Today’s Topic: Component Circuit Modelshscdlab/pages/courses/microwaves/EE40458_… · Today’s...
Transcript of Today’s Topic: Component Circuit Modelshscdlab/pages/courses/microwaves/EE40458_… · Today’s...
Today’sTopic:ComponentCircuitModels
• Recall:– Keydifferencebetweenmicrowave/RFdesignand“regularold”circuitdesigniswhetherfinitepropagaConvelocityoflightissignificantornot
• Roughidea:– IfpropagaCondelayincomponentismuchsmallerthanthesignalperiod,then“regularold”approachprobablyok—instantaneousresponse
– Ifnot,thenwhat?– Andcanwebemoreprecise?
DelayEffects
• Acouplecircuitstoconsider:
• Thebasics:KVL,KCL:– KCL:currententeringandleavinganodeisequal– KCL:voltagearoundaloopiszero
• “regularold”designsuggeststhesetwoarethesame:Vout(t)=Vin(t)*R2/(R1+R2)
• Isthistruefortheoneontheright?– Yes?No?Maybe?SomeCmes?!SomeCmes.
DelayEffects
• What’sthedifference?– ChangesinVin“takeCme”toshowupatVout– Takes(atleast) ℓ/csecondsforsignaltogofromAtoB(assumingpropagaConatc—aswe’llsee,propagaConiso^enslowerthanthis)
– Socurrents,voltagesatAandBnotequal• Thisisonewaytolookattheoriginofinductancefromnon-magneCccircuits
– But“regularold”circuitdesigntreatsA&Basasinglenode—uhoh
DelayEffects–InPictures• LookatinputstepfuncCon:
– Note:propagaConisn’tone-way;signalreachesend,“reacts”toKCL/KVLatnodeB,andthisnewdisturbancepropagatesbacktosource.CanhavemulCplebounces
– Delayincurrent—lookslikeinductance
So:LayoutMaders
• Fromtheseideas,canseethatgeometriclayoutofacircuitcanmader—verydifferentfrom“normal”– Iffrequencyhighenough,orℓlongenough,signalat“A”verydifferentthanat“B”duetothedelay,andbouncesbackandforthtoreachsteady-state
• “Timedomainpicture”—focusedondelay.Easytounderstand,butrarelyusedinmicrowavedesign
" Instead,we’llmostlyworkinthefrequencydomain(phasors,complexV,I)
• Timedelayisembeddedinthephase-frequencyrelaCons
FrequencyDomainEffects• MostcommunicaConsystemssendinformaConusinga
carriersignalwithmodulaCon– ToroughapproximaCon,signallookslikeasinewave,withsome
wigglesadded– Afewpictures:
– Ifvelocity=v,Cmedelay=ℓ/v;wavelengthλ=v/f
FrequencyDomainEffects
• SosignalissCllasinewave—justhasdifferentphase(andamplitude)atdifferentposiCons– Delaywillgetsubsumedintothisdependence
• Givesrisetosomerulesofthumbaboutcomponentsizesandfrequencies– Acommonone:ifℓ<λ/10,cansCllused“lumpedelement”circuitdesigntechniques
– OK—butsomecareisrequiredtounderstandwhatthisreallymeans(andwhatitdoesn’t—misusedalot…)
– Example:considercomponentoflengthℓ=λ/10.End-to-endphasedifference?• Δ Phase=2πℓ/λ(rad)or360ℓ/λ(deg)• 36°!Notsosmall,certainlynotnegligible
CircuitElementModels• Soevenifℓ<λ/10,clearly“regularold”circuitdesignisnotok;
36°phasechangesareimportant• Ifℓ>λ/10,clearlythingsareevenfurtheroff• Whatcanwedo?Switchdevicemodels.Threelevels:
– Lowfrequency(ℓ<λ/100):phasedifferencesnegligible:20242OK– “lumped”circuitmodels(λ/100<ℓ<λ/10):canusemorecomplicated
models,butkeepKVL,KCLaround– Distributedcircuitmodels(ℓ>λ/10):needsomethingelse—spaCal-
dependentcircuittheory• Sowhataboutthatλ/10ruleofthumb?
– Itdoesn’tmeanyoucanignoretheseeffects;meansyoucansimplifyhowyoutreatthem
– Don’tneedafullelectromagneCcsanalysis,can“patchup”regularcircuitdesignapproaches
– AlternaCveview:“moresophisCcated”circuitelementmodels(butitissCllreallyjustaband-aid)
CircuitModelsforComponents• Startwithworkhorsepassives:R,L,C• Lowfrequencyregime(ℓ<λ/100):
– Easy:justlikeEE20242:V=I*R,V=jωL*I,I=jωC*V– Nothingnew
• “lumpedelement”models(λ/100<ℓ<λ/10)– Phase/delayisimportant,needtoaugmentourtreatmenttocapture
that,butwouldlikeittobesimple– We’llworkupmodelsforcomponents
• Evenawireisn’tsosimple—notanidealshort– Idealshort:phasedelay=0;wireoflengthλ/10,phasedelay~36°
– Fix:modelasinductance.Empiricalformula(veryhandy…)
L(µH)=(0.002ℓ)ln(4h/d)– ℓ=length(incm),d=diameter,h=heightabovegroundplane
ShortWireL(µH)=(0.002ℓ)ln(4h/d)
– ℓ=length(incm),d=diameter,h=heightabovegroundplane• Anumericalexample:
– #22wire(likeforabreadboard):d=25.3mils=0.0643cm(aside:microwavepeopleuse“mils”alot;1mil=0.001”.Yes,inches)– h/dinrangefrom10/100(insideln,sonotsosensiCve)" L=7.4nH/cmto12nH/cm
• Doesthismader?nHseemssmall…• Putthisinacircuitcontext.Assumeh/d=100(12nH/cm)
– At10MHz:impedanceofwireisjωL=~j1Ω/cm– At100MHz:impedanceofwireis~j10Ω/cm– Dependingonwhattherestofthecircuitlookslike,thiscouldbe
nothing,oritcouldbeabigdeal(isitinserieswith25Ω?Or1000Ω?– Noteitcanstarttomaderatquitelowfrequencies(below100MHz)
OtherComponents:R,L,C• Butfirstsomevocabulary:
– Impedance,admidance,reactance,susceptance—besurewe’reallonthesamepage
• Z(impedance)=R(resistance)+jX(reactance)• Y(admidance)=G(conductance)+jB(susceptance)• Y=1/Z• Careful:
– G≠1/R,B≠1/X!– Probablyobviousifyouthinkitthrough,butsotempCng…
• Resistor:lumpedelementcircuitmodel
LumpedElementRExamples• Small-ishresistor:
– 50Ω– C=1pF– L=10nH
~5mmofwireoneachend)
– Realpartnotchangedmuch,butsignificantimaginarypart
SmallResistor—anotherlook• Sameresistor,same
data—but|Z|andangle
• Overallmagnitudestronglyaffected;significantphase
– Whatyouseedependsonwhatyoulookfor
LargeResistor• Largeresistor:
– 10kΩ– 1pF– 10nH
– Realpartfallsoffacliff,imaginaryparthasbignegaCvepeakatverylowfrequencies;bigresistorsdon’tworkwellatRF…
LargeResistor–anotherlook• Mag/angleviewo^en
easiertointerpret• |Z|fallingfromshuntC• Phase–>90°--capacitor
– Conclusion:bigresistorsdon’tworkwellatRF…
“Intermediate”RExample• Intermediateresistor:R=100Ω,C=1pF,L=10nH
• RealpartfallsmoredramaCcallythansmallR,lesssothanlarge
• Imaginarypartcomparabletorealpartathighfrequencies
“Intermediate”R–anotherlook• Intermediateresistor:R=100Ω,C=1pF,L=10nH
• Note:|Z|canbelargerorsmallerthanDCresistance• NotcapturedbyeitherapproximaCon—needfullmodel• LifeisnotsosimpleatRF…
Capacitors• Real-worldcapacitorsaren’tidealeither…
• Performance:C=0.01µF,L=20nH(1cmofwireateachend)
Capacitors• Notebigdip(headstozero—hugeholeonlogplot)and
abruptflipinphase
• Belowfs–reactance<0(likeC);abovefs,reactance>0(L!)• IdealC:X=-1/(ωC)–straight-linepartbelow~40MHzorso• Thisbehaviorcanbeaproblemorahelp—butyouhaveto
knowitisthere!
Inductors• InpracCce,inductorsareo^entheleastidealofcommon
passives.
• Performance:L=10µH,C=0.5pF,R=5Ω
• OK…X>0atverylowfrequencies(X=ωL),butnotveryideal
Inductors• Let’scompare:modelvs.idealL• Zoominonlow-freq.
range• PlotXforidealL
(X=ωL,L=10µH)andfullmodeltogether
• Matchesonlyatverylowfrequencies
• Bigpeak(inrealandimaginarypart;alsoin|Z|
• Sameformulaasfsforcapacitor,butverydifferentbehavior• BehaviorislousyifyouwantedX=ωL• GreatifyouwantaDCshortandRF“open”—calleda
“choke”;probablyactuallymoreuseful…
ImpactonCircuits?• So—doesanyofthismadermuch?A^erall,whatwereally
careaboutiswhetherthecircuitdoeswhatwewantornot• Example:RFlow-passfilter
• Simplepi-networkfilter,easilydesignedusingstandardfilter
synthesistoolsinCADpackages(ADS)• ValuescomputedautomaCcallyfromfilterspecificaCons
RealFilterPerformance• Includethefullmodel
foreachcomponent• ParasiCcstakenfrom
typicalsurface-mountvalues
• Um…thingsarenotsogood
RealFilterPerformance• Comparison:
• Passbandisnarrowerthanbefore—ifwewantedsignalsabove1GHztogetthrough,um…
• “Second”passbandat5GHzandabove—ifwewantedtoblocksignalsthere,weblewit
• What’swrong?
Recap:LumpedElementModels
• Havedeveloped“lumpedelement”equivalentcircuitmodelsfortypicalR,L,C,pluswire
• Reliesonℓ<λ/10,sonotapropertyonlyofthecomponent,butalsoofthesignals
• Sidenote:beverycauCousofvendorclaims.Theyaren’tlying,butyouneedtounderstandwhattheymean…lookatanexample:
• hdp://www.usmicrowaves.com/res/ceramic/alumina_ceramic_al2o3_99ghz_thin_film_chip_resistor_re1020t10.shtml
DatasheetDetails• Here’sthetempingpart:99.47GHz!Thatshouldbegreatformy
mm-wavecircuitat94GHz,right?• Here’stherealthing:
• 0.032pF!1/ωC=50Ωat99.47GHz.Oh.• So:at99.47GHz,Z≠50Ω.Z=50Ω||-j50Ω.|Z|=35.4Ω,
ang(Z)=-45°.Ooh.At50GHz?|Z|=44.7Ω,ang(Z)=-27°• Caveatemptor?Ofcourse…justdotheanalysisfirst,cuta
purchaseordersecond.Theydidn’thideanything…