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by Tom Lecklider, Senior Technical Editor Tweet 0 Spectrum Analyzers Get Real From the SonyTek products of the 1980s and 1990s through today’s handheld H500/SA2500 and RSA5000 and RSA6000 series bench instruments, Tektronix (Tek) has led the development of realtime spectrum analyzers. Of course, the competition hasn’t been standing still, and recently Agilent Technologies and Rohde & Schwarz (R&S) introduced realtime analyzers. Anritsu’s MS269xA and MS2830A series of spectrum/signal analyzers feature a builtin realtime vector signal analyzer. Swept Wide Span DPX Spectrum Display Learn how realtime spectrum analysis techniques can help make precise and selective measurements of complex and elusive signals. Then see how to take the vital next steps to complete the solution with post processing analysis, signal replay, and transfer of captured signals to other analysis or simulation tools. 0 Like

Evaluation Engineering Special Report_ Spectrum Analyzers

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  • 10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers

    http://old.evaluationengineering.com/specialreports/201306/instruments/SpectrumAnalyzersGetReal.html 1/10

    byTomLecklider,SeniorTechnicalEditor Tweet 0

    SpectrumAnalyzersGetReal

    FromtheSonyTekproductsofthe1980sand1990sthroughtodayshandheldH500/SA2500andRSA5000andRSA6000seriesbenchinstruments,Tektronix(Tek)hasledthedevelopmentofrealtimespectrumanalyzers.Ofcourse,thecompetitionhasntbeenstandingstill,andrecentlyAgilentTechnologiesandRohde&Schwarz(R&S)introducedrealtimeanalyzers.AnritsusMS269xAandMS2830Aseriesofspectrum/signalanalyzersfeatureabuiltinrealtimevectorsignalanalyzer.

    SweptWideSpanDPXSpectrumDisplay

    Learnhowrealtimespectrumanalysistechniquescanhelpmakepreciseandselectivemeasurementsofcomplexandelusivesignals.Thenseehowtotakethevitalnextstepstocompletethesolutionwithpostprocessinganalysis,signalreplay,andtransferofcapturedsignalstootheranalysisorsimulationtools.

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  • 10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers

    http://old.evaluationengineering.com/specialreports/201306/instruments/SpectrumAnalyzersGetReal.html 2/10

    CourtesyofTektronix

    Spectrumanalyzershavebeenaroundforalongtimeseereference1foranexampleofaHewlettPackard1964microwaveanalyzerbutrealtimeinstrumentsaresignificantlydifferent.AccordingtoanAgilentrealtimespectrumanalyzertechnicaloverviewdocument,Inaspectrumorsignalanalyzerwithadigitalintermediatefrequency(IF)section,realtimeoperationisastateinwhichallsignalsamplesareprocessedforsomesortofmeasurementresultortriggeringoperation.Inadditiontogapfreeanalysis,arealtimeRFanalyzermaybedefinedashavingfourmorekeyattributes:highspeedmeasurements,consistentmeasurementspeed,frequencymasktriggering,andadvancedcompositedisplays.2

    Othercompanieshavesimilardefinitions,thedistinguishingfactorbeingthedigitalsignalprocessing(DSP)thathasreplacedthefinalIFsection.

    Traditional,analogdesignsfrequencyconverttheinputsignaltoasuitableIFwithabandwidthlargerthanthewidestbandwidthIFfiltertheresolutionbandwidth(RBW)filter.Inthefrequencyconversionprocess,amixerisdrivenbytheRFinputsignalandasweepinglocaloscillator(LO).WhentheoutputofthemixerfallswithintheIFbandwidth,itisamplifiedandappliedtotheRBWfilter.

    Thepurposeofthefilteristodiscriminatebetweenfrequenciesthatareclosetoeachother,sothefiltersrejectionsomedistanceawayfromthecenterfrequencyisimportant.AnalogdesignsuseadiodedetectortoconverttheRBWfilteroutputenvelopetoabasebandsignalthatmaybefurtherfilteredbyavideobandwidth(VBW)filterbeforedisplay.

