ADVANCED RADAR ANALYSISTools for Measuring Modern radars––APPLICATION NOTE

Application Note

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Table of ContentsIntroduction ....................................................................3

Time and Frequency Domain Analysis ..........................4Instrument Considerations ................................................4Signal Characteristics ........................................................4Measurement Considerations ............................................4 Oscilloscope Measurements ........................................5 Pulse Measurements ................................................6 Oscilloscope Triggering .............................................6 Manual Timing Methods ...........................................7 Automated Oscilloscope Timing Measurements........7 Observing Time Varying Behaviors in the Frequency Domain ........................................10 DPX™ Live RF Spectrum Display ..............................11 Automated measurements on an RSA .......................11 Related Measurements ..............................................13Measurement Equipment Selection Chart .......................14

Example Measurements ...............................................14 Instrument Operation .................................................14 Thedifferencebetween Amplitudevs.TimeandTimeOverview ..................15 The Trace Decimation Control .................................15

Examining Intended Signals ........................................17 Monitoring Example: Weather Radar using RSA Series ....................................................17 Troubleshooting Incidental Modulation Example: Wideband chirp using a DPO7000 Series oscilloscope and SignalVu ......................................19

Unintended Signals ......................................................21 Spurious .................................................................21 Transient Signals (DPX Live RF Spectrum Display) .............................21 Frequency Mask Trigger (FMT) in the RSA Series ...............................................24 Signals in both Time and Frequency Domains ....25 Event Isolation and Analysis ................................26

External Threats ...........................................................26 Interference ............................................................26

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Advanced Radar Analysis

IntroductionIndesigningmodernelectronicwarfareandradarsystems,youfacesignificantchallenges.Youmustdevelopsolutionswiththeflexibilityandadaptabilityrequiredfornext-generationthreatdetectionandavoidance.Tosucceed,youneedcapable tools for the generation and analysis of extremely complexpulsepatternsandyouneedtovalidatedesignswithadvanced scanning methodologies – tools that can handle complexradarbaseband,IFandRFsignalsaswellasidentifymulti-systeminterference.

Withtoday’srapidadvancesinradartechnology,developingand manufacturing highly specialized and innovative electronicproductstodetectradarsignalstakesleading-edge

technology and tools. Tektronix innovative test equipment reduces testing uncertainty during the design process and deliversconfidenceintheintegrityofincreasinglycomplexdesigns.TektronixArbitraryWaveformGenerators,Real-timeSpectrumAnalyzersandHigh-BandwidthOscilloscopesofferthecapabilitiesyouneedtomanagetherequirementsofmodernradarapplications.Real-timevisibilityofadvancedpulse compression systems and the generation and analysis of all digital dynamic signal types help you create highlyreliable,cost-effectivesystemdesignsfordefenseand commercial electronic systems. The broad portfolio of Tektronix generation and analysis tools represents a scalable architecture that can protect your investments and speed your design development.

Application Note

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Time and Frequency Domain Analysis

Instrument Considerations

Tounderstandthebestmeasurementdevice,weneedtounderstandthesignalswearedealingwith.First,weassumea sensor (Radar) or an ECM is mostly characterized as a time domainphenomenon,asfundamentallywearedealingwiththe transmission of a quantity of electromagnetic energy that illuminatesatarget-wethenmeasurethetimeittakesforthereflectedresidualenergytoarrivebackatthereceiver.Thesignalcouldbeacontinuouswaveorasequenceofpulseswithaspecificmissiongoal.However,pulseriseandfalltimes,thetypeofmodulationandthebehaviorofthetransmitteramplifierandmostimportantlythefrequencyoftransmission can create a broad range of responses that need to be considered.

Timedomainmeasurementsaretraditionallyperformedwithoscilloscopeswhilespectrumanalyzersarebestsuitedforfrequencydomainmeasurements.However,ourproblemisunique;inthatwehavetimedomainbehaviorswewanttoobserve,buttheyareexhibitedinthefrequencydomain.Whilesweptspectrumanalyzersofferwidefrequencyanddynamicranges,theirabilitytocharacterizetimedomaindataislimited.Oscilloscopesofferexcellenttimedomainanalysis,butlackindynamic range especially at high frequencies. Advancements howeverinanalogtodigitalconvertertechnologiesandinmeasurement instrumentation architectures such as FFT based analyzers or Vector Signal Analyzers capturing both the phaseandamplitudecomponentsofthesignal,oritsvector)nowallowforwideinstantaneousbandwidthcapturesofhighdynamic range time domain data in the frequency domain. Thisiskeywhendealingwithatransient/pulse-basedtimedomainsystemwithtransmissionfrequenciesinthegiga-hertzranges such as a radar or ECM.

Real Time Spectrum Analysis drives to the next level of insight. Theadvantageofreal-timemeasurementcapabilityistheability to capture transient events in the frequency domain; realtimemulti-domaintriggering,time-selectivespectrumanalysis,continuouswidebandwidthwaveformstorageandhigh-qualitypersistentdisplaysthatprovidethehigherlevelsofmeasurement capability and insight.

Signal Characteristics

Asdiscussed,twotypesofsignalsneedtobecharacterized,CWandPuled,witheachhavingitsownmissionadvantagesand disadvantages.

CW Radar -Continuous-waveradarisatypeofradarsystemwhereaknownfrequencyofcontinuouswaveradioenergyistransmittedandthenreceivedfromanyreflectingobjects.Continuous-wave(CW)radarisexcellentforcalculatingvelocityusingtheDopplereffectbycomparingthefrequencyshiftofthereceivedsignalwiththatofthetransmitted.Test equipment needs to have the required broadband performancetocapturetheCWsignalwithenoughfrequencyresolution to analyze the Doppler frequency shift.

Pulsed Radar-Apulsedradaremitsshortandpow¬erfulpulses and in the silent period receives the echo signals. It is excellent for determining range by measuring the time differencebetweenthetransmittedpulsesandthereceivedpulses.TheDopplereffectcanalsobeobservedtomeasurevelocity,howeveritisusuallycalculatedovermultiplepulses.

Both types of radar could employ various types of modulation.

Measurement Considerations

For CW radar the instrument must be able to capture the transmissionfrequencyplusthereflecteddopplershiftedfrequency.Forpulsedradartherearetwooptions–thefirstiscapturethelowerfrequencyenvelopeinformationsuchasPRI(PulseRepetitionInterval)usingadetector,orcaptureallthesignalsinformationwithabroadbandinstrumentsuchasawidebandwidthOscilloscopeorSpectrumAnalyzer.ThedarkerlineinFigure1showsthetimedomainenvelopeofthepulseandthelighterlinesshowthesinusoidalenergythatfundamental makes up the pulse.

Figure 1. The autonomy of a pulse.

