DistributedAmplifierBasedVoltageControlled Oscillator.docx
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DistributedAmplifierBasedVoltageControlled Oscillator.docx
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DistributedAmplifierBasedVoltageControlledOscillator
DistributedAmplifierBasedVoltageControlledOscillator
TeamMembers:
TimMacShane
DaneStivers
ToddBeyer
MonicaStudnicki
Advisor:
DrPrasadN.Shastery
Fundedby:
DeptofECEBradleyUniversity
Abstract
Voltagecontrolledoscillators(VCOs)areanessentialtotheworldofcommunications.Theyworktoprovideflexibilityandstabilityforpublic,military,andspacecommunicationssuchascellularphones,radar,andsatellitecommunications.AsthebandwidthoffrequenciesthattheVCOcantunetoincreases,itsusefulnessincreases.Thereforeitisimperativetoachieveahightuningstandard.Distributedamplifiersareknownquitewellfortheirabilitytocreateahigh,steadygainoveralargeoperatingfrequencyrange.Themainobjectiveofthecapstoneprojecttitled“DistributedAmplifierBasedVoltageControlledOscillator”istostudythefeasibilityofusingadistributedamplifiertoincreasethebandwidthatwhichtheoverallsystemcanoperate.
TableofContents
TOPICS
Introduction
SystemSpecifications
SystemLayout
SubsystemDesign
∙∙ DistributedAmplifier
∙∙ PowerDivider
∙∙ GainBalancing
∙∙ TunableFilter
Inter-connectivity
ResultsAnalysis
ProposalforFutureResearch
Bibliography
AppendixA
AppendixB
AppendixC
AppendixD
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Introduction
Oscillatorsarethemostimportantcomponentsinmodernwirelesscommunicationsystems.Theyestablishtransmissionandreceptionfrequenciesforallwirelessdevicesincludingcellulartelephones,radios,TVs,andsatellites.Thereareseveraltypesofoscillatorsthatareusedincommercialandmilitarysystems.ThemostcommontypeofoscillatorusedtodayistheVoltageControlledOscillator(VCO).VCOsthataretunableoveranoctave[1]frequencyrangeareavailablecommercially.
Thedistributedamplifier(DA)isatypeofamplifierthatcanbedesignedtooperateoveranextremelywidefrequencyrange.ThefocusofthisresearchprojectistoexaminethefeasibilityofusingaDAasthebasisofaVCO.IftheDAprovestobeaviablebaseforaVCO,VCOsthataretunableoveradecade[2]couldthenbedeveloped.ADAbasedVCOhasnotyetbeensuccessfullyimplemented.
ThegoalofthisprojectistodesignaDAbasedVCOthatcanbetunedoversomespanoffrequencieswithintherangeof1to3GHz.TheVCOoutputpowermustbestableoveritsentiretuningrangeandshouldhaveanoutputimpedanceof50.Thetop-leveldesignoftheVCOisshowninFigure1.TheDAandtheTunableBandpassFilter(BPF)aresubsystemsthatarebeingdesignedspecificallyforthisapplication.APowerDividerandFeedbackAmplifierarebeingoutsourced.Theprojectisthereforedividedintotwosubsections.Eachsubsectionisthendividedintotwoparts,makingfoursectionstotal.TwomembersofthestudentteamareworkingontheBPFdesignandincorporatingtheFeedbackAmplifierintotheoverallsubsystem;theothertwomembersoftheteamareworkingontheDAdesignandincorporatingthePowerDividerintotheoverallsubsystem.Eachcomponentwillbetestedindependentlyandthenbeinterconnectedwithintheirspecificsubsystemfortesting.ThetwosubsystemswillthenbeconnectedonceagainfortestingwiththeultimategoalofbuildingtheentireVCOcircuitononemicrostriplayout.Thisreportwillcontaintheentiresystemspecifications,completesystemschematic,eachcomponentdesignandtestresults,theinterconnectivityresults,furtherworksuggestions,andsomecomparisonstopreviousworkdone.