    Mostcompaniesmakingspectrumanalyzersgraduallyhavereplacedanalogcircuitrywithdigitalformanyreasons.Eliminatingdriftovertimeandtemperatureaswellasimprovingaccuracyareamongtheusualreasonsthatthisgeneraltrendhasoccurredininstrumentdesign.Forspectrumanalyzers,therearealargenumberofadditionalbenefits.

    DigitalRBWFilters

    ThedigitalIFoverviewpresentedinanAgilentapplicationnotestated,ApartialimplementationofdigitalIFcircuitryisimplementedintheAgilentESAEseriesspectrumanalyzers.Whilethe1kHzandwiderRBWsareimplementedwithtraditionalanalogLCandcrystalfilters,thenarrowestbandwidths(1Hzto300Hz)arerealizedusingdigitaltechniques.3

    RatherthansweeptheLO,inESAEseriesinstruments,theLOstepsin900HzincrementswhennarrowRBWsareselected.Sufficientstepsmustbetakentocovertherequiredanalysisspan.TheoutputfromamixerwithafixedLOremainsatimedomainsignalbutcenteredatadifferentfrequency.AftersamplingbyanADC,FFTfiltersproducethefinalRBWfiltereddata.

    AnalogimplementationsofnarrowRBWfilterspresentseveralproblemsthatthedigitalapproachsolves.Analogfilterscontainenergystorageelementsthatmustcharge/dischargetoaccommodatetransients.This

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    meansthatsweepingtheLOmustbedoneslowlytoavoiddistortion.Inaddition,theselectivityofanarrowbandwidthanalogfilterismuchlessthanthatofanequivalentdigitalfilter.

    Asdescribedinreference4,the[R&SFSPandFSUanalyzerswere]equippedwithFFTfiltersforthenarrowbandwidths.MultiplenarrowbandFFTswereconcatenatedtoatracerepresentingtheselectedfrequencyspan.AsthecomputingtimefortheFFTswassmallcomparedtothesettlingtimefornarrowRBWfilters,theFFTmethodprovidedagreatspeedadvantageoverthetraditionalsweepmethod.FurtherdigitalprocessingimplementstheequivalentoftheVBWfunctionandscalingbeforedisplay.

    MakingthenextsteptoacompletelydigitalIFhasbecomepossiblebecauseofadvancesinhighspeedADCsandDSPhardwaresuchasFPGAs.Figure1showsablockdiagramofthealldigitalIFinAgilentsPSAseriesinstruments.ThisisagoodexampleofthecontinuingrelocationoftheADCtoapositioneverclosertotheRFinputinaswepttunedinstrument.

    Figure1.PSASeriesAllDigitalIFBlockDiagramCourtesyofAgilentTechnologies

    ThePSAachieveda10MHzbandwidthusinganADCclockedata30MHzrate.AlthoughtheADCresolutionwaslimited,thePSAimplementedanautorangingschemethattookadvantageofthetimedelaythroughthemultipoleantialiasfiltertovaryanalogpreADCgainaswellasdigitalpostADCgain.ThisschemeeffectivelyincreasedtheADCdynamicrangeonthefly.

    ReturningtoFigure1,theHilberttransformblockfollowingtheADCseparatesthesamplesintoIandQstreams.BecausethephysicalRFinputanditssampledrepresentationareanalytic,theimaginary(I)partofthedataistheHilberttransformofthereal(Q)part.Afterdigitalfiltering,theIandQsignalsareconvertedtoa

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    magnitudeandphaserepresentation,VBWfilteredandscaledtocorrespondtopowerorvoltage,anddisplayed.

    FornarrowRBWs,theLOissteppedratherthanswept,providingatimedomainsignalattheinputtotheRISCprocessorFFTblock.RBWfilteringanddecimationfornarrowspansareaccomplishedthere.