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Advanced Radar Analysis

Another important factor is to ensure the instrument has enoughbandwidthtocapturetherise/falltimescorrectly.Forexample,animpulseradarmayhaveaveryshortdurationpulse therefore a very broadband oscilloscope may be the best tool to capture the pulse and characterize its parameters such as overshoot and rise and fall times. Very fast transition timesorveryshortduration(sub-nanosecondorshorter)canbeaccuratelyseenona70GHzbandwidthoscilloscopesuchas the DPO70000SX family.

Figure 2. The DPO70000SX ATI Performance Oscilloscope delivers the industry’s most accuratecaptureofhigh-speedsignalbehaviortoverify,validateandcharacterizeyounest generation designs.

Formeasuringon/offratioawideamplitudeisrequired,Understanding the dynamic range of the instrument is key for thismeasurement,sometimesreferredtothedynamicrangeofaninstrument,abandcanbeexpressedindecibels,actualbitsoreffectivenumberofbits(ENOB).

Forexample,theTektronix4,5and6SeriesMSOoscilloscope has a 12 Bit analog to digital converter (ADC) andcancapturesignalswithupto8GHzinbandwidth.TheMDO4000ChasanoptionalSpectrumAnalyzerbuiltinwithanRFfrequencyrangeupto6GHznotonlydoesthisallowsfor cross domain triggering but also provides higher dynamic range for frequency domain measurements The 5 and 6 Series can be used alongside a USB spectrum analyzer such as the RSA300orRSA500familieswhenwiderfrequencycoverageandbetterdynamicrangeisrequired,especiallyforfrequencydomain measurements.

Figure 3. The 5 Series MSO and RSA500 can be used together to perform multiple domain measurements.

If the pulse has vector modulation as used in data link orcommunicationsystems,thismayrequirespecializeddemodulation measurements such as error vector magnitude (EVM). Advanced frequency and vector analysis is provided the SignalVu PC that can connect to both Oscilloscopes and Spectrum Analyzers.

Oscilloscope Measurements

The oscilloscope is the fundamental tool for examining varyingvoltageversustime.Itisverywell-suitedfordisplayingthe shape of baseband pulses. The origin of oscilloscope performance parameters traces back to characterizations ofearlyradarpulses.Today’sreal-timeoscilloscopeshavebandwidthupto70GHz,andaredesignedtocaptureanddisplayeitherrepetitiveorone-shotsignals.

Modern Oscilloscope triggering systems are very highly developed and can trigger on both analog and multiple channels of digital data. For example the 5 series oscilloscopes eight input channels can be assigned as trigger inputs for both analog signals such as the envelope of an individualpulse,adefinedstreamofpulseenvelopesoreachchannelcanbeassignedtoafurthereightdigitalinputs,providing the capability to trigger on multiple parallel data lines orwords.UtilizingtheexternaltriggerRFdevicessuchastheRSA300 or RSA500 families of spectrum analyzers can be triggered to perform frequency domain measurements based onreal-timeanalogordigitaldomainevents.

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Figure 4.FastAcqshowsasingletoo-narrowpulseoutofmanytensofthousands. Figure 5. Discovery of a single transient glitch in a train of pulses.

Pulse Measurements

The FastAcq feature of the oscilloscope operates on live time-domaindatausingDPX™acquisitiontechnology.Allfrequencydomainmeasurementsaremadeonthetime-sampled acquisitions of stored data.

The FastAcq display on the oscilloscope can discover basebandpulsetime-domaintransienterrors.Figure4showsjustonesinglepulsethathasanarrowerpulsewidththaneven hundreds of thousands of correct pulses. The blue color on the temperature scale representation of signal persistency representstheleastfrequentoccurrence,whiletheredareasare the parts of the signal that are the same every time.

TheFastAcqcapabilityontheDPO,DSA,andMSOSeriesprovidesatime-domaindisplaywithahighwaveformcapturerate. The DPX acquisition technology processor operates directlyonthedigitalsampleslivefromtheA/Dconverter.Itdiscoversrapidvariationsorone-shoteventsinthetime-domain display.

Forwidebandmeasurementsusinganoscilloscope,FastAcqcan be used to see even momentary transient events using the voltage vs. time display.

Figure5showsaone-timetransientinblue.Forthisdisplay,bluerepresentsverylow-occurrencetransients,whileredrepresentspartsofthewaveformthatareconstantlyrecurring.

Oscilloscope Triggering

One of the most highly developed capabilities of the oscilloscope is triggering. Recent advances in oscilloscope trigger have enabled methods of triggering an acquisition or measurement based on the voltages and voltage changes in one or more channels.

Theserangeincomplexityfromsimpleedgeorvoltage-level triggering to complex logic and timing comparisons for combinations of all of the input channels available. Pattern recognition,bothparallelandserial,triggeringon“runt”or“glitch”signalsandeventriggeringbasedoncommercialdigitalcommunications standards are all available in oscilloscopes.

The5and6SeriesMSO,DPO7000andDPO/MSO/DSA70000Seriesoscilloscopesallowtheusertospecifytwodiscretetriggereventsasaconditionforacquisition.Thisisknownasatriggersequence,orPinpoint™triggering.Themainor“A”triggerrespondstoasetofqualificationsthatmayrangefroma simple edge transition to a complex logic combination on multipleinputs.Thenanedge-driven“BDelayed”triggercanbespecifiedtooccurafteradelayexpressedintimeforevents.

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Advanced Radar Analysis

TheB-triggerisnotlimitedtoedgetriggering.Instead,theoscilloscopeallowstheB-triggertolook,afteritsdelay period,foraconditionchosenfromthesamebroadlistoftriggertypesusedintheA-trigger.AdesignercannowusetheB-triggertolookforasuspectedtransient,forexample,occurringhundredsofnanosecondsafteranA-triggerhasdefinedthebeginningofanoperationalcycle.BecausetheB-triggeroffersthefullrangeoftriggeringchoices,theengineercanspecify,forinstance,thepulsewidthofthetransienttheywanttofind.Over1,400possibletriggercombinationscanbequalifiedwithPinpointtriggering.Sequences can also include a separate horizontal delay after theA-triggereventtopositiontheacquisitionwindowintime.

TheResetTriggerfunctionmakesB-triggeringevenmore efficient.IftheB-eventfailstooccur,theoscilloscope,ratherthanwaitingendlessly,resetsthetriggerafteraspecifiedtimeornumberofcycles.InsodoingitrearmstheA-triggertolookforanewA-event,sparingtheusertheneedtomonitorandmanually reset the instrument.

The system can detect transient glitches less than 200 ps wide.Advancedtriggertypes,suchaspulsewidthtrigger,canbeusedtocaptureandexaminespecificRFpulsesinaseriesofpulsesthatvaryintimeorinamplitude.Triggerjitter–acrucial factor in achieving repeatable measurements – is less than 1 trillionth of a second (1 ps) rms.

Forbasebandpulses,thetriggersbasedonedges,levels,pulsewidth,andtransitiontimesareofthemostinterest.Iftriggeringbasedoneventsrelatedtodifferentfrequenciesisneeded,thentheRSASeriesspectrumanalyzerisrequired.

