evaluation engineering special report_ spectrum analyzers
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Spectrum AnalyzerTRANSCRIPT
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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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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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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10/07/2015 EvaluationEngineeringSpecialReport:SpectrumAnalyzers
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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