全套电磁兼容资料9

1CableShieldingAgainstEFieldGroundedshieldU2NC1SCSGRU1Conductor1ShieldConductor2+-C2S2CableShieldingAgainstEFieldGroundedshieldEquivalentCircuitU1C1SCSGC2SShieldUN20=3CableShieldingAgainstEFieldPoorshield’sgroundconnection:resonancebetweenshieldcapacitanceCSGandparasiticinductanceLpofgroundconnectioncanactuallyincreasenoisevoltagepickupbytheinnerconductorU1C1SCSGC2SShield0UN2≠Lp4CableShieldingAgainstEFieldCenterconductorestendingbeyondshieldU2NC1SCSGU1Conductor1ShieldConductor2+-C2SC12C2G5CableShieldingAgainstEFieldCenterconductorestendingbeyondshieldC1SCSGU1C2SC12C2GS2G212121N2CCCCUU++=ThesameasunshieldedcableexceptforareducedvalueofmutualcapacitanceC12andanincreasedvalueoftotalcapacitancetogroundC2S+C2G6CableShieldingAgainstEFieldForgoodelectricfieldshielding,itisnecessarytominimizethelengthofthecenterconductorthatextendsbeyondtheshieldandtoprovideagoodgroundontheshield7CableShieldingAgainstHFieldUngroundednonmagneticshieldLM12I1RU1+R1LM1SR28CableShieldingAgainstHFieldUngroundednonmagneticshield:equivalentcircuitU2NLM12I1R+U1+R1LM1S-+-1S1MSNILjUω=R29CableShieldingAgainstHFieldShieldgroundedatbothendsI1R2RShieldcurrentcanflow,thusinducingasecondnoisevoltageoninnerconductorISU110CableShieldingAgainstHFieldU2NR+I1R2ShieldShieldgroundedatbothends-Idealshieldactsasashort-circuitedtransformersecondary11MagneticCouplingBetweenShieldandInnerConductorNofieldinsidecavity++++++++++MagneticfluxlinesCurrentflowingintoshieldTubularconductorHp:Uniformcurrentdistributionaroundthecirconference12MagneticCouplingBetweenShieldandInnerConductor++++++++++MagneticfluxlinesCurrentflowingintoshieldNofieldinsidecavityCenterconductorShieldandcenterconductorcouplewiththesamefluxLS=LM2S13MagneticCouplingBetweenShieldandInnerConductorNoisevoltageinducedininnerconductorduetoshieldcurrentISUSNRSLM2SU2SNLSSSSNSLjRUIω+=ω+ω=ω+ω=jLRjUULjRLjUUSSSNSN2SSS2MSNSN214MagneticCouplingBetweenShieldandInnerConductorSNSN2UUωSScLR=ω1ω+ω=ω+ω=jLRjUULjRLjUUSSSNSN2SSS2MSNSN215EffectofShieldonMagneticCouplingLM12I1RU1+R1LM1SR2LM2SISRSLS16EffectofShieldonMagneticCouplingI1RU1+R1R21S1MSNILjUω=NoisevoltageinducedonshieldLM1SUSN+17EffectofShieldonMagneticCouplingI1RU1+R1R2U21NLM12++112MN21ILjUω=Noisevoltageinducedonconductor2byconductor118EffectofShieldonMagneticCouplingSS2MSN2ILjUω=Noisevoltageinducedonconductor2byshieldcurrentI1RU1+R1R2U2SN++LM2S+19ShieldingEffectivenessTotalnoisevoltageinducedinshieldedconductor2(LM12=LM1S)ωSScLR=ω12MLjω1N2IUSS12MLRL1SS12MSN2N21N2IRLj1LjUUUω+ω=−=20ShieldingtoPreventMagneticRadiationShieldgroundedatoneendMagneticfield++++++++++ShieldgroundedatbothendsTopreventradiationfromaconductorgroundedatbothends,theconductorshouldbeshielded,andtheshieldshouldbegroundedatbothends21ShieldingtoPreventMagneticRadiationI1U1R1ISIGI1U1R1ISIGRSLSLMTheshieldcurrent,equalandoppositetotheconductorcurrent,producesafieldwhichcancelsthatcausedbytheconductorSS1SLjR1IIω+=22ShieldingAgainstMagneticFieldsKeypoint:reduceareaofthereceptorLoopI1U1RLIG=I1I1U1RLIG=I1I1U1RLIS=I1ShieldgroundedatbothendsatωRS/LS23ShieldingAgainstMagneticFieldsAtlowfrequenciestheshieldshouldnotbeoneofthesignalconductor,duetothevoltagedroponshieldresistanceRScausedbyshieldnoisecurrentISUin+-GroundpointatdifferentpotentialsRSLMLSISUin+-SSinIRU=24ExperimentalDataMagneticfieldshieldingofvariouscables(allshieldcutofffrequenciesarebelowthetestfrequencyof50kHz)PowerAmplifier50kHzoscillatorL1L21MΩ100Ω10Vpp+-L1=10turnsofAWG20wire9-indiameterL2=3turnsconcentricwithL17-indiameter25CircuitGroundedatBothEnds100Ω1MΩ•Referencestructurewithoutanymagneticfieldshielding.Thenoisemeasuredacrossthe1MΩresistorwas0.8V.Thiswastakenasreferencevalue(0dB)26CircuitGroundedatBothEndsShieldgroundedatoneand:att=0dB100Ω1MΩ•Nomagneticfieldshieldingprovided27CircuitGroundedatBothEndsShieldgroundedatbothends:att=27dB100Ω1MΩ•Somemagneticfieldprotectionbecausethefrequencytestisabovetheshieldcutofffrequency.Thegroundloopformedbytheshieldgroundedatbothendspicksupalargenoisecurrentinducedbythemagneticfield(noisevoltagedropintheshield)28CircuitGroundedatBothEndsTwistedpair(6turnsperfoot):att=13dB100Ω1MΩ•Magneticfieldreductionduetothereducedreceptorarea.Onceagain,thelargegroundloopcouplesnoiseinthecircuit29CircuitGroundedatBothEndsTwistedpairwithshieldgroundedatoneend:att=13dB1MΩ100Ω•Nofurtherattenuationprovidedbytheshieldgroundedatoneend,ascomparedtotheunshieldedtwistedpair30CircuitGroundedatBothEndsTwistedpairwithshieldgroundedatbothends:att=28dB100Ω1MΩ•Theshieldgroundedatbothendsdivertssomeofthemagneticallyinducedground-loopnoisecurrentawayfromthesignalconductors.31CircuitGroundedatOneEndShield:att=80dB1MΩ100Ω•Nogroundloop.Veryeffectiveduetothesmallreceptorarea(theshieldcanberepresentedbyanequivalentconductorlocatedonitscenteraxis)32CircuitGroundedatOneEndTwistedpair(6turnsperfoot):att=55dB100Ω1MΩ•Lowerattenuationprovidedascomparetotheshieldcase.Duetoelectricfieldcoupling.Moreturnsperfootgivelessmagneticpickup33CircuitGroundedatOneEndTwistedpairplusshieldgroundedatoneend:att=70dB100Ω1MΩ•Theshieldgroundedatoneend