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1、ChelmsfordAmateurRadioSocietyAdvancedCourse TransmittersPart-2-Mixers&ModulationMixersinTransmittersNeedtorecallandexpandIntermediatecourseonmixersandmodulationinTransmitters.IngeneralVFOs,CrystalOscillatorsandSynthesisersdonotdirectlygeneratethefinalRFOutputfrequency.Mixersareusedtocombinet
2、woormorefrequencysourcesaspartofthemodulationandup-conversionscheme.Needtounderstandthatspuriousoutputscanalsooccur,aswellasdeviationissuesonFM.MixersTwofrequenciescanbecombinedinamixercircuitResultisthecreationofsumanddifferencefrequenciesMixer10MHz+1MHz=11MHzand10MHz–1MHz=9MHz10MHz~1MHz~NO
3、TE:Overdrivingamixer,underdrivingtheLOport,orpoorsuppressionofharmonicscanresultinotherunwantedspuriousmixerproducts.IfPSUisolationispoor,mainshumsidebandscanalsobeinadvertentlyaddedMixerasaModulatorMixermaybeusedasanAMmodulatorAMhascarrierandtwosidebandsMostpowerisinthecarriersignalExcessiv
4、eaudiocausesover-modulationMixer1.401MHzUppersideband1.399MHzLowersideband1.400MHzCarrier1.4MHz~1kHz~DCoffsetunbalancesmixerandcausescarriercomponent.DCoffsetBalancedMixersABalancedMixerisusedtocreateSSBasitwillnullifythecarriercomponenttoleavethetwosidebandsRFIsappliedtocentretapswhichresul
5、tsinnullnetcarrierAMcanbegeneratedbydeliberatelyunbalancingitAudioInputDoubleSidebandOutputRFCarrierInputAM&SSBBandwidthSSBhasanumberofadvantagesNocarrier,sopowerisnotwastedHalfthebandwidthofAMNoRFpowerwithoutmodulatingaudioSmallerPSULessheat+300HzCarrierLowerSidebandUpperSideband-3kHz-300Hz
6、+3kHzSSB:2.7kHzBWAM:6kHzBWCarrierandUnwantedSidebandissuppressedcomparedtonormalAM,reducingbandwidthFMModulatorColpittsOscillatorwithFrequencyModulationbyVaricapDiodeL1andC1setnominalfrequency,whichisvariedbyCDFMBandwidthismorecomplexassidebandsextendwiderthanAMC3470pFC4680pFC1150pFR2330R110
7、0kTR1+9VC2220pFOutputVaricapDiodeC522pFL110uHL2RFBlockAudioInDCBlockCDDCBiasTransmitterUpConversionSSBgenerationandothermodulationisperformedatloworintermediatefrequenciestoeasedesign,filteringetcThismustbeup-convertedtofinalRFfrequencybyanothermix