    TheMovetoRealTime

    Swepttunedanalyzers,whethertheyuseadigitallyimplementedIFornot,canmissatransientburstoccurringatafrequencyfarawayfromthatbeingsweptatanyinstant.NarrowanalogRBWsaggravatethesituationbecauseoftheassociatedslowsweepspeed.AnalyzersthatimplementnarrowRBWsviaFFTfiltershavemuchfastersweepsbutstillcanmisstransients.

    Instead,realtimeanalyzersoperateinasteppedfashion,dealingwithblocksoftimedomaindatainbandwidthsaswideas160MHzinAgilentsN9030AKRT2PXAsignalanalyzer,110MHzinTeksRSA5000andRSA6000instruments,and40MHzintheR&SFSVR.

    Alloftheusualanalogproblemsmustbeaddressedintheoveralldesignofarealtimeanalyzer,suchasthenumberandpositioningofthemixersandIFfrequencies,LOpurity,preselection,anddynamicrange.AssumingagoodRF/microwavesolutionaheadofthefinalmixer,therealtimecapabilitiesarebasedontheanalogtodigitalconversionfidelityandtheDSPthatfollows,inparticulartheFFT.

    ComparedtoFigure1,Figure2showsaverydifferentviewofthecircuitfunctionsfromtheADCtothedisplayfoundinAgilentsPXArealtimesignalanalyzer.SomesimplifiedsignalanalyzerblockdiagramsdepicttheADCfollowedbyalargeFPGAindicatingthatthehighspeedhardwareisimplementedinthatway.Somecalculationshelptoexplainafewofthedetails.

    AgilentsADCissampledata400MS/sratetoachievethedesired160MHzbandwidth.Theminimumtransientdurationthathasa100%probabilityofintercept(POI)is3.57s,whichisabout150nslongerthanthereciprocalofthe292,968FFTs/srate,3.413s.ThismeansthattheADChastimetoconvert

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    Figure2.SimplifiedRealTimeSystemBlockDiagramSource:AgilentTechnologies

    morethan1,000samplesforeachFFTandthattransientswillbeincludedinatleastoneFFT.

    AsimilarcalculationforTeks110MHzbandwidthRSA6000witha300MHzclockrateand292,969FFTs/syieldsthesame3.413sperFFT,andtheclockrateishighenoughtoacquiremorethan1,000samples.Minimumtransientdurationis3.7s,soagain,atleastoneFFTwillseeatransientthatlong.FortheR&SFSVR,theminimumsignaldurationis24sforthecorrectpowertobedisplayed.

    WhileNationalInstruments(NI)doesnotofferadedicatedrealtimespectrumanalyzerproductoutofthebox,asNikhilAyer,productmanagerRF/microwavetest,explained,OurmodularPXIvectorsignalanalyzerscanbeextendedwithuserdefinedFPGAprocessingtocreatesuchsystems.NIFlexRIOFPGAmodulesofferDSPfocusedFPGAsforrealtimesignalprocessing,peertopeerdatastreamingforhighbandwidthbasebanddatamovement,andabstractgraphicalprogrammingthroughNILabVIEWFPGAsoftware.WhencombinedwithoneofourVSAssuchasthePXIe5665orPXIe5667,userscancreatearealtimespectralanalyzeroraddressanyotherapplicationwhichrequiresrealtimesignalprocessing.NIvectorsignaltransceiverstakethisastepfurtherbecausetheyaresoftwaredesignedinstruments.

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    ColorGrading

    Itsobviousthatthingshavetooperatequicklytocatchtransients,butwhatdoyoudowithallthedata?ItsimportanttorememberthatthemodulationonthefrequencyshiftedRFinputisdynamic:itcouldbeconstantlychanging.So,eachsuccessiveFFTshowsthestateofthesignalasitchangesintime.Thisiswherethevariousformsofcolorgradedpersistenceaddtremendousvaluebyhelpingyoutodistinguishamongfrequentlyandlessfrequentlyappearingsignalsandevenseesignalshiddenbehindothers.

