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Vl01.25No.6JOURNALOFELECTR0NICSfCHINANovember2008

    ;AHIGHPERFORMANCEUWBLFMWAVEFORMGENERATOR

    ;ZhuMingbo

    ;(DeptofSpaceRemoteSensing,InstituteofElectronics,ChineseAcademyofSciences,Beijing100190,China)

    ;(DeptofElectronicsEngineering,NavalAeronauticalandAstronauticalUniversity, ;Yantai264001,China)

    ;AbstractInthispaper,thedesignandimplementationofahighperformanceUltra-WideBand

    ;(UWB)LinearFrequencyModulation(LFM)waveformgeneratoratVeryHighFrequency/ultraHigh

    ;FrequencyfVHF/UHF)bandareintroduced.Firstly,thedesignideasforahighperformanceUWB

    ;LFMwaveformgeneratoraredescribed.Then.agenerationschemeforUWBLFMwaveformsis

    ;presentedaccordingtothebasebanddigitalgenerationmethodcombiningwiththebandwidthex

    ;tensionmethodviafrequencydoubling.Anexperimenta1systemhasbeenimplementedandtested.The

    ;resultsshowthattheUWBLFMwaveformgeneratorachievesveryhighperformance. ;KeywordsUltra-WideBand(UWB);LinearFrequencyModulation(LFM);Signalgeneration

    ;CLCindexTN958.3

    ;DOI10.1007/s1176700700214

    ;I.Introduction

    ;AsignalisreferredtoasUItra-WideBand

    ;fUWB)oneswhenitsinstantaneousrelative

    ;bandwidthexceeds25%.UWBsignalshavealready

    ;beenusedincommunicationsystems,andwillbe

    ;usedinradarsystemsinthenearfuture.Thereare

    ;twomainkindsofUWBsignals,i.e.,impulseand

    ;modulatedones.UWBLinearFrequencyModula-

    ;tionfLFM1waveformsbelongtothemodulated

    ;signals.

    ;Duetoitsgoodpulsecompressioncharacteris.

    ;tics.LFMwaveformshavebeenwidelyusedinhigh

    ;resolutionradars,suchasSARsandISARs.The

    ;UWBLFMisakindofwaveformcommonlyusedin

    ;UWBradars.IfitoperatesintheVHF/UHFband

;andcombineswiththesyntheticaperturetech

    ;nique.UWBradarusingLFMwaveformwillhave ;FOliagePENetration(FOPEN1andGroundPEN

    ;etrationfGPEN)capabilities,andwillbeableto ;detectandimagecoverttargets.Thusitisofgreat ;importancetostudythegenerationofaUWBLFM ;radarsigna1.

    ;1Manuscriptreceiveddate:January25,2007;reviseddate ;January11,2008.

    ;Communicationauthor:ZhuMingbo,bornin1971,male, ;Ph.D.,associateprofessor.DeptofElectronicsEngi? ;neering,NavalAeronauticalandAstronauticalUniversity, ;Yantai264001,China.

    ;Emaihmb

    ;zhu@126.com.

    ;Atpresent,thestudiesonUWBsignalgenera- ;tionaremainlyconcentratedinthatoftheimpulse ;waveforms[1-~1.

    ;Refs.f6,71introducedawideband

    ;waveformgenerationmethodforradarapplications, ;whichuseddigitalgenerationinthebasebandand ;quadraturemodulationintheIntermediateFre- ;quency(IF).Ref.[8]alsousedthismethodtogen

    ;crateUWBLFMsignalsforwirelesscommunica- ;tions.

    ;Inthispaper,thedesignandimplementationof ;ahighperformanceUWBLFMwaveformgenerator ;viatheabovemethodatVHF/UHFbandforradar ;useiSintroduced.Itsmaincharacteristicisthat ;predistortioncompensationtechniqueisusedto ;improvetheperformanceoftheultimatewave- ;forms.

    ;II.DesignIdeasandScheme

    ;1.Designideas

    ;Itisofconsiderabledifficultytogenerateahigh ;performanceUWBLFMwaveformwithitsin

    ;stantaneousrelativebandwidthexceeding85%at ;VHF/UHFband.Toreachthataim,thedesign ;ideashavebeendeterminedasfollows:itisessential ;fortheUWBLFMwaveformgeneratortohave ;predistortioncompensationcapabilitysoastoen

    ;surethatwecanachieveaUWBLFMwaveform ;withhighperformancewiththeexistingcompo- ;nents.

