However,youliketoviewittakenoticeofthe
separateVERT/divandHORIZ/divsettingsshowninpurple.Youcanadjust
thesebypushingtheMATHbuttonandthenusingthehorizontaland
verticalscaleknobs.IftheFFTisn’tshownonthescreen,movetheVERTICAL
positionknobuntilyouseethezeropointandthenthesignal.
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TherearetwooptionsforverticalunitsontheFFT:mVrmsand
dBVrms.Whenlookingtoviewsmallsignalsalongwithmuchlargersignals
dBVrmsisbeneficialbecauseitprovidesalogscale.Forthosenotusedto
logarithmicscalinglookatthescreenshotabove.Inthesplitscreenmode
shownthereare8verticaldivisionsdedicatedtotheFFT.Herearethevalues
atthetopofeachdivisiongiventhe200mVrms/divsettingandtheposition
ofthezeromarkerontheleftatthetopofthebottomdivision:
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DivisionsdownfromtopofFFT
screen
Valueattopofthatdivision
0(topofpurpledisplay)1.4Vrms
11.2Vrms
21Vrms
3800mVrms
4600mVrms
5400mVrms
6200mVrms
70mVrms
8(verybottomofthescreen) ‐200(negativenumbersarenotused
byamVrmsscale)
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So,inthismodetheFFThasabout1.4Vrmsoftotalrangecapability.Youcan
changethesensitivity,butifyouwantedtoseea10Vrmsfundamentalanda
10mVrmsharmonicatthesametimeitisn’tpossibleonalinearscale.

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IfyouchangetheunitstodBVrms,herearethevalues:
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Whenthesettingsare20dBVrms/divandthereferenceismovedtoone
divisiondownfromthetopoftheFFTdisplay:
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Divisionsdownfromtop
ofFFTscreen
dBVrmsfromposition
marker
Valueattopofthat
division
0+10dBVrms 3.16Vrms
10dBVrms 1Vrms(attheposition
marker)
2‐10dBVrms .316Vrms
3‐20dBVrms .1Vrms
4‐30dBVrms .0316 Vrms
5‐40dBVrms .01 Vrms
6‐50dBVrms .00316mVrms
7‐60dBVrms .001mVrms
8‐70dBVrms .000316mVrms
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Tounderstandthisfully,theformulatoderivethedBVrmsvalueisasfollows:
dBVrms=20*log(Vrms/VREF)
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whereVREFinthiscaseis1Vrms,soitcanbesimplifiedto:
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dBVrms=20*log(Vrms)
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Consequently,whenlookingataspotonthescreenthatisknowntobeX
dBVrmstheactualVrmsvalueexpressedbysignalatthatlocationis:
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Vrms=10^(X/20)
Forexample,‐20dBVrms=10^‐1=0.1Vrms

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Sointhiscaseevery20dBisafactorof10xthevoltage.Usually,dBisused
intermsofWattsormilliWatts(dBm)inwhichcaseevery10dBisafactorof
10xthepower,butbecauseoftherelationshipbetweenpowerandvoltage
notethat20dBisafactorof10onyourscopesdisplay.
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ThehorizontalunitsperdivisionoftheFFTarealwaysinfrequency.The
abovescreenshotshows1.25KHzperdivision.Inthisdisplay,theleftsideof
thescreenisalwaysatDCor0Hz.Eachdivisiontotherightthenmarksthe
setvalue.Inthiscasethedivisionswouldbe0Hz,1.25KHz,2.5KHz,3.75
KHz,5KHz,andsoon.
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Howtosaveawaveform'sFFT?
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TosaveanFFTtothescopeoraUSBdrive,theFFTmustbesavedasa
referencewave“REF”asinthefollowingprocedure:
Pressthe“REF”button.Whenthesidemenuappears,setthesourcetobe
theFFT.Thenusethe“Save”optioninthemenutosavelocallyorexternally.
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RIGOL TECHNOLOGIES USA INC
877-4-RIGOL-1 How-To Guide
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Tosavethewaveform,selectsave.
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TosavetheFFTwaveformtoaUSBdrivepressthe
“Location”buttontheLocationnowswitchesto
“External”asinthefigurebelow:
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Uponselecting“Save”selectthefiletosavethewaveformtoandthenexit
uponcompletion.
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Referencewaveformscanthenberecalledatalatertimetocomparesignals.

RIGOL TECHNOLOGIES USA INC
877-4-RIGOL-1 How-To Guide
[email protected]
www.rigolna.com

Insummary,whilenotasaccurateasaspectrumanalyzerorwithasmuch
range,theFFTfunctionoftheoscilloscopecanbeusedtoviewthe
frequencydomaininformationforasignal.Withavarietyofcapabilitiesto
changeranges,units,anddisplaymodeaswellassavingandrecalling
waveforms,Rigoloscilloscopescanmakefrequencymeasurementseasily
andquicklyusingtheincludedFFTfunction.
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Formoreinformationorquestionscontactusdirectlyat:

RigolTechnologiesUSA
7401FirstPlace
SuiteN
OakwoodVillage,OH44146

Youcanreachusdirectlyat:
(440)232‐4488
877‐4‐RIGOL‐1
inf[email protected]om
www.rigolna.com(RigolNorthAmericawebsite)
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