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1、ChelmsfordAmateurRadioSocietyAdvancedCourse TransmittersPart-2-Mixers&ModulationMixersinTransmittersNeedtorecallandexpandIntermediatecourseonmixersandmodulationinTransmitters.IngeneralVFOs,CrystalOscillatorsandSynthesisersdonotdirectlygeneratethefinalR
2、FOutputfrequency.Mixersareusedtocombinetwoormorefrequencysourcesaspartofthemodulationandup-conversionscheme.Needtounderstandthatspuriousoutputscanalsooccur,aswellasdeviationissuesonFM.MixersTwofrequenciescanbecombinedinamixercircuitResultisthecreationo
3、fsumanddifferencefrequenciesMixer10MHz+1MHz=11MHzand10MHz–1MHz=9MHz10MHz~1MHz~NOTE:Overdrivingamixer,underdrivingtheLOport,orpoorsuppressionofharmonicscanresultinotherunwantedspuriousmixerproducts.IfPSUisolationispoor,mainshumsidebandscanalsobeinadvert
4、entlyaddedMixerasaModulatorMixermaybeusedasanAMmodulatorAMhascarrierandtwosidebandsMostpowerisinthecarriersignalExcessiveaudiocausesover-modulationMixer1.401MHzUppersideband1.399MHzLowersideband1.400MHzCarrier1.4MHz~1kHz~DCoffsetunbalancesmixerandcause
5、scarriercomponent.DCoffsetBalancedMixersABalancedMixerisusedtocreateSSBasitwillnullifythecarriercomponenttoleavethetwosidebandsRFIsappliedtocentretapswhichresultsinnullnetcarrierAMcanbegeneratedbydeliberatelyunbalancingitAudioInputDoubleSidebandOutputR
6、FCarrierInputAM&SSBBandwidthSSBhasanumberofadvantagesNocarrier,sopowerisnotwastedHalfthebandwidthofAMNoRFpowerwithoutmodulatingaudioSmallerPSULessheat+300HzCarrierLowerSidebandUpperSideband-3kHz-300Hz+3kHzSSB:2.7kHzBWAM:6kHzBWCarrierandUnwantedSideband
7、issuppressedcomparedtonormalAM,reducingbandwidthFMModulatorColpittsOscillatorwithFrequencyModulationbyVaricapDiodeL1andC1setnominalfrequency,whichisvariedbyCDFMBandwidthismorecomplexassidebandsextendwiderthanAMC3470pFC4680pFC1150pFR2330R1100kTR1+9VC222
8、0pFOutputVaricapDiodeC522pFL110uHL2RFBlockAudioInDCBlockCDDCBiasTransmitterUpConversionSSBgenerationandothermodulationisperformedatloworintermediatefrequenciestoeasedesign,filteringetcThismustbeup-convertedtofinalRFfrequencybyanothermix
