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1、Maxim>AppNotes>Wireless,RF,andCableKeywords:smithchart,RF,impedancematching,transmissionlineJul22,2002APPLICATIONNOTE742ImpedanceMatchingandtheSmithChart:TheFundamentalsAbstract:TutorialonRFimpedancematchingusingtheSmithchart.Examplesareshownplottingreflectioncoefficients,impedancesandadmittanc
2、es.AsamplematchingnetworkoftheMAX2472isdesignedat900MHzusinggraphicalmethods.Triedandtrue,theSmithchartisstillthebasictoolfordeterminingtransmission-lineimpedances.WhendealingwiththepracticalimplementationofRFapplications,therearealwayssomenightmarishtasks.Oneistheneedtomatchthedifferentimpedan
3、cesoftheinterconnectedblocks.Typicallytheseincludetheantennatothelow-noiseamplifier(LNA),power-amplifieroutput(RFOUT)totheantenna,andLNA/VCOoutputtomixerinputs.Thematchingtaskisrequiredforapropertransferofsignalandenergyfroma"source"toa"load."Athighradiofrequencies,thespuriouselements(likewirei
4、nductances,interlayercapacitances,andconductorresistances)haveasignificantyetunpredictableimpactonthematchingnetwork.Aboveafewtensofmegahertz,theoreticalcalculationsandsimulationsareofteninsufficient.In-situRFlabmeasurements,alongwithtuningwork,havetobeconsideredfordeterminingtheproperfinalvalu
5、es.Thecomputationalvaluesarerequiredtosetupthetypeofstructureandtargetcomponentvalues.Therearemanywaystodoimpedancematching,including:●Computersimulations:Complexbutsimpletouse,assuchsimulatorsarededicatedtodifferingdesignfunctionsandnottoimpedancematching.Designershavetobefamiliarwiththemultip
6、ledatainputsthatneedtobeenteredandthecorrectformats.Theyalsoneedtheexpertisetofindtheusefuldataamongthetonsofresultscomingout.Inaddition,circuit-simulationsoftwareisnotpre-installedoncomputers,unlesstheyarededicatedtosuchanapplication.●Manualcomputations:Tediousduetothelength("kilometric")ofthe
7、equationsandthecomplexnatureofthenumberstobemanipulated.●Instinct:ThiscanbeacquiredonlyafteronehasdevotedmanyyearstotheRFindustry.Inshort,thisisforthesuper-specialist.●Smithchart:Uponwhichthisarticleconcentrates.Theprimaryobjectiv