Manual Timing Methods

Traditionalmeasurementsofpulseswereoncemadebyvisual examination of the display on an oscilloscope. This is accomplishedbyviewingtheshapeofabasebandpulse.Themeasurementsavailableusingthismethodweretimingandvoltageamplitude.Thesemeasurementsweresufficient,aspulsesweregenerallyverysimple.

Thebasebandpulseswereusedtomodulatethepoweroutputoftheradartransmitter.IfitwasnecessarytomeasuretheRF-modulatedpulsesfromthetransmitter,thenasimplediodedetectorwasoftenusedtorectifytheRF signal and provide a reproduction of its baseband timing andamplitudefortheoscilloscopetodisplay.Generally,theoscilloscopedidnothavesufficientbandwidthtobeabletodirectlydisplaytheRF-modulatedpulses,andifitdid,thepulsesweredifficulttoclearlysee,andwasevenmoredifficulttoreliablygenerateatrigger.

Forthesebasebandpulsemeasurements,themeasurementtechniquefirstusedwastovisuallynotethepositionon-screen of the important portions of the pulse and count the numberofon-screendivisionsbetweenonepartofthepulseand another. This is a totally manual procedure performed by theoscilloscopeoperatorandassuchwassubjecttoerrors.

Automated Oscilloscope Timing Measurements

WiththeadventofAnalog-to-Digitalconverters,theprocessoffindingthepositionon-screenbecameoneofdirectlymeasuring the time and voltage at various portions of the pulse.

Nowtherearefullyautomatedbasebandpulsetimingmeasurementsavailableinmodernoscilloscopes.Single-buttonselectionofrisetime,falltime,pulsewidth,andothersarecommon.However,mostofthesemeasurementsdo not focus on the measurement envelopes of modulated radar signals.

Whenusedonpulse-modulatedcarriers,thesemeasurementsareoflimitedutility,becausetheyarepresentedwiththecarrier of the signal instead of the detected pulse. This results inpulsewidthmeasurementsthataremadeonasinglecarriercycle,andrisetimesofthecarrierinsteadofthemodulatedpulse. Detectors may be used on the input of the oscilloscope to remove the carrier and overcome this.

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Pulse RepetitionFrequency

Figure 6. Spectrumplotmeasurementsofpulsewidthandrepetitionrate.

Whenchoosingthecorrecttool,mostengineersusanoscilloscopeswhenperformingtimedomainmeasurements,but spectrum analyzers are best suited for frequency domain measurements.Asdiscussedearlier,radarandEWisunique,in that time domain behavior is exhibited in both the time and frequencydomain.Asweptspectrumanalyzerofferswidefrequencyanddynamicranges,butitsabilitytocharacterizetimedomaindataislimited.Oscilloscopesofferexcellenttimedomainanalysisandtriggercapability,butlackindynamicrange,especiallyathighfrequencies.

Advancements in analog to digital converter technologies and in measurement instrumentation architectures such as FFT

based analyzers or Vector Signal Analyzers (the instrument capture both the phase and amplitude components of thesignal,oritsvector),nowallowforwideinstantaneousbandwidthcapturesofhighdynamicrangetimedomaindatainthefrequencydomain.Thisiskeywhendealingwithatransient/pulse-basedtimedomainsystemwithtransmissionfrequenciesinthegiga-hertzrangessuchasaradarorECM.

Real Time Spectrum Analysis drives to the next level of insight. Theadvantageofreal-timemeasurementcapabilityistheability to capture transient events in the frequency domain; real timemulti-domaintriggering.

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Advanced Radar Analysis

Figure 7. Alternate instrumentation block diagrams.

Figure 7 puts it all into context. All architecture use a super heterodyne process to convert high frequency signals to a lowerfrequencyforanalysis.Thesweptspectrumanalyzeressentialonlymeasuresthepowerinafilter(ResolutionBandwidthFilter)ataspecificfrequencypointderivedfromthe frequency of the Local Oscillator (LO). It can display a rangeofpowervsfrequencyby‘sweeping’theLOandthex-axisofdisplay.Thisallowsforverybroadfrequencydisplayswithexcellentdynamicrange,butthetimedomainacquisitionbandwidthislimitedtothatoftheresolutionbandwidthfilter.AVectorSignalAnalyzerusesafixedlocaloscillator,withamuchlargerIFfilter(insteadoftheRBWfilterbank).Thisallowsustoacquirelargeamountsoftimedomaindata,

then either display it as time domain data translate it to the frequency domain utilizing an Fast Fourier Transform. A Real TimeSpectrumAnalyzerconceptualisthesameasaVSA,howevermoreprocessingisperformedearlierinthesignalchainallowingforfasterprocessingofFFT’sandtriggersthusproviding the enhanced measurement capability required for a successful test.

WhatthisallmeansisthattheRTSAarchitectureallowsusto observing time varying behaviors in the Electromagnetic Spectrum,analysisinthetimeandfrequencydomainthesystemsundertestbehavior,whilesimultaneouslyrecordingthe events to a hard disk array.

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Attenuator

Low-Pass

IF Filter

External

FreeRun

Down Convert& Filter

Real-TimeI-Q Out

(Option 55)

Band-Pass

26.5 GHz1 Hz Up to 800 MHz

Turning Range Acquisition Bandwidth

100 or 300 Msps

Input

LocalOscillator

Post Capture Processing

Live Signal Processing

Real-Time Bandwidth Display Processing

DPX™

Displays

X-Y

X-Y

Real-Time Digital ProcessingRF Down-Converter

Micro-

Processor

Trigger

ADC Corrections

Amp./Phase Capture

Pixel Bu�er

Memory

MemoryDPX™

DDC/Decimation

FMT

Variable BWLevel

RSA7100 + Option 110

Figure 8 SimplifiedRSAblockdiagram.

Observing Time Varying Behaviors in the Frequency Domain

As the RTSA architecture acquires many time domain recordsandconvertsthemtofrequencyinrealtime,wecansimultaneouslyobservetimeandfrequency.Onewaytodisplaythisiswithaspectrogram.Aspectrogramaddsthedimensionoftimewhilestillallowingyoutoobservefrequencyandamplitude.Thexaxisrepresentsfrequency,theyaxisrepresents time and amplitude is represented by color. Usually redforhighpowerandgreenforlowpower.

An alternate method is to visualize this measurement by emulating the anomalies of a cathode ray tube (CRT) common as the display on a spectrum analyzer before the turn of the

century.ACRTutilizesamagnetically-controlledelectrongunthatfireselectronsonaphosphor-coatedscreen.Thescreenilluminatesandthenslowlydecaysovertime.

Figures9and10showatypicaldisplayusingaphosphoremulationtechnique.Withoutphosphoremulation,thescreeninFigure10wouldjustshowthelargeLFMsignal,withtheCWsignal“poppingout”thetopontheleft.Butwithphosphoremulation,youcanseeasecondlowerpowerLFMoverlappedinfrequency.Inaddition,severalsingle-frequencypulsedcarriersandtwocontinuouswave(CW)interfererscanbeobserved.Ofcourse,thebandwidthandacquisitionspeedsare much fast than a turn of the century spectrum analyzer.