    ;ZHUeta1.AHighPerformanceUWBLFMWaveformGenerator775 ;2.Generationscheme

    ;Ingeneral,theremainlyexisttwokindsof ;methodsforLFMwaveformgeneration,namelythe ;passivemethodandtheactivemethod.Thepassive ;oneusuallyusesSAWcomponentsanditisdifficult ;togenerateanLFMwaveformwithlargeproductof ;durationandbandwidth.Itiseasy,however,to ;generateanLFMwaveformwithlargeproductof ;durationandbandwidthbytheactivemethod.In ;thepast,theactivemethodusuallyusesanalog ;components.suchasVCO.Becauseoftheinherent ;defectsofanalogcomponents,boththefrequency ;stabilityandthemodulation1inearityofthe ;waveformsgeneratedbythisanalogactivemethod ;axerelativelypoor.Therapiddevelopmentof ;digitaltechniquesmakesitpossibletogeneratean ;LFMwaveformbythedigitalmethod.Comparedto ;allthepreviousones,thedigitalgenerationmethod ;hasmanydistinctadvantages,suchasgoodflexi- ;bility,highreliability,andconvenientcompensa- ;tionforwaveformdistortion.Now,thedigital ;methodhasalreadybeentheprimarymethodfor ;thegenerationofLFMwaveformswithhighquality ;and1argeproductofdurationandbandwidth. ;Accordingtothedesignideaspresentedpreviously, ;wehavedeterminedtoadoptdigitalmethodto ;designandimplementthegenerationoftheUWB ;LFMwaveform.

    ;Limitedbythespeedofdigitalcomponents, ;especiallythespeedofdigitalmemoriesand ;COUnters.highqualityIWrBLFMwaveformscur

    ;rentlycannotbedirectlygeneratedatVHF/UHF ;band.However,wecanuseadigitalgeneration ;methodinbasebandcombinedwiththebandwidth ;expansionthroughfrequencydoubling.Namely, ;firstweusedigitalmethod(D/Aconversion)to ;generatetheIandQorthogonalanalogLFM ;basebandcomponentswiththerequiredduration ;directly,thenwecanmakethemhavedeterminate ;IFandbandwidththroughquadraturemodulation, ;andatlastwecangeneratetheUWBLFMwave- ;formwiththerequiredfrequencybandandband

    ;widththroughfrequencydoublingandfrequency

;conversion.

    ;Atpresent.thedigitalmethodsusedforLFM ;waveformgenerationmainlyincludewaveform ;storagemethodandDirectDigitalSynthesize ;(DDS).TheformergeneratesanalogwavefcIrms ;directlythroughA/Dconversionoftheprestored ;waveformsamples.Thismethodcanbeusedina

    controlledmannerandsocantakefuU ;program

    ;advantageofthesupportofsoftwaretocalculate ;andloadthesamplesofarbitrarywaveforms.The ;latter,namelytheDDSmethod,generatesanalog ;waveformsthroughphaseaccumulation,amplitude ;recallandA/Dconversion.TheDDScomponents ;thatcangenerateLFMwaveformsareusuallyre- ;ferredtoasDDCS.HowevermostoftheDDCS ;productsatpresentarelackofflexibilityofam- ;plitudeandphasecompensation.Accordingtothe ;samedesignideas,wehavedeterminedtoadopt ;waveformstoragemethodtoimplementthegen

    ;erationofthebasebandLFMwaveformsothatthe ;amplitudeandphaseofthewaveformcanbepre- ;distortedconveniently.

    ;3.Blockdiagramofthegenerationscheme ;Accordingtowhatdiscussedabove,wedecideto ;designandimplementtheUWBLFMwaveform ;generatorthroughdigitalbasebandgeneration ;combinedwithbandwidthexpansionviafrequency ;doubling.Theblockdiagramofthegeneration ;schemeisshowninFig.1.Asdepicted,thegenera- ;tionofthebasebandwaveformuseswaveform ;storagemethod.

    ;Fig.1Blockdiagramofthegenerationscheme ;III.DesignandImplementation

    ;Accordingtothegenerationscheme,theband- ;widthofthebasebandwaveformsapproaches ;40MHz.thebandwidthofthefinalUWBLFM ;waveformafterquadraturemodular:ionandtwo ;stagesoffrequencydoublingwillexceed300MHz. ;ToensurethequalityoftheUWBLFMwaveform. ;first,carefulconsiderationshavebeengiventothe ;choiceofcomponentsandthedesignandlayoutof ;thecircuits.Then,asymmetricalstructurehasbeen ;776JOURNALOFELECTRONICS(CHINA),Vo1.25No.6,November2008

    ;adoptedtoensurethecoherenceoftheamplitude

    ;andphaseoftheI/Qcomponents.Finally,the ;controlpartoftheUWBLFMgeneratorhasbeen ;designedwithFPGAtoimprovetheintegration ;andreliability.Moreover,theamplitudeandphase ;distortionoftheUWBLFMwaveformgenerator ;hasbeencompensatedcarefully.Bydoingso,the ;qualityofthefinalUWBLFMwaveformhasbeen ;greatlyimproved.

