Radar Transmitter4ppt.ppt

simplifiedradarblockdiagram,display,keycomponentsofaradarsystemtransmitterelectronicdeviceusedtogeneratethemicrowaveemenergytransmittedbytheradarreceiverelectronicdeviceusedtodetectthemicrowavepulsethatisreflectedbytheareabeingimagedbytheradarantennaelectroniccomponentthroughwhichmicrowavepulsesaretransmittedandreceived,thetransmittedwaveisvariedandrangeisdeterminedbyobservingthelagintimebetweenthismodulationandthecorrespondingmodulationofthereceivedechoes.,cwradars,targetspeedmeasurements,rangemeasurements,dopplershift,frequency-modulation(fm),small,low-powerversionsofcwdopplerradarsareusedas:speedsensors(policeradar)vehicledetectorsfortrafficcontrolproximityfuzesinrockets,bombs,andprojectiles.intheseapplications:therangetothetargetisusuallysmallthelossinsensitivitybecauseoftheuseofasingleantennaisacceptable.,anx-banddopplertransceivermechanicaltuningcoarselysetsfrequency,whereasfinetuningandafccanbeprovidedbymodulatingtheoperatingvoltage.(u.s.armyphoto.),dopplershift,m/a-comgunnplexerdopplertransceiver,whichpacksatransmitter,ferritecirculator,andmixerintoasinglemodule.,theadditionofanantenna,frequencymeter,andadirect-current(dc)powersourcecompletestheradar.,blockdiagramforasimplesingle-antennacwdopplerradarbasedonadopplertransceiver.,agunnoscillatoristhebasictransmitter,whichiscoupledtoasingleantennathroughthecirculator.transmitterpowerreflectedbackfromtheantennaportactsasthelocaloscillatorintothesinglebalancedmixer(anadjustablescrewallowsintentionalstanding-waveratio(swr)mismatchtoforceanadequatelevelofreturnsignal).,cwradar:w.r.tpulsedradar-lesscomplex-lowcost-loweroperatingvoltage,andinsomecases(highpower)usestwoantennas(wastesinarea),thepulsedradartransmitter:generatespowerfulpulsesofemenergyatpreciseintervalshigh-powermicrowaveoscillator(magnetron)microwaveamplifier(klystron),suppliedbyalow-powerrfsourcemodulator:properly-timed,high-amplitude,rectangularpulsehigh-poweroscillatorswitchestheoscillatoronandoffmicrowavepoweramplifieractivatestheamplifierjustbeforethearrivalofanelectromagneticpulsefromaprecedingstageorafrequency-generationsource.,pulsedradar,inamplifiers,themodulatorpulseissuppliedtothecathodeofthepowertubeandtheplateisatgroundpotentialtoshieldpersonnelfromshockhazardsbecauseoftheextremelyhighvoltageinvolved.themodulatorpulsemaybemorethan100kvinhigh-powerradartransmitters.radartransmittersproduce:voltages,currents,andradiationhazardsthatareextremelydangeroustopersonnel.safetyprecautionsmustalwaysbestrictlyobservedwhenworkinginoraroundaradartransmitter,commonfeaturesofradartransmitter,itisusuallylargefractionofradarsystemhighcostlargesizeheavyrequiressignificanteffortsitrequiresamajorshareofsystemprimepowerandmaintenance,becauseradarsarerequiredtogeneratesomuchpoweroutputmostpeopleprefertokeepawayfromit,rangebutthegreaterthenumberofitemsthatareincludedineachofthetwopaths,themorechanceexistsforphasedifferencestooccurbetweenthetwopathsasafunctionoffrequency,temperature,orcomponenttolerances.,therefore,combiningchainsismoredifficultthancombiningsinglestagesandisusuallyavoided.,solidstateamplifiers(ssas),comparedwithtubes,solid-statedevicesoffermanyadvantages:,1.nohotcathodesarerequired;therefore,thereisnowarmupdelay,nowastedheaterpower,andvirtuallynolimitonoperatinglife.2.deviceoperationoccursatmuchlowervoltages;therefore,powersupplyvoltagesareontheorderofvoltsratherthankilovolts.thisavoidstheneedforlargespacings,oilfilling,orencapsulation,thussavingsizeandweightandleadingtohigherreliabilityofthepowersuppliesaswellasofthemicrowavepoweramplifiersthemselves.3.transmittersdesignedwithsolid-statedevicesexhibitimprovedmeantimebetweenfailures(mtbf)incomparisonwithtube-typetransmitters.modulemtbfsgreaterthan100,000hhavebeenmeasured.,4.nopulsemodulatorisrequired.solid-statemicrowavedevicesforradargenerallyoperateclass-c,whichisself-pulsingastherfdriveisturnedonandoff.5.gracefuldegradationofsystemperformanceoccurswhenmodulesfail.thisresultsbecausealargenumberofsolid-statedevicesmustbecombinedtoprovidethepowerforaradartransmitter,andtheyareeasilycombinedinwaysthatdegradegracefullywhenindividualunitsfail.6.extremelywidebandwidthcanberealized.whilehigh-powermicrowaveradartubescanachieve10to20percentbandwidth,solid-statetransmittermodulescanachieveupto50percentbandwidthormorewithgoodefficiency.7.flexibilitycanberealizedforphasedarrayapplications.forphasedarraysystems,anactivetransceivermodulecanbeassociatedwitheveryantennaelement.rfdistributionlossesthatnormallyoccurinatube-poweredsystembetweenapoint-sourcetubeamplifierandthefaceofthearrayarethuseliminated.,singlessamodulebroadbandwidth,lowpower,moderategain,lownoise,lowefficiencydevicessmallsize,lowcostmanufacturingprocessidealforuseasdriversforhighpowersourcestwobasictransistortypesbjtsandfetsbothareusedat3ghzforpoweramplifiersbutfetsdominateathigherfrequenciesbotharelimitedinfrequencybytransittimeeffectsthataresimilartothoseencounteredbyvacuumtriodesnewmaterialsgaasandganproducehighermobilitycarriersandhigherbreakdownvoltagetoextendtheperformanceenvelopofsolidstateamplifiers,mostradaroscillatorsoperateatpulsevoltagesbetween5and20kilovolts.theyrequirecurrentsofseveralamperesduringtheactualpulsewhichplacessevererequirementsonthemodulator.thefunctionofthehigh-vacuumtubemodulatoristoactasaswitchtoturnapulseonandoffatthetransmitterinresponsetoacontrolsignal.thebestdeviceforthispurposeisonewhichrequirestheleastsignalpowerforcontrolandallowsthetransferofpowerfromthetransmitterpowersourcetotheoscillatorwiththeleastloss.thepulsemodulatorcircuitsdiscussedinthissectionaretypicalpulsemodulatorsusedinradarequipment.,pulsemodulator,gas-filledtubes,insometubes,theairisremovedandreplacedwithaninertgasatareducedpressure.thegasesusedincludemercuryvapor,neon,argon,andnitrogen.,theyarecapableofcarryingmuchmorecurrentthanhigh-vacuumtubes,andtheytendtomaintainaconstantirdropacrosstheirterminalswithinalimitedrangeofcurrents.,theelectronstreamfromthehotcathodeencountersgasmoleculesonitswaytotheplate(ionization),iftheplatevoltageisverylow,thegas-filleddiodeactsalmostlikeanordinarydiodeexceptthattheelectronstreamisslowedtoacertainextentbythegasmolecules.,increaseplatevoltage(ionizationpoint)firingpotential,thevalueoftheplatevoltageatwhichionizationstopsiscalledthedeionizationpotential,orextinctionpotential,thyratrongas-tubemodulator,itconsistsofapowersource(ebb),acircuitforstoringenergy(l2,c2,c3,c4,andc5),acircuitfordischargingthestoragecircuit(v2),andapulsetransformer(t1).inadditionthiscircuithasadampingdiode(v1)topreventreverse-polaritysignalsfrombeingappliedtotheplateofv2whichcouldcausev2tobreakdown.withnotrigger