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Advanced Radar Analysis

Figure 9. Interference to a pulse. Figure10. Multiple chirps in one band.

Figure 11. The automated pulse measurements results table.

DPX™ Live RF Spectrum Display

The RSA Series enables DPX™ Live RF spectrum display on upto800MHzacquisitionbandwidth.ThenameDPXcomesfromtheconceptofa“DigitalPhosphorTechnology”display,whichre-createstheslow-fadememoryeffectofaCRTphosphor.FortheRSA,thehardwareprocessorperformsupto292,969FFTspersecond.Thisgivescontinuousreal-timevisibilityofvaryingRFsignalswithoutinterruption.Figure9isaradarpulsewithaninterferingcarriersweepingthroughthepulsed spectrum.

Thecontinuousnon-interruptedvisibilityguarantees100%probability of intercept of signals or transients as short as 3.7 µs because there is no dead time. The DPX spectrum display is observing all the time.

Interference to radar pulses and situations of multiple signals on the same frequency can all be discovered. As mentioned earlier,figure10isaDPXspectrumdisplayofachirpthathasasecondlowerpowerchirpoverlappedinfrequencyaswellasseveralsingle-frequencypulsedcarriersandtwoContinuous Wave (CW) interferers.

Automated measurements on an RSA

While the RSA Series can be used in a manner similar tothesweptspectrumanalyzertomeasureineitherthefrequencydomainorthetimedomain,automatedpulsemeasurementsareavailablethatoffermuchmoresignaldetailand measurement repeatability. The Advanced Signal Analysis suite,Option20,includesthepulsemeasurementcapability.Forthisanalysis,theanalyzerstoresadigitalacquisition,findsthepulseswithinitandmeasuresafullsetofparametersforeachpulse.Timing,frequencyandphaseparametersareall measured. Any selected set of these parameters can be displayed or exported.

These measurement results can be further processed to display trends and analyze these trends.

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The RSA Series pulse measurement suite provides a comprehensive set of pulse parameter measurements for upto800MHzbandwidth,includingreadoutsoftiming,distortions,amplitude,frequency,phaseandpulsetime.

Someofthemeasurementsarespecificallyforchirpedpulses,includingFrequencyDeviation,FrequencyError,PhaseDeviationandPhaseError.Anyoftheparameterswithanumeric result can also have these results plotted versus pulse number,givingvisibilityoftime-trendsoferrors.Andlastly,these trends can be input to an FFT to determine if the errors haveacoherentfrequencyoferrorvariation,possiblyleadingto discovery of the cause of the variation.

Figure 9.Pulsemeasurementresultstable,withsevenparametersselectedfordisplay.

Anyorallofthemeasurementswithnumericresultscanbeincluded in the pulse table display as seen in Figure 9.

The automated pulse measurements operate on a record of datacapturedinabandwidthselectableuptoa800MHz,whichistunableanywhereuptotothe26.5GHzRFcoveragefor the RSA7100.

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Advanced Radar Analysis

Figure 10. SignalVuvectorsignalanalysissoftwareshowinga1GHzwidechirpandtableofpulseparameters.

Related Measurements

TheSignalVuvectorsignalanalysissoftwarethatincludesOptionSVP-AdvancedSignalAnalysishasthesameautomated pulse measurement functionality of the RSA Series.ItcanbeinstalledonDPO/MSO5000,DPO7000andDPO/DSA/MSO70000Seriesoscilloscopes.Thiscombinestheextremelywidebandwidthavailablefrom,forexample,theDPO70000Seriesat33GHz,withthespectrumanalysisandfully automated Pulse Measurement Suite from the RSA Series spectrum analyzers.

Wheninstalledonanoscilloscope,theinternalsoftwarelimitsonsettingfrequencycoverage,bandwidth,andrecordlengthautomaticallyadjusttousethefrequencyandmemorylimitsoftheoscilloscopeonwhichitisinstalled.TheSignalVuvectorsignalanalysissoftwareprovidescontroloftheoscilloscopeattenuator,gain,samplingrateandsweepratecontrolsbyallowingtheusertocontrol“ReferenceLevel,”“FrequencySpan”inamannerwithwhichspectrumanalyzerusers are familiar.

Thevectorsignalanalysissoftwarecanuseanyoftheinputchannels of the oscilloscope. It can also use the oscilloscope’s “mathtrace”capabilitytocombinedatafrommultiplechannels and use math functions on the data.

Themeasurementswillhavethesamedynamicrangeastheoscilloscope,whilemaintainingitsfullbandwidth.Figure10showsaspanof1.25GHz,andachirpedpulsethatis1GHzwide.Thefrequencyspancapabilityislimitedonlybythebandwidthcapabilityoftheoscilloscopeonwhichthesoftwareisinstalled.

JustliketheRSA,thepulsemeasurementsareperformedondataintheacquisitionmemoryonacontinuouscapture/analyzecycle,oronwaveformsstoredontheinstrumentharddisk drive.

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Table 2.Measurementequipmentoverview.

Measurement Equipment Selection Chart

The measurement tasks and the pulse parameters that are importantforeachtaskhavebeenpresented,aswellasthecapabilities and limitations of the various test equipment. Table 2 summarizes the test instrument choices based on the measurement characteristics needed.

Example Measurements

Instrument Operations

WhetherusinganoscilloscopeoraReal-timeSpectrumAnalyzer,therearesomecontrolsprovidingflexibilityofoperationwhichmayneedtobesetdifferentlyfromthedefaulttoallowpropersignalacquisitionandpulsemeasurements.Usuallyitbecomesnecessarytoadjusttheseonlywhenoperatingoutsideofoneormore“default”instrumentsettings.Measurementresultscanbeaffectedbyadjustingtheseadvanced settings.

The more common of these include: Delay after Trigger (delays the start of acquisition after a trigger has been recognized); Acquisition Time and Acquisition sample length (normally these are automatically coupled to settings of Span,Bandwidth,andtypeofmeasurementinuse);Units

ofVolts,WattsordB(thedefaultisdB,butsometasksmay require alternate units to be selected); Alignments (the internal alignment routines may be disabled if needed for a measurementforwhichinterruptionduetointernalalignmentprocedureswouldbeunacceptable).

Thetime-domaindisplays(Frequency,Phase,andAmplitudeversusTime)haveanadditionalmeasurementilterwhosebandwidthcanbeadjusted.

All the measurements made in the RSA5000 and RSA7100 relyongettingsignalswhichhavepassedthroughanIFfrequencyconvertersystem.Forthe110MHzbandwidthoptionoftheRSA5000andRSA7100,theIFfilterisapproximately120MHzwide.