    TeksInstaVuoscilloscopesfromthe1990sintroducedcolorgradedpersistence,subsequentlyenhancedandrelabeledasDPXtechnology.Thebasicideaistorunsuccessiveshortacquisitionsataveryhighratelotsofwaveforms/s.Datafromeachacquisitionisaddedtoasamplenumbervs.signalamplitudepixelmapinwhichacounterateachlocationkeepstrackofhowmanytimesthewaveformhasgonethroughthatpoint.

    Themapisreadoutata30Hzratetoavoiddisplayflicker,andthedataisdisplayedasarangeofcolorscorrespondingtotheaccumulatedcountateachpixel.Thesameschemesuitsdisplayofhighrate,shortFFTsandhasbeenadoptedbythemajorrealtimesignalanalyzermanufacturers.

    OverlappedFFT

    Foracoupleofreasons,representingveryshorteventsischallenging.Iftheeventoccurspartlyinonedataacquisitionblockandpartlyinthenext,theFFTsonthosetwoblockswilleachshowreducedamplituderesults.Also,becauseawindowingfunctionisappliedaheadoftheFFTcalculation,thepositionofashorteventwithintheblockofADCdatamatters.Eventsneartheendsoftheblockwillbereducedinamplitude.

    TheoverlappingofFFTssolvesbothproblems.RatherthancomputingtheFFTofonedatablockandthenthenext,thesecondblockmaycontainthelast80%ofthefirstblockandonly20%newdata.Successivelyrepeatingthisprocesscausesthepositionofthecapturedeventtomovethroughthecenterofthewindowingfunctionandeliminateserrorscausedbyeventssplitbetweendatablocks.

    RealTimevs.SweptTuned

    SweptAnalyzersRemainRelevant

    Realtimesignalanalyzersclearlyhavetheedgefordifficultsignalssuchasthosethathopandoccupyalargebandwidth.And,transientburststhatinterferewithanothersignalmaybeveryhardtoidentifywithoutarealtimeanalyzer.Nevertheless,theyarentalwaysthebestsolution.

    AccordingtoDarrenMcCarthy,A&DtechnicalmarketingmanageratR&S,ThecontinueduseofsweptanalysishasanimportantmissiontosupportlegacydefenseATEequipmentandthecontinuedcodeemulationoftestmethodswrittenmanyyearsagothatwerebasedonanalogdetectionandtiming.Therearepracticaleverydayusesofsweptanalyzersthatarenotreadilyreplacedbyrealtimeanalysis:

    Trackinggeneratorsandsourcecontrolofsweptsignalgeneratorsarenotintegratedintorealtimeanalysismethods.

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    VideooutputstypicallyusedtheanalogdetectorsmanyrealtimeanalyzersnolongersupportvideooutputmeasurementmethodsusedinsomeEMCtestmethodsandrasterscanningapplications.[Sweptanalyzerssupport]visualdetectionofpulserecurrencefrequency(PRF)rateswhilepulsemeasurementsaretypicallyavailableusingdigitalacquisition,theviewingofthepicketfencescreatedbyasweptanalyzercouldgiveatraineduseraninstantaneousunderstandingofPRFrates,staggeredpulserateinterval,orothertimingartifactsacrossthespectrumofapulse.

    RigolTechnologiesdoesnotmakearealtimeanalyzerbuthastakenadvantageofADCandFPGAadvancestodeveloplowcostswepttunedspectrumanalyzers.ChrisArmstrong,generalmanageratRigolTechnologiesUSA,explained,Inbuildingamodernsweptanalyzer,westartedbyreenvisioningtheIFsectioninanalldigitalapproach.ThecuttingedgedigitalcomponentsthatareavailableallowedourengineeringteamtobuildadigitalFPGAbasedplatformtocontrolthefrontendandgeneratehighspeed,accuratefrequencyanalysiswithanimpressivecostadvantage.Additionally,theFPGAfoundationallowsustoincrementallyaddfeaturesandimproveeveryaspectoftheinstrumentovertimesincewearenotlimitedbyanyexpensivetochangeASIChardware.