    ;1.Basebanddigitalgeneration

    ;Thehardcoreofthewaveformstoragemethod ;forbasebandgenerationismemoryandD/Acon

    ;verter.SRAMsarechosenasthewaveformstorage ;devicessoastomakethedigitalgenerationof ;basebandprogrammable.Accordingtothesam

    ;plingtheorem,theD/Aconversionrateshouldbe ;atleastfourtimeshigherthanthehighestfrequency ;ofthebasebandsignal,i.e.,theoversamplingrateis

     ;notlessthan4.Furthermore,asynchronouscoun

    ;terneedstobedesignedtoreadoutthewaveform ;hasbeenimprovedthroughcarryinginadvance, ;andtheconsistencyamongbitsofthecounterhas ;

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    ;(a)Icomponent

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    ;(c)Thebasebandspectrum

    ;Fig.2ThebasebandLFMwaveforms

    ;alsobeenenhancedbysynchronouslatching.This ;basebandparthasthecapabilityofgeneratingar

    ;bitrarywave~Irmsthroughprogramming.The ;basebandLFMwaveformsgeneratedareshownin ;Fig.2withtheirbandwidthnear40MHz.

    ;2.Widebandquadraturemodulation

    ;Quadraturemodulationiscrucialtothegen- ;erationofwidebandLFMsignalswhenthemodu

    ;lationfrequencyandtherelativebandwidthare ;bothveryhigh.Thetwomaintechnicalindexesfor ;evaluatingquadraturemodulatorsarecarrierand ;imagesuppression.Itiswellknownthatthecarrier ;leakageisresultedfromtheDCbiasoftheI/Q ;components,andthemirrorimageisresultedfrom ;thedifferencesbetweentheamplitudeandphaseof ;theI/Qcomponents.Thecarrierleakageandmirror ;imagecanresultinseriousspuriouscomponents ;afterfrequencydoubling.whichwillbadlyaffectthe ;pulsecompression.Soitisnecessarytoimplement ;thequadraturemodulationwithbigcarrierand ;imagesuppression,z.e.toensuretheconsistencyof ;theDCbias.amplitudeandphaseoftheI/Q ;components.

    ;Numericalsimulationisdonetoevaluatethe ;effectsofthecarrierandimagecomponentsonthe ;PeakSidelobeRatio(PSR1ofthematchfiltering ;afterfrequencydoubling,wherethefrequency ;doublingiSbasedonatimedomainmultiplication ;mode1.Whenthefrequencydoublingisfourfold.for ;Hammingweightedmatchfiltering,itshowsthat ;thecarrierandimagesuppressionafterquadrature ;modulationmustbelessthan-39dBctoensurethe ;PSRofpulsecompressionlessthan40dBc.In

    ;implementation,thecarrierandimagesuppression ;arecontrolledunder50dBc.Afterquadraturemo-

    ;dulation,thebandwidthoftheIFsignalapproaches ;80MHz.TheIFLFMsignalanditsspectrumgen

    ;eratedafterquadraturemodulationisshownin ;Fig.3andFig.4.

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    ;Fig.3TheIFLFMsignal

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    ;ZHUetatAHighPerformanceUWBLFMWaveformGenerator ;3.Widebandfrequencydoublingandconversion ;ToensuretheultimateUWBLFMsignalhaving ;excellentamplitudeandphasecharacteristics.the ;analogpartsofthegeneratorshouldmainlyresolve ;thenonlineardistortionofthephasecharacteristic, ;mainlyensurethehighlyflatnessofthepassband ;andthehighlysuppressionoftheattenuationband. ;Inthefrequencydoublingstage,twostagesofduple ;frequencydoublerwithexcellentlinearcharacter

    ;isticsareadoptedtoreducespuriouscomponents ;andtoensurethewidebandcharacteristics.Inthe ;frequencyconversionstage,widebandlinearmixer ;withhighisolationandhighIFareadopted.Fur

    ;thermore,thephasecharacteristicoffiltershas ;greateffectonthelinearityofthegeneratorphase ;characteristics.soitisnecessarytodesignallthe ;filtersbythemaximum1inearphaserequirement.or ;bythemaximumflatenvelopedelaydemand.The ;directlygeneratedUWBLFMsignalisshownin ;Fig.5.

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