第三講微帶電路

BasicsofmicrostripcircuitAddressedbyXinhuaYu11.1Thedevelopmentofthemicrostripline1.1.1ThetransmissionlineThetransmissionlineisusedtoguideelectromagneticwavetotravelinacertaindirectionandtoformmanykindsofmicrowaveelements,suchasfiltersanddividers.Itconsistsofasystemconstructedwithconductoranddielectric.Itscommonformsareparallellines,coax,waveguide,stripline,microstripandsoon.2Coaxialcable(coax)rectangularwaveguideparallellinesMicrostripCoplanarstripsFigure1.1Severaltransmissionstructuresstriplinecoplanarwaveguide(CPW)3Parallellinesaresuitabletotransmitlowfrequencysignalandunsuitablehighfrequencysignalduetolargeradiationlossesathighfrequencies.Thissituationresultsintheadventofcoaxandwaveguide.4Coaxandwaveguidearediscardedintheintegratedcircuitfortheirlargesizeandheavy!Soit’sthemicrostrip’sturntoworkintheintegratedcircuit.Themicrowaveintegratedcircuitcanbedividedtwotypes:

TheLumpedintegratedcircuit:dimensionfarsmallerthantheoperationalwavelength;Itselementsarealllumpedtype;1.1.2ThetransmissionlinetypeThetrendofminimizingthecommunicationsystemmadetheadventofmicrowaveintegratedcircuits.5

Thedistributedintegratedcircuit(referredtoasmicro-stripcircuit):Itsdimensionisatthesamelevelastheo-perationalwavelength;Itselementsconsistofdistribut-edparametermicrostrip;1.2MicrostripFigure1.2Configurationofamicrostripline6Figure1.3Conceptualevolutionofamicrostripfromatwo-wireline7Figure1.4Microstripconfiguration

TheconfigurationopenatthetopmakesamicrostripveryconvenientforuseinMICswherediscretelumpeddevices(activeorpassive)mustbemountedintheintegratedcircuit.8Figure1.5VarioustransmissionlinesderivedfromamicrostripInvertedmicrostripSuspendedmicrostripSlotmicrostripMicrostripwithdouble-layerdielectric91.2.1MicrostripfieldconfigurationamicrostripStructurecannotsupportapureTEMwave.Continuityofthetangentialcomponentoftheelectricfieldalongadielectric-airinterfaceyieldsDielectricsideoftheinterfaceAirsideoftheinterface(1.1)Figure1.6MicrostripconfigurationUsingMaxwell’sequations,onemaythuswrite10(1.2)Expanding(1.2)andusingthecontinuityofthenormalcomponentofthemagneticflux,weobtain(forthecasewhen)(1.3)

AsεrisnotequaltounityandHy≠0,(1.3)impliesthattheexpressionontheleft-handsideshouldbeanonzeroquantity,whichcanonlybetrueifHzisnonzero.ThuswenotethatforMaxwell’sequationstoholdgoodfortheconfigurationofFigure1.6,thelongitudinalcomponentofHmustexist.11

SimilarargumentscanbeadvancedtoshowthatEz,thelongitudinalcomponentoftheelectricfield,isalsoanonzeroquantity.ThefringingcomponentsExandHxatthedielectric-airinterfacethatleadtothenon-TEMnatureofthemicrostripmode.微波線中存在的是TE-TM混合波場，其縱向場分量主要是由介質(zhì)-空氣分界面處的邊緣場Ex和Hx引起的，由于微帶線基片厚度h遠小于傳輸波長，因此縱向場分量與導(dǎo)體帶和接地板之間的橫向分量場相比很小，所以微帶線中傳輸模的特性與TEM模相差很小，稱之為準(zhǔn)TEM模。xzFigure1.7Microstripfieldconfigurationy121.2.2MethodsofMicrostripAnalysis

Theanalysismethodsareaimedatdeterminingthecharacteristicimpedanceandpropagationconstant(phasevelocityandattenuationconstant).Figure1.8Variousmethodsofmicrostripanalysis

*pureTEMconsideredonly*Suitabletocircuitatlowerfrequencies(belowX-band)Hybridnatureofthemodeofpropagationconsidered13Quasi-staticmethodPureTEM

mode，introducehomogeneousmediumwitheffectivedielectricconstantεe

toreplacethehybridmediumofamicostrip.Modespropagatinginamicrostriparequasi-TEMmodeandhighordermodesincludingwaveguidemodeandsurfacewavemodeswhichmustbesuppressed.Figure1.9Fieldconfigurationofmicrostrip14wthFigure1.10FieldconfigurationofmicrostripFigure1.11MicrostripwithhomogenousdielectricMainparameter:thecharacteristicimpedanceandthephasevelocity15(t=0)(1-1)(1-2)1.2.3MicrostripAnalysisusingquasi-staticmethod16WhenTheeffectofnon-zerothicknesscanberepresentedbybroaderwidthWeundertheconditionWecanbedecidedbyWhere(1-3)(1-4)17Where

Whenandaregiven,thewidthWcanbegotby1.2.4Microstripdesignusingquasi-staticmethod(1-5)(1-6)18Table1-119

Example1:Getthewidthandlengthof

amicrostriptoresultinaphaseshiftof90degreesat2.5GHz.

Thesubstrate’sparametersareandthecharacteristicimpedanceofthemicrostripis50ohms.e0erFigure1.12MicrostripwithExample120Solution：Formicrostripwithacharacteristicimpedance50W，byconsultingTable1.2orcalculatingby1-2,3,4，t/h=0.01,Sothelengthofthe900

phaseshiftlinecanobtainbyThus,21例2試計算復(fù)合介質(zhì)微帶電路50歐姆傳輸線寬度W。已知εr=9，h=1mm，t=0.01mm。解：采用（1.3）式計算。A=2.057B=3.948W/h=1.06W=1.06mm22

Thecomparedwithwaveguidearethe

generallylowerpowerhandlingcapacity,andhigher

losses.Also,unlikewaveguide,microstripisnotenclosed,andisthereforesusceptible

tocross-talkandunintentionalradiation.

Microwavecomponentssuchasantennas,couplers,filters,powerdividersetc.canbeformedfrommicrostrip,theentiredeviceexistingasthepatternofmetallizationonthesubstrate.1.2.5Disadvantagesofmicrostrip1.2.6Applicationsofmicrostrip

Thelossesinclude

conductorloss,dielectriclossandradiationloss.23MicrostripantennaarrayinapplicationMicrostripantennaMicrostripantennaarrayRadiationpatt