    Armstrongaddedthatsweptspectrumanalyzersareusedwherewidebandanalysisoratrackinggeneratorisneeded.TheamplituderesponseofawidebandRFfilterisshowninFigure3.Thesinglesweepextendsformorethan80MHzandwasmadewithRigolsDSA815TG1.5GHzspectrumanalyzerandbuiltintrackinggenerator.

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    Figure3.FilterBandwidthMeasuredwithTrackingGeneratorCourtesyofRigolTechnologies

    AgilentsRichardOverdorf,applicationsengineerandproductplanner,saidthatsweptanalysiswaspreferredforvirtuallyallcasesinwhichastaticsignalenvironmentexisted.ForconstantorCWsignals,sweptoperationgivestheusermoreflexibilitywithwiderspansandRBWchoices,andrealtimegenerallyisoflessvalue,hesaid.

    Forexample,Overdorfcontinued,Averycommonmeasurementthatcanbecomesignificantlymoretimeconsumingandexhibitdegradeddynamicrangewitha[realtime]stitchedFFTiswidespanspuriousmeasurements.[Inarealtimeanalyzer],highorderimagerejectfiltersgenerallyareusedwithassociatedamplitudeandphaseripple.

    Thevendorsparticipatinginthisarticleagreedthataerospaceanddefense,surveillance,andradarapplicationsofteninvolvedshorttimedurationpulses.Thesechallengingsignalsrequireaninstrumentwithbothgoodlowlevelsensitivityandthebenefitsofrealtimeoperation.

    Preselection

    Ifinputsignalsareacceptedacrosstheentirefrequencyrangeofaspectrumanalyzer,itsvirtuallycertainthatunwantedsignalswillinterferewiththoseinthebandbeinganalyzed.Toavoidthisproblem,conventionalsweptanalyzersuseatunableYIGbandpassfilteraheadofthefirstmixerstage.Notonlydoesthisapproacheliminateinterference,italsominimizesradiationfromtheLOsourcethroughtheinputconnectoranimportantconsiderationinsecureapplications.

    Unfortunately,YIGfiltersarelimitedtoabout50MHzbandwidth,whichisfartoonarrowfortheverywidebandanalysisrequiredfortodayssignals.Evenwithinthe50MHzbandpass,thephasechangessignificantlyneareachedge.Further,becauseofthehysteresisassociatedwiththemagnetictuning,thefiltercenterfrequencycannotbeshiftedtoadifferentvalueandthenreturnedexactlytotheinitialsetting.ATekarticle5considers35to40MHzasthepracticalYIGpreselectorbandwidth.AgilentsPXAsignalanalyzerusesaYIGpreselectorwithabandwidththatvariesfrom46to74MHzdependingontheanalyzerbandwidthandthetunedcenterfrequency.

    Bypassingthepreselectionfilterhasbeentheonlyoptionavailableformakingverywidebandmeasurements.Forexample,option067forAnritsusMS2830Aprovidespreselectorbypassthatsupports125MHzwidebandmeasurementsto43GHzonanappropriatelyconfiguredinstrument.

    Instead,TekhasdevelopedaswitchedfilterbanktoreplacetheYIGpreselectionfilter.AlthoughTekdoesntspecifydetailsofthefilters,theTektronixComponentSolutionsgroupsellsaninechannel8to22GHzswitchedfilterbankthatwasdevelopedbythesamemicrowavetechnologygroupbehindTektronixRealTimeSpectrumAnalyzersasstatedinthedatasheet.

    Thefiltershavebetween1.5and2.0GHzbandwidthsandareclaimedtoprovideverystableandrepeatable

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    performance.Theyareintendedforsuperheterodynesystemswherethemixerinputbandmustbeasubsetoftheinputfrequencyband,accordingtothedatasheet.Becausethefilterperformancedoesnotconflictwithwidebandwidthmeasurements,theydonotneedtobebypassed.