Due to the nature of frequency conversion schemes that are thiswide,thefilterusedmusthaverelativelysharpfrequencyresponseroll-offatthefilteredges.Forpulsesthismaycauseasmuchas40%overshoot.ForthepurposesofpulsemeasurementsthereisaneedforaGaussianresponsefiltertominimizethisovershootonapulse.ThisGaussianfiltercanbeaddedtoflattenpulseresponseattheexpenseofbandwidth.Anon-Gaussianfiltercanalsobeselectedifnarrowerbutflatresponse is desired.

Instrument Series Frequency Range Bandwidth Rise Time (20%/80%) / Min Pulse Width Detection

3rd Order IntermodulationDistortion (typical)

Spectrum Analyzers

RSA5000 1 Hz to 26.5 GHz 25 MHz std;40 MHz opt

85 MHz110 MHz

40 ns/120 ns25 ns/120 ns12 ns/50 ns10 ns/50 ns

<-84 dBc @ 2 GHz

RSA7100A 16 kHz to 14.2/26.5 GHz 320 MHz std;800 MHz opt.

-85 dBc @ 100MHz to 3.4GHz-65 dBc @ 3.4GHz to 6GHz-80 dBc @ 6GHz to 26.5GHz

Oscilloscopes

DPO7000 DC to 3.5 GHz 500 MHz1 GHz

2.5 GHz3.5 GHz

310 ps/16 ns200 ps/8 ns100 ps/3 ns95 ps/2.25 n

-40 dBc @ 2 GHz

DSA/DPO/MSO70000 DC to 33 GHz 4 GHz6 GHz8 GHz

12.5 GHz16 GHz20 GHz25GHz33 GHz

65 ps/2 ns43 ps/1.3 ns33 ps/1 ns

23 ps/640 ps21 ps/500 ps17 ps/400 ps17 ps/400 ps17 ps/400 ps

-55 dBc @ 2 GHz-48 dBc @ 10 GHz-50 dBc @ 18 GHz

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Advanced Radar Analysis

Figure 11 .TraceDecimationissetforamaximumof100,000pointsontheupper-rightparametervs.timedisplay.

FortheRSAhardware,therearesomehardwarecontrolstobeconsidered,including:Internal/Externalfrequencyreference,usedforlockingthefrequencyreferencesothatthe analyzer measures frequency referred to the external systemreference;externalgainorloss,usedtoaccountforanattenuatorwhichmaybeusedtolowertransmitterpowertothelimitsoftheRSAhardware;andattenuatorlimitswhichpreventtheinternalattenuatorfrombeingsettozero,therebyprotectingtheanalyzer’sfront-endmixerfromdamagebylarge signals.

FortheSignalVuvectorsignalanalysissoftware,someoftheoscilloscopecontrolsmayinteractwiththesoftwarecontrolsor settings. While the default setting locks the oscilloscope controlstotheanalysissoftwaresettings,itispossibletooverridethissettingandmanuallysetsamplerate,inputattenuation,etc.Thiswillcausethesoftwaretochangespan,RBW,andReferenceLeveltoaccommodatethemanualoscilloscope settings.

The difference between Amplitude vs. Time and Time Overview

Atfirstglance,itwouldappearthatthesetwodisplaysmightwellbethesamething.Theyarenot.

TheTimeOverviewisaverysimplemagnitudedisplaywhichhasasitssourceallofthedecimatedI/Qsamplepairsoftheacquisition. It is here that the subset of the full acquisition intendedtobeanalyzedcanbeseenandadjusted.Theportion of the acquisition that is used to create the Spectrum Displayisalsoshown,andcanbeseparatelyadjusted.

Theamplitudevs.timedisplayisoneoftheanalyseswhichusethe“analysis”subsetoftheacquisitionrecord.Thisdisplayisequivalenttothe“zero-span”settinginthetraditionalswept

analyzer.Thisdisplayhasitsown“TimeDomainBandwidth”filterwhichcanbeusedtoreducethebandwidthofthismeasurement for noise reduction or glitch reduction.

The Trace Decimation Control

Anotheroneoftheselesser-usedcontrolsisthe“TraceDecimation”setting.Particularlyforthe“parametervs.time”displays,theacquisitionrecordbeinganalyzedcanbeverylarge.Itcanbeaslargeastheentireacquisitionmemory,ascomparedtothe“PulseTrace”whichcanhaveonlythesamples for one pulse. When there are several million or more acquisitionsamples,thetraceprocessingmaybecomeslow.Thisusuallyhappenswhenthedigitizingrateisveryhighatthe same time as the acquisition record length is set very long.

Thetraceprocessorhasauser-selectedTraceDecimationthatwillsetalimitonthenumberofresultingtracepointsallowedinthese“parametervs.time”displays,providingfaster display results.

Inmostcases,100,000pointsshouldbeplenty.Butinthecaseofaradarpulsewithverysmalldutycycle,theremay be a need to zoom in on the display much more than normal.InFigure11,theacquisitionwas18.722mslong,andhas936,112samplestakenat40MHzsamplerate.These acquisition parameters can be found in the acquisition controltab.Themaximumtracepointsaredecimateddownto100,000asseeninFigure11.Buttoseejustonepulseintheamplitudevs.timedisplayintheupperrightofthescreen,thedisplayhasbeenzoomedto10.36µswidetoseejustonepulse.Thisprovidesonly10.36/18722*100,000=55pointsforthedisplay.Ascanbeseenhere,55pointsisnotenoughto clearly see the character of the pulse.

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Figure 12 .Theselectionissetto“NeverDecimate”thetracepoints.

The solution is to remove the limit on trace points as seeninthelowermiddleofFigure12.Nowthefullsetofsamples(936,122)ofthedigitizedrecordisavailablefortheun-zoomedtrace,providing10.36/18722*936,122=405pointsintheon-screenzoomeddisplay.

Nowtheamplitudevs.timetracehasenoughpointstoshowthetruecharacterofthedigitizedpulse,visibleinthe upper right of Figure 12.

The number of points in the trace may not be the same asthepointsthatareseenon-screen.TheLCDscreenis only capable of normal personal computer display resolution(inthiscase1,024pointshorizontally).Ifthechosen display has more points than can be displayed ontheLCD,thetracemustbefurthercompressedforthedisplay.ThisisdonebytheWindowsoperatingsystem.InFigures11and12,thetracehasonly40%ofthefullscreensize,or410pixels.Therefore,inthis case the computer display has not needed to

furthercompressthe405-pointtrace.Therewouldbeconsiderablecompressionofthe100,000pointsifthetraceweredisplayedwithoutzoom.

Evenwhentheon-screendisplayisfurthercompressedbyWindows,measurementaccuraciesarenotaffected.Themarkersandothermeasurementsoperateonthefull“traceresolution.”Thefull-resolutiontracescanalsobeexportedforfurther analysis or record keeping.

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Advanced Radar Analysis

Examining Intended Signals

Monitoring the intended signals emanating from a radar systemmaybenecessarytoassurecompliancewithregulationsaswellastoconfirminteroperabilitywhenmultiplesystems are installed in close proximity to each other.

