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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers

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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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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



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



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



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.



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.



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

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

Armstrongaddedthatsweptspectrumanalyzersareusedwherewidebandanalysisoratrackinggeneratorisneeded.TheamplituderesponseofawidebandRFfilterisshowninFigure3.Thesinglesweepextendsformorethan80MHzandwasmadewithRigolsDSA815TG1.5GHzspectrumanalyzerandbuiltintrackinggenerator.



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



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.



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

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NationalInstruments

RigolTechnologies

Rohde&Schwarz

Tektronix