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FMTranceiver.pdf

FMTranceiver.pdf

上传者: frankcfu 2012-08-25 评分 0 0 0 0 0 0 暂无简介 简介 举报

简介:本文档为《FMTranceiverpdf》,可适用于IT/计算机领域,主题内容包含DesignandConstructionofFMTransmitterandReceiverFinalReportDavidChenAbstrac符等。

DesignandConstructionofFMTransmitterandReceiverFinalReportDavidChenAbstract:OurFMtransmitterandreceiverarebuiltwithdiscreteanalogcomponentsandintegratedontwocircuitboardsThemodulationschemeusesasuperheterodynesetup,inwhichtheintermediatecarrierisatkHzandthetransmissioncarrierisatMHzUsingmWofDCpower,thetransmitteroutputsadBmsignalcenteredatMHzwithkHzbandwidthUsingabouthalfasmuchpower,aroundmW,thereceivercandetectincomingsignalsatpowersaslowasdBmThisreceptivityleveltranslatestosuccessfulaudioreceptionatdistancesofalmostkmfromthetransmittingantennaatopPackardDesignTheory:Transmitter:Thetransmitterisdesignedtotakeasignalintheaudiorange(Hz–kHz)andprepareitfortransmissionthroughtheairAsignalpathforthetransmitterisdepictedinFigureTwomodulationstagesperformtheupconversionTheVCO(implementedthroughtheLM)convertsthebasebandsignalintothefrequencyofasquarewaveWithonlyaDCinput,theVCOissettooutputatexactlykHzTocontrolboththeDClevelandthemaximumACvariationattheVCOinput,anaudioamplifierisplacedbetweentheVCOandtheaudiosourceFollowingtheVCO,themixerupconvertstheVCOoutputtothetransmissionfrequencyatMHzAcrystalprovidesthemixerwithitsMHzlocaloscillatorreferenceThemixeroutputcantechnicallybeusedfortransmission,butitisgenerallytooweaktobesentfarApoweramplifierthatprovidesdBofpowergainisplacedbetweenthemixerandtheantennatoboosttheactualoutputSincepoweristheprimaryconcernatthisstage,impedancematchingbetweenthemixerandthepoweramplifierisneededtominimizetransferlossWithadequateoutputpower,thistransmitterisabletosendsignalsouttodecentdistancesFigureStagesofFMtransmitterReceiver:ThereceiverisdesignedtoworkwiththetransmitterSpecifically,thereceivermustpickupanoisyandattenuatedtransmission,selectivelyfilterandamplifysignalsinthespectralregionsaroundMHzandkHz,andultimatelydemodulatethetransmittedsignalbacktoarecognizableaudiosignalThisprocessisillustratedbyablockdiagraminFigureThesignalofftheantennafirstenterstheLNAAccordingtoFriis’Equation,noisethroughthisfirststagehasthegreatestimpactonthesystem’sSNRThus,muchcareistakentoensurethattheLNAhasthebestSNRofallstagesinthereceiver,throughacombinationofhighgainandlownoisefigureThesignalisthensentintothemixer,whichdownconvertsthesignaltoanintermediatefrequencyofkHzFilteringandaddinggainismucheasieratthislowercarrierfrequencythanatMHzAtthispoint,asecondorderButterworthbandpassfilter,centeredatkHzwithkHzbandwidth,isusedtoattenuatenoisefarawayfromkHzTwononinvertingamplifierstogetherprovidedBofgainUnfortunately,thefilterdoesnotlimitnoisearoundkHzandtheamplifiersintroducenoiseoftheirown,sotheIFstagewillnotboosttheSNRasmuchastheLNAdoesThegainprovidedbytheIFstage,though,doeshelpsendastrongersignaltothePLLIfthePLLcanlockontothemodulatingsignalembeddedinthefrequencyofitsinput,thePLLwilloutputthedemodulatedaudiosignalWithsomeadditionalgainandcrudelowpassfilteringfromtheaudioamplifier,weshouldbeabletohearthetransmittedsignalonaspeakerFigureStagesofFMreceiverCharacterizationofTransmitter:OverallCharacteristics:SystemlevelcharacteristicsforthetransmitteraresummarizedinTablesandThetransmitterisabletooutputneardBmpoweracrossaΩloadSincetheantennahasapproximatelyimpedance(asmeasuredonthenetworkanalyzerandSWRmeter),almostallofthedBmistransmittedWemustdifferentiatebetweenthesystem’sinputandoutputbandwidth,becausetheVCOtranslatesvoltagesintofrequencyTheinputbandwidthwasfoundtobealloftheaudiorange(Hz–kHz)allsignalswithinthisrangecouldbeamplifiedbytheaudioamplifierandthenconvertedbytheVCOThelowerboundofHzistightbecauseDCblockingcapacitorspreventoperationclosertoDC,buttheupperboundofkHzismuchlooserTheoutputbandwidth,ontheotherhand,islimitedbytheVCOWhentheVCOiscenteredatkHz,itssquarewaveoutputcanswingbetweenkHzandkHzThus,theΩoutputbandwidthiskHz(listedasthesystembandwidthinTable)andremainsunchangedgoingthroughthemixerandpoweramplifierstagesInordertooutputatdBm,thesystemconsumesmWofpowerAtthisrateofusage,ahrmAVbatterycanbeexhaustedinhoursPowerconsumptionforeachstageislistedinTableClearly,thepoweramplifierusesthebulkofthepower,anecessarybutseeminglywastefuldrainDistortionneverbecameaseriousissuewiththetransmitter,aslongastheaudiosourcewaslimitedtomVamplitudeandtheVCOinputwaslimitedtomVamplitudeTheseconstraintskepttheaudioamplifierTHDlessthanandtheVCOoutputstablywithinkHzofkHzcenterfrequencyOutputPowerdBm(acrossload)ΩVBatteryLifehours=minutesSystemBandwidthkHz(dBcutoffs)TransmittableAudioRangeHz–kHzTableKeyfiguresofmeritfortransmitterOverallAudioAmplifierVRegulatormWmWmWVCOMixerPowerAmplifiermWmWmWTablePowerconsumptionoftransmitterblocksAudioAmplifier:Thefunctionoftheaudioamplifieristwofold:)tobiastheDCleveloftheVCOinputatVsothereismaximumfrequencyswingonbothsidesofkHz,and)tolimittheACamplitudeoftheVCOinputtomVProperDCbiasingisaccomplishedusingaΩkpotentiometeractingasavoltagedividerSincethelargestaudiosignalweappliedhadamVamplitude,wechosetosetthegainoftheamplifieratdBVCO:FigureVCOfrequencyvsinputvoltageFigureVCOoutputforHzinputWeusedatimingcapacitorofpFandapotentiometerofΩktoshifttheVCOcenterfrequencytokHzOthervaluesofcapacitanceandresistancecouldalsobeusedtoachievethekHzcenter,buttheycausednonlinearfrequencyshiftsattheVCOinputinresponsetolinearchangesattheinputvoltage,asdiscoveredinLabFigureshowsthefrequencybehavioroftheVCOthrougharangeofinputvoltagesBasedonthistest,wechoseaDCbiasofVattheinputtotakefulladvantageofthelinearregionThefrequencytovoltagesensitivitykoftheVCOismeasuredtobekHzV,lowerthanpredictedAtypicalinput(HzmVamplitudesinusoid)producestheoutputdepictedinFigure,wherewecanseethebandwidthisclosetokHzandthepeakoutputpowerisdBmMixer:ThemixercorrectlyupconvertedthekHzsignalfromtheVCOtoMHz(seedepictioninFigure)Unfortunately,intheprocess,themixerdidnotprovidethedBofconversiongainpromisedbythespecificationsheetInstead,themixerattenuatedthesignalbydBWewerecarefultoselectcouplingcapacitorsattheinputandoutputtorepresentlowimpedanceatkHzandMHz,respectively,butbeyondthat,wecouldhadnootherdesigncontroloverthemixercircuitWeacceptedthisunexpectedlossandfocusedourenergiesonthepoweramplifierFigureMixeroutputforkHz,dBminputFigureNetworkjoiningmixerandPAPowerAmplifier:ThepoweramplifierisdesignedtosupplyuptodBofgaintocompensatefortheweakmixeroutputOtherthanthebasicGaliamplifier,thereisanRFchokebranchconsistingofainductanceinserieswithaHµΩresistance(multiplediscretecomponentshadtobeusedtoachievethesevalues)Toensuremaximumpowertransferbetweenthemixerandthepoweramplifier,thematchingnetworkinFigurewasusedThenetworktransformstheoutputimpedanceofthemixer(includinganFcouplingcapacitor)intoapproximatelyAdditionalmatchingwasunnecessary,sinceboththeinputandoutputimpedancesoftheGaliweremeasuredonthenetworkanalyzertobeveryclosetoΩΩCharacterizationofReceiver:OverallCharacteristics:SystemlevelcharacteristicsforthereceiveraresummarizedinTablesandThesystemgainisdB,consistingofdBfromtheLNAanddBfromeachoftheIFamplifiersConsideringtheexactsystembandwidthisdifficultbecausetherearemultipleblockswithdifferingfrequencyresponses,suchasthematchingnetworksfavoringMHzversustheIFbandpassfilterfavoringkHzWechoosethemostunambiguousbandwidthconstraint:thekHzdBbandwidthoftheIFfilteraroundkHzcenterTheminimumdetectablesignalofdBmwasmeasuredbyalternatingbetweentwosingletoneinputs(HzandkHz),loweringthepowergradually,andsubsequentlydeterminingwhenwecouldnolongerhearthealternatingtonesnorseetheirpeaksonthespectrumanalyzerUnlikethetransmitter,thereceiverisamobiledevice,sobatterylifebecomesagreaterconcernWewereabletolimitpowerconsumptiontomW,translatingtohoursofusageonahrmAVbatteryTheLNAusesalargefractionofthepower,butthisisaworthwhileinvestmenttoobtainhighgain(andthushighSNR)througharguablythemostimportantblockonthereceiverSystemGaindBVBatteryLifehours=minutesSystemBandwidthkHzMinimumDetectableSignaldBmTableKeyfiguresofmeritforreceiverOverallLNAVRegulatormWmWmWIFAmplifiersMixerPLLmWmWmWAudioAmplifiermWTablePowerconsumptionofreceiverblocksLNA:FigureLNAusingshuntshuntfeedbackFigureMatchingnetworksforLNAandmixerPowerGaindBNoiseFiguredBSignaltoNoiseRatiodBdBCompressionPointdBm(measuredatinput)IPPointdBm(measuredatinput)InputImpedance(j)Ω(aftermatching)OutputImpedance(j)(aftermatching)ΩMaximumReceivableDistancekm(westsideofEscondidoVillage)TableKeyfiguresofmeritforLNATheimportanceoftheLNAtotheoverallfunctionalityofourreceivercannotbeoverstatedThisstagehasthehighestSNRofallstagesandcanpotentiallyhavethegreatestimpactonoverallsystemnoiseaccordingtoFriis’equationFigureshowsthatweusedtheshuntshuntfeedbacktopologypresentedinlectureThecollectorandemitterresistancesaredesignedtodrawmAofcurrentthroughthetransistorInactuality,theentireLNAcircuitisusingclosetomABiasresistancesofΩkarechosentodrawacurrentmuchsmallerthanthecollectorcurrentandmuchlargerthanthebasecurrentOriginally,thefeedbackresistancewassetat,butweexperimentallyfoundthatΩkΩMmovedthecollectorcurrentandbiasvoltagesclosertodesiredlevelsThefeedbackcapacitancewewilltalkaboutshortlyThiscircuitisabletoprovidedBofgainwithanoisefigureofdB,resultinginanSNRofdB,whichismuchhigherthantheSNRofsubsequentstagesOneofthemosttimeconsumingandbafflingchallengesweovercamethisquarterwasthematchingofLNA’sinputandoutputimpedancestoΩTheproblemwasmadedifficultbytheseverecouplingbetweentheLNA’sinputandoutputAssoonaswematchedtheinputclosetoΩandstartedmatchingtheoutput,wedestroyedtheinputmatch,andviceversaWereachedapointwherewecouldmatcheitherportwithaccuracybutcouldnotmatchbothsimultaneouslyItwasonlywhenwediscoveredthemainculpritforthestrongcoupling,thefeedbackcapacitor,thatwemadeprogressOuroriginalfeedbackcapacitanceofpFwastoolargeWhenthepFwasreplacedwiththecurrentpF,thecouplingminimizedsignificantly,totheextentthatwecouldindependentlymatchtheinputandoutputThefinalinputandoutputimpedancesare(j)and(j)ΩΩusingthenetworksshowninFigureThesevaluesareverysensitivetoparasiticeffects,soourwiringandconstructionhadtofollowstringentstandardsMixer:Again,themixerfailedtoprovidetheexpecteddBconversiongain,butfortunatelyitdidnotattenuatethesignalpassingthroughitBothoftheinputsarematchedtoΩasshowninFigureTheoutputofthemixerterminatesintothefirstIFamplifier’sinput,whichhasverylargeimpedance,somatchinginthatcasedidnotmakemuchsenseThelinearitycharacteristicsofthisstagearedBmfordBcompressionanddBmforIPIFFilterandAmplifier:ThepassivefilterintheIFsectionprovidesfrequencyselectivityofkHzaroundkHzcenter,asdepictedinFigureUnfortunately,thefiltercannotremovenoiseinthepassbandEachamplifierinthisstagecontributesdBgainatkHzbutalsoraisesthenoisefloorbydB,resultinginanSNRofonlydBThisindicatesthattheIFstageisnotasefficientastheLNAatamplifyingthesignalwithoutintroducingmorenoiseFigureFrequencyresponseofIFbandpassfilterPLL:MinimumDetectableSignaldBmLockRangekHz–kHzCenterFrequencykHzSensitivitykHzVLinearitytotalharmonicdistortionTableKeyfiguresofmeritforPLLAfterdeterminingthecorrectbiascomponents,thePLLdidnotpresentanyproblemsinthelatterintegrationstepsItsminimumdetectablesignalhasdBmpowerThus,givenaweakdBmontheantennaandassumingdBgainpriortothePLL,thedBmsignalcanbedemodulatedLockrangewasfoundtobekHz–kHz,slightlylargerthanpredictedTheimplicationofthisdiscrepancyisthatthePLLwilllockontomoresignalsaroundkHzbutwillalsointroducemorenoiseintothefinaloutputNonlinearityofthePLLisnegligiblegiventheverysmalltotalharmonicdistortionAudioAmplifier:TheaudioamplifierbetweenthePLLandthespeakerisnecessaryfortworeasons:)thePLLoutputmaybetooweaktobeheardclearlyonthespeaker,and)thePLLoutputcontainsaleakageofthekHzsignalfromtheinput,resultinginhighfrequencyperturbationsontopofanotherwisecleanaudiosignalTothePLLoutputofdBmpower,weaddeddBofgainthroughtheaudioamplifierToeliminatethekHzleakage,weusedalowpassfilterwithakHzcutoffinthefeedbackpathoftheamplifierThelowpassfilterresultedinremarkablycleanaudiooutputsFieldTests:Withthetransmitterandreceiverbothfunctioning,weconductedthreefieldtests,thelastofwhichwastheclassfieldtestonSPAMDayFirstWalk:Mypartnerwentoutintheevening,aftertherainsubsidedOurtransmitterwasconnectedtotheroofantennaandsentoutasimplemusicpieceHewasabletogetgoodsignalreceptionasfarasEncinaHall(lessthankm)Alongtheway,henoticedthatdeadspotsoccurredmostfrequentlyinthepresenceoftreesSometimes,standinginfrontoftallbuildingshelpedreception,probablybecausesomesignalsbouncedoffthebuildinginthedirectionofthereceiverOneproblemthatplaguedthelatterhalfofhistestswasafaultybatterclip,causingthereceivertobepoweredonlyintermittentlySecondWalk:BothmypartnerandIparticipatedinthissecondfieldtestWewentinadifferentdirectionthistime,towardsthelakeAstheelevationbeneathourfeetincreased,thereceptionimprovedbecausetherewasaclearerlineofsightbetweenourreceiverandtheantennaatopPackardInparticular,atthelake,wecouldpickupadecentsignalTreesoftendisruptedreception,whilebuildingfacesagainhelpedreceptionInfact,oneofthebestlongrangesiteswasinfrontofamusicbuildingwhichwasevenafewhundredmetersfurtherthanthelakeHavinglostreceptioninaforestedregion,weheadedbacktowardsthecenterofcampusBythetimewereachedWhitePlaza,weregainedaclearaudiosignalFortherestofthetrip,wewalkeddownSerraStreetandhadthegoodfortuneofbeingabletoreceiveaudioasfarastheSerraElCaminointersection(aboutkm)ThereweredeadspotsalongthewayandtheSNRdegradedwithdistance,buttheaudiosignalwasstilldiscernibleonElCaminoFurtheronElCaminointhedirectionofCaliforniaAvenue,welostoursignalbutpickedupsomeinterestingexcerptsfromtalkshowsThesestraytransmissionswereespeciallystrongnearlamppostsandotherlargemetalpolesOuraudiosignalreturnedbrieflyinfrontofahotelnearStarbucks,maybebecausewehadabetterlineofsightawayfromthetreesThirdWalk(SPAMDay):SPAMDaywasmostenjoyableinthatwewereabletowalkasaclassandreceiveatransmissiontogetherWewereabletoreachthegasstationandstillreceiveaclearsignalFurthertowardsElCamino,theSNRdegradedfasterthanitdidonourpreviousfieldtest,sowestoppedhalfwaybetweenthegasstationandElCaminoSeveralfactorscouldhavebeendetrimentaltoreception:)theweatherwasnotasfavorable,)thenewlycreatedmetalenclosureforthereceiveractuallyhurtreception,or)theBNCoutputtoaloudspeakerisnotasoptimalasawireconnectiontoaheadphoneConclusion:BlockbyblockdesignandconstructionoftheFMtransmitterandreceiverwastherightapproachWebecomefamiliarwiththebenefitsandlimitationsofeachstageandcouldoptimizethemindividuallyinthefirstfewweeksWhenintegrationoccurred,thefirsthandknowledgeoftheseparateblocksbecameaninvaluablepartofdebuggingThemainachievementofthisprojectisthesuccessfulconstructionoftwosystems,showninFiguresandOurtransmitterisabletooutputatdBm,whileourreceiverisabletodetectsignalsasweakasdBmandasfarawayaskmOurtwocircuitsarebynomeansidealGivenmoretime,wewouldfixsomeissueswehavenoticedduringconstructionSincetheLNAprovidesthebestSNRoutofallstagesonthereceiver,wecanincreaseitsgainbydrawingmorecollectorcurrentandtrytolowerthenoisefigurebyexperimentingwithnewresistancevaluesTheIFamplifierscurrentlycontributetoomuchnoiseintheregionaroundkHzWecantryadifferentopampanddifferentvaluesofbiasresistors,aswellassharpenthecutoffofthepassivebandpassfilterbyusingahigherorderButterworthtopologyNeithermixerisprovidinganyconversiongain,sowewouldtryusingadifferentmixerchipThereiscertainlyanimpedancemismatchbetweentheVCOoutputandmixerinput,whichwewouldsolvebyimpedancematchingonetotheotherorbothtoΩAlso,withmoretime,wewouldexplorethetransmissionroutethroughtheColpittsoscillator,toincreasethetransmissionpowerwithoutsacrificingstabilityFigureTransmittercircuitFigureReceivercircuit

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