    NIalsohasdevelopedaswitchedfilterbankthePXIe5693RFpreselectormodulewith16fixedtunedsuboctavebandpassfiltersthatcoversthefrequencyrangefrom20MHzto7GHz.ThemoduleincludesnotchfilterstosuppressTVband(55MHzto75MHz)andFMband(88MHzto108MHz).Preamplifiersfollowingthepreselectionfiltersallowforalownoisefigurewithoutcompromisingdistortionperformance.

    WhatElseCanItDo?

    Havingsolvedthebasicsignalmeasurementrequirementsdigitally,manyopportunitiesexistwithinarealtimesignalanalyzerforincreasedfunctionality.Amajorfeatureisfrequencymasktriggering,whichcomparesaspectrummasktosuccessiveFFTsastheyareperformed.Thismeansthatyoucantriggertheinstrumentwhensomepredefinedcombinationoffrequenciesoccursinthesamewaythatanoscilloscopetriggersonatimedomainwaveformmask.

    TeksMattMaxwell,productmanagerfortheRTSA,saidthatyouneedtobeabletotriggeronanysignalthatyoucanseeontheDPXdisplaywithintherealtimebandwidth.Notonlyisthisafrequencyselectivetrigger,butthisallowsyoutotriggeronsignalsunderneathothersignals.

    Digitaltechniquesalsohavebeenappliedtooscilloscopedataprocessing,bringingscopesandsignalanalyzersmuchclosertogether.Forexample,theR&SRTOK11optionpermitsI/QdatatobeextractedfromdigitallymodulatedsignalsthattheRTOhasacquired.TheI/QdatacanbeexportedtoanalysissoftwaresuchasMatlaborLabVIEWwithanadjustablesamplingrate.Similarly,Agilents89600VSAsoftwarecanprovideverywidebandanalysisforsignalscapturedonahighspeedoscilloscope.

    Anritsuhasusedanapplicationsolutionapproachinitssignalanalyzerdesign.Manyoptionsareavailableincludingextensiverecordingandplayback/signalgeneration.AsexplainedbyPatrickWeisgarber,businessdevelopmentmanageratthecompany,Combiningthe[features]alongwitharealtimevectorsignalanalyzerinasingleinstrumentprovidesengineerswithanexpandedanalysiscapability.Italsohascostandspacebenefitsbecauseiteliminatestheneedformultipleinstrumentsandtheirassociatedexpensesandbenchrealestate.

    R&SsMcCarthysummeduptherecentadvances.Forseveralyears,mostallspectrumandsignalanalyzerplatformshavehadbothsweptandvectorsignalanalysiscapability.Thishasbeenaplatformevolutionfromspectrumanalyzervendorsforoveradecade.Whiledifferentbandwidthsforvectorsignalanalysiscapabilitytypicallyareavailablefortimedomainvectoranalysis,withenoughplatforminsight,moderndesignscanaddrealtimeanalysistovectorsignalanalysishardwarewithasoftwareoption.

    References

    1.Lecklider,T.,SpectrumAnalyzersDigitallyEnhanced,EEEvaluationEngineering,June2012,p44.

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    2.RealTimeSpectrumAnalyzer(RTSA)PXAXSeriesSignalAnalyzerN9030AKRT1&N9030AKRT2,AgilentTechnologies,TechnicalOverview59911748EN,February2013,p.3.

    3.SpectrumAnalyzerBasics,AgilentTechnologies,ApplicationNote150,59520292,August2006.

    4.Ramian,F.,ImplementationofRealTimeSpectrumAnalysis,Rohde&Schwarz,WhitePaper,p.3.

    5.TheGrowingImportanceofSwitchedFilterPreselection,Tektronix,January2010.

    ForMoreInformation

    AgilentTechnologies

    Anritsu

    NationalInstruments

    RigolTechnologies

    Rohde&Schwarz

    Tektronix