Monitoring Example: Weather Radar using RSA Series

After understanding the importance of signal acquisition and tracedecimation,thisexampledemonstratedhowweusetimeoverviewandamplitudeversustimemeasurementstoisolate the pulses measurements to be made. This example isasetofobservationsofaweatherradarat9.6GHz.Thisradarispulsed,andhasarotatingantennascanningthefull360degrees.Withseveralofthemeasurementwindowsseparatelydisplayed,themarkersinallsimultaneouswindowsare correlated across the multiple displays.

Foraground-basedmonitoringposition,themeasuredpowerofthepulseswillvaryasthetransmitantennaswingsacrossthe monitor receiver antenna.

HeretheTimeOverviewwindowshowsthepulsesand theiramplitudechangesastheantennasweepsacrossthemonitor antenna. The marker has been placed on the highest amplitudepulsewhichcorrespondstothetimewhenthetransmitantennawaspointingdirectlyatthemonitorantenna.

Figure 13.TimeoverviewoftheWeatherRadar.

Figure13,thefoursmallboxesneartheupperleftofthe displaywiththeletterAinonepair(forAnalysis)andtheletterS in the other pair (for Spectrum) select the entries for the programming of the portion of the acquisition record over

whichthepulseAnalysisandtheSpectrumanalysiswillberespectively performed. Which pulses out of the total acquisition record contribute to each of these sets of measurements can be separately selected.

ThebluebaronthetopoftheTimeOverviewmeasurementindicatestheAnalysisTime.NotethattheAnalysisLengthandOffset(the2bluebuttons)willapplytoalldisplaysonthe instrument except the Spectrum Display. The Spectrum LengthandOffsetcanbesetwiththeredbuttons.The minimumSpectrumLengthsettingwillbeautomaticallydetermined by the amount of samples necessary to realize therequestedRBWsettingwheninAutomode.Forexample,narrowRBWfilterwillrequirealongerSpectrumTime.TheSpectrumLengthandOffsetareindicatedbytheredbaratthe top of the plot.

Figure 14.Amplitudevs.timewithnotracedecimation.

Theamplitudevs.timetracewindowcanbesettoanalyzeany part or all of the acquired record. This display is the equivalentofthezero-spanmodeinsweptspectrumanalyzers. Since this particular acquisition is long enough to assurecapturingthefullantennarotationacrossourposition,there is quite a lot of data. The trace decimation may need to be disabled if zoom is needed.

Themarkerseenhereistime-correlatedwiththeoneintheTimeOverviewwindow.Thismeansthatifeithermarkerismoved,theotheronewillmovetoexactlythesametimeposition.Thisallowsmeasurementsofdifferentparameterstocorrelate all at exactly the same time.

Application Note

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Figure 16.PulseTraceoftheWidthshowingthepulselinesandcardinalpoints.

Figure 15 .ThePulseTableshowingthemeasuredresults.

Pulse Trace is selected as the display in Figure 16 and Width is entered as the parameter to display. The readout says pulse numberfourisdisplayed.Thisselectionwillbemadeinthenextwindow.Forthisdisplay,theunitswerechangedfromdBtoVolts.Thisgivesalineardisplayandnowthepulselinesandcardinalpointsaredisplayed.Theyaredisabledwheneverthedisplayhasanon-linearscale,asthelineswouldalsogonon-linear.

Thisisarealoff-the-airradarmeasurement,sotheantennausedandsomelocalobjectsmayhaveanimpactonpulsethe distortions seen here.

Nowthatthepulsesarefound,atablecanshowalloftheparameterstobereported,asseeninFigure15.Notetheone cell is dark blue. When the mouse is clicked in any resultscellitwillbecomebluetosignifythatithasbeenselected,andthepulsetracewindowwillconfigureitselfand graphically display that particular pulse parameter. If theparameterbeingexaminedinthePulseTracewindowischangedinthatwindow,thebluehighlightselectioninthePulse Table moves accordingly.

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Advanced Radar Analysis

Figure 17.The300nspulsewidthmodeoftheWeatherRadar.

Figure 19.Waveformwith16pulsesshown.

When there is a need to verify many pulse measurements atonce,thePulseMeasurementSuitegivesrapidandcompleteanswers.

Troubleshooting Incidental Modulation Example: Wideband chirp using a DPO7000 Series oscilloscope and SignalVu

This is an example of using the SignalVu vector signal analysis softwareonanoscilloscopetomeasurearadarsignalwitherrorsinthetransmittedsignal.Itisachirpedpulsewithchirpwidthofabout500MHz,whichrequiresthegreaterbandwidthof the oscilloscope.

Figure 18.Thevoltagewaveformofa500MHzwidechirpedpulse.

AsimpleviewofthemodulatedpulsewaveformisseeninFigure18.

Withmultiplepulsesincludedinonewaveform,itisnoteasytoseedifferencesbetweentheindividualpulses.Iftherearedifferences,theyaremoresubtlethancanbeseenfromavoltagewaveform.

Thisparticularweatherradarradarhastwomodeswithdifferentpulsewidths.Thenextpulseexaminedisthe300nspulsewidthinFigure17.

Application Note

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Figure 20. Spectrum display of the 500 MHz chirp using SignalVu vector signal analysissoftware.

Figure 22. Plot of the trend of the amplitudes of the measured pulses.

Figure 21.ThePulseTableonSignalVuvectorsignalanalysissoftwareshowingtheparameters of the 500 MHz chirp.

TheSignalVuvectorsignalanalysissoftwareusesthedigitizedvoltagewaveformstoredintheoscilloscopeandusesthesamesoftwareastheRSASeriestomakealloftheRFmeasurements.ThefrequencyspectrumisshowninFigure20. This pulse chirps from 250 MHz to 750 MHz.

NextthePulseTableisselectedwithbasictimingandamplitude measurements.

TheAverageONpowerofthepulsenumbershaveverysmallvariationsshowing.Buttheydonotappearatfirstglancetobesignificant.Thegraphicalplotofthenumericalresultswillbe more indicative if there is in fact a coherent variation. Figure 22showstheTrendPlotoftheAvgONparameter.

Tobeabletoseethesmallvariations,theverticalscalehasbeen“zoomed”toasensitivityof0.1dBperdivision.Atthissetting there is very clearly a periodic variation of the pulse amplitude.Thetotalpeak-to-peakamplitudevariationis about 0.07 dB.

Thehorizontalscaleissimplythenumberofthepulsewhoseamplitudeisplottedvertically.Theeffectivewaytofullyanalyzethe variation is to use the FFT of the trend data from this plot.

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Advanced Radar Analysis

Figure 23. FFT of the trend of the pulse amplitudes.

Selecting“FFT”insteadof“Trend”inthedrop-downboxinthelowerleftofFigure23bringsupthefrequencyspectrumofthemeasuredparameter.Thespectrumplotdistinctlyshowsapeakdisturbanceat4kHz,whichis53dBbelowtheaveragevalue of the amplitude. Something is modulating the pulse powerata4kHzrate.Thefrequencyofthedisturbancecanassist in the troubleshooting of components or subsystems withintheradarcausingthisproblem.

ToconvertthisdBreadingtotheamplitudevariation,workbackwardsthroughthedecibelformula:V=antilog(dB/20).Minus53dBequatestoanRMSvoltagevariationof0.22%.Thisis0.63%peak-to-peak.Thisagreesverycloselywiththe“eyeball”valueseeninthepreviousFigure22.

A very subtle disturbance of this radar transmitter has proven easy to analyze.

Unintended Signals

Whenexamininganinstalledradarsystem,oneimportanttaskis to check for emissions that do not help the radar and very wellmaycauseinterference.TheuseforradarsmustconsiderspectrumallocationsaswellasothernearbyRFequipmentand facilities. Unintended signals may also provide increased visibilityofradar,oranunwantedsignaturewhichcanbeusedto identify the radar.

Spurious

There may be signals radiated from a radar system that willcreateinterferencetoothersystems.Thesemaybeatfrequencies outside the assigned channel of the main radar transmitter signal. Some of these signals may be active all the time and some may be active only during pulse transmission. This requires stepping across the entire frequency range of the analyzer.ThewidebandoscilloscopewithanFFTspectrumplotcanprovideasingleviewofbothin-channelandout-of-channel emissions.

Transient Signals (DPX Live RF Spectrum Display)

Somesignalsmayonlybepresentduringthepulse,butbeonadifferentfrequency.Somesignalsmaybecompletelyunrelatedtothepulse,andmaybeonanyfrequencyincludingwithinthepulsefrequencyitself.

Todiscoversignalssuchasthese,amonitorisneededwhichiscontinuousandalsoonethatcanshowasinglesignaldeviation out of the continuous examination.

The DPX Live RF Spectrum Display can do exactly that. This technologyisimplementedintheRSAhardware,soitcannotbeusedbytheSignalVuvectorsignalanalysissoftwareonstoredacquisitionrecords.TheDPO/DSA/MSOSeriesoscilloscopes also utilize DPX technology for voltage vs. time traces.

Application Note

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DisplayColor

Grading

1

3

3

1

1

3

3

1

1

34 4 4444

1

3

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1

1

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Memory

DiscreteFourier

Transform

Real-Time Spectrum Analysis(DFT Computation is Completed Before Next Sample Set)

Transform Sample SetsRF DFT Spectrums

292,969 FTT/s >14,000 FFT/Frame 10 - 33 Frame/s1 Hz 100 Msps

or300 Msps(Opt. 110)

26.5 GHz

Pixel Histogram Temp. Grading

Analog RFTo Digital

Conversion

Figure 24.TheDPXspectrumdisplayhardwareengineontheRSA.

Refer to the previous Figure 5 for the RSA block diagram. TheA/Dconverteroperatescontinuously.SamplesofwhateversignalisintheIFarepassedtothehardwaresignalprocessorwithoutinterruption.Thesesamplesareprocessedatupto292,969seamlessspectrummeasurementspersecond.

ThesespectrummeasurementsaredoneintheDPXrun-timeengine.Thenextstepusesasmallbuffermemory(virtualscreen)intowhichabitmapofthespectrumdisplayisplaced.Thenaseachspectrumdisplayisproduced,anotherbitmapisaddedtothepixelbuffer,onepixelatatime.Thiscreatesabitmapinwhicheachpixelcontainsasinglenumberrepresentingthenumberoftimesthatthespectrumtrace“hit”that location on the virtual screen.

Once the main display monitor is ready to accept the next update,thebufferpixelsareconvertedtothedifferentcolorsthatrepresentthedensityofhits–blueforveryfewhitsuptored representing many hits. In this fashion the DPX spectrum displayinformationisupdatedtothedisplaymonitorwithoutmissingthepresenceofevenoneofthe48,000spectrummeasurements per second.

One last step is for the DPX spectrum display processor to checktheuserentryfor“persistence.”Ifthepersistenceissettominimum,thenthepixelmemorywillbezeroedoutbeforethenextsetofspectraisentered.Ifthepersistenceishigh,thentheolderdatawillslowlybedividedoutsothattheeffectfromanoldspectrumeventwillslowlyfadeaway.However,whenthepersistenceissettoInfinite,thepixelhistogrambehindtheimagewillbeacompletehistogramandnot fadeaway.

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Advanced Radar Analysis

Figure 25. RSA measurements of the Frank Coded pulse.

NowtheDPXspectrumdisplayisusedto“discover”someotherwiseinvisibleartifacts.ThisFrankCodedpulsehassomeveryinterestingcharacteristics.Eachpulsewasprogrammedtohavecontrolledtransitiontimes.ThisshouldsomewhatlimitthebandwidthofaCWpulse.ThepulsetimingscanbeseeninthepulsetableinthelowerrightofFigure25tobe100µswidthand11µsriseandfalltimes.However,thetransitionsbetweenthedifferentsegmentsofphasearenotphase-continuous.Thiscreatesveryshort-timewidefrequencytransient spectral components at these transitions.

TheentireSin(x)/xspectrumofthepulseoccupiesonlytheverycenterofthe110MHzwidestandardSpectrumDisplayinthelowerleftofFigure25.Thisdisplaydoesnotshowthatthere is extended spectral energy present due to the phase discontinuitiesincorrectlyallowedatthetransitionsbetweenthesegmentsofdifferentphasesofthemodulation.Thesephase transitions are only a very small percentage of the

pulseduration.Astandardspectrumdisplaycannotshowthespectralresultsoftheseverynarrowtransitions.

The DPX spectrum display in the upper left dramatically showstheinfrequentwidebandsplatterduetothephasetransitions. The dark blue of these excursions denotes that they are very infrequent compared to the central portion of the pulse spectrum.

The spectrum trace is the result of one measurement (depending on the spectrum settings) that may happen once foreachpulse,ormayintegrateformanypulses.TheDPXspectrumdisplaywith292,000individualspectrapersecondhas(forthis100µspulsewidthand120µspulseperiod)atleastsixspectrummeasurementsperpulsewithnogaps.Thisprovidesdiscoveryofallthespectralenergyincludingthewidesplatter from the phase transitions.

DPXspectrumdisplayallowsdiscoveryofinfrequentorotherwiselowprobabilityofinterceptsignals.

Application Note

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Figure 26. AFrequencyMaskthattriggeredonthesmallsignalinbetweentwo large ones.

Frequency Mask Trigger (FMT) in the RSA Series

Therun-timeprocessenginethatisusedforDPXspectrumdisplaycanbeusedtomonitortheacquisitionbandwidthwithoutinterruptionandtriggertheinstrumentontheincoming frequency spectrum.

Fortriggeringonspecificfrequenciesatspecificamplitudes,Tektronix invented the Frequency Mask Trigger (FMT). The usercandrawamaskonthespectrumdisplay.Thismaskcoulddefinea“Triggerifasignalentershere”mask.AnexampleofamaskisinFigure26.Thisoneisdrawnsothatinbetweentwoofthelegitimatesignalsthereisatriggerareadrawn.ThemagnitudeoftheFFTofasignaliscomparedagainthemaskasfastasnecessarytosatisfyNyquistcriteria.Nowifasmallsignalintrudesonthemask,atriggerisgeneratedwhichwillcapturethesignalintomemory.

The signal is being continuously digitized and fed into the acquisition memory. When the data reaches the end of the memory,itseamlesslystartsatthebeginningofmemoryand replaces the oldest data. In this circular fashion there is alwaysanentirememoryofcaptureddatabeingupdated.Whenthetriggeroccurs,theinstrumentmarksthetriggerlocation in memory.

In the same manner as digital oscilloscopes or Logic Analyzers,thereareuser-specifiedpre-triggerandpost-triggermemorysizes.Oncethetriggerlocationismarkedinmemory,theacquisitionwillcontinueuntilthepost-triggeramountofmemoryisfilled.

Inthismannerthememorycanbeoptimallyconfiguredtocontain a seamless capture of the events both before and after the trigger.

Themaskcanbedrawnarbitrarilywithasmanyfingersorothershapedareasasdesired.Alternatively,the“Autodraw”functioncanbeusedtoautomaticallydefinethespectrumfromexistingtracesinDPXspectrumorSpectrumviewswithauser-definablefrequencyandamplitudeoffset.Thesepointscanthenbemanuallymodifiedaroundspecificfrequencyevents of interest.

Thismaskcantriggeronaverysmallsignalatonefrequency,whilepreventingthesurroundinghugesignalsfromgeneratingatrigger.AnormalpowertriggerwillalwaystriggeronanysignalwithintheIFbandwidthwhichisabovethetriggerthreshold.TheFMTwillonlytriggeronthefrequenciesspecifiedbytheuser.

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Advanced Radar Analysis

Figure 27. Spectrumanalyzerandfrequencyvs.timeviewsofthePLLunlocktransient. Figure 28. DPX spectrum display of the PLL unlock transient.

Signals in both Time and Frequency Domains

Some signals may exhibit transient behavior in both the time and frequency domains simultaneously. One example of this is a Phase Locked Loop (PLL) that is marginal in its lock and occasionally unlocks. When it unlocks it may quickly snap back and not unlock again for many hours. Such transient behaviorcanbeverydifficulttocapture.

But complicating the troubleshooting is the likelihood that whenunlocked,theoscillatormaysweepoverawidefrequency range. Such a transient in both time and frequency requires a tool like DPX spectrum display to understand the character of the fault.

Figure 27 is such an occasionally unlocking PLL. Without DPX spectrumdisplaythereisvirtuallynowaytoevendiscoverthatthereisaproblematall.Thetransientunlockandre-locktakes 150 µs total. This transient repeats about once per

secondandthedutycycleofthetransientis0.015%.Thisiswhythespectrumtraceintheupperwindowhasonlytheintended carrier and not a clue that there is a problem.

Thelowertraceisafrequencyvs.timedemodulationview.Ifaproblemcanbelocated,thenitsnaturecanbeseen.NexttheDPXspectrumdisplayisusedtolookforanytransient problems.

WiththeDPXspectrumdisplay,theproblemisrapidlyvisible.Theverylowdutycycleisthereasonforsuchadimbluecolorforthetransientfrequencieswhiletherestofthespectrumthatiscontinuousshowsasverybrightred.

Oncetheproblemisdiscoveredtoexist,andthefrequenciesareknownwhereitisdoingitswork,theFMTcanbesetuptocapturearecordonlywhenthetransientslipsintotherightpart of the blue display components.

Application Note

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Figure 29. FMTcapturesthetransient.Nowfrequencyvs.timeanalyzesit.

Event Isolation and Analysis

The upper trace in Figure 29 is the same spectrum trace as before,butnowithasaFMTsetupintheblueboxuppermostinthedisplay.Thatrepresentswherethetriggerfoundthesignal,andthespectrumplotdisplaysthefrequenciespresentat that time.

Thefrequencyvs.timedisplaynowshowsthenatureofthetransient. The display had been zoomed in to see the detailed shape of the transient. Markers can be used to take exact measurements if needed.

External ThreatsThere may be many signals external to radar or other electronicsystemswhichwillcauseproblems.Whenasystemutterlyceasestofunction,itwillbeknownthatthereis a problem. But some signals may be more subtle such thattheradarsimplygetsthewrongresult.Thismaywellbemuchworsethanoutrightfailure.

Inthecaseofthemoresubtleinterference,amethodisneededtodiscoverthatthereisaproblem,triggeronit,captureandthenanalyzeittolearnwhat,andpossiblywhereitis.

Dependingonthetypeofexternalproblem,thetechniquesfordiscoveringandlocatingitmaydiffer.

Interference

Interferencetoapulsedsignalmaybehardtofind.Thenextexampleisthecaseofa“scanningmonitorreceiver”located near a radar installation. The Local Oscillator (LO) inthereceiverwasradiatingaharmonicphysicallyneartheradar receiver. As the scanner hopped through the band it wasmonitoring,theharmonicwashoppingthroughtheradarpulse frequency

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Advanced Radar Analysis

.

Figure 30. Ordinary analyzer display of the spectrum in the area of the radar.

However,initiallyitwasonlyknownthateveryfoursecondsorsotheradarexperiencederrors.Figure30showsthespectrum as measured by an ordinary spectrum analyzer or VSA.Onlytheoutlineofthesine(x)/xpulsespectrumisvisible.Iftheradarcouldbeturnedoffothersignalsmightbeseen,butinthiscasetheinterferencewasonlypresentforlimitedperiodsoftime,sotheradarwouldneedtobeturnedoffforseveral days.

An ordinary analyzer does not have enough spectrum measurements,nordoesitcombinethemsoastobesurenottomisstheeffectsfromanyoneofthem.TheDPXspectrumdisplaycanseeallofthespectrumeffects.NotehowthesteppingLOharmoniciseasilyvisibleeveninsidethespectrumlobesofthepulsetransmitter.Whenviewingthison a real RSA Series spectrum analyzer (not a still image as shownhere)theLiveRFviewreallymakestheinterferencemorevisibleyet,asthevisibilityisimprovedbyitsmovementinside the stationary pulse spectrum.

Signal

Interference

Figure 31.TheRSAwithDPXLiveRFspectrumdisplaycanseetheinterferingsignalhopping underneath the radar pulse.

For Further InformationTektronix maintains a comprehensive, constantly expanding collection of application notes, technical briefs and other resources to help engineers working on the cutting edge of technology. Please visit tek.com

Copyright © 2016, Tektronix. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks or registered trademarks of their respective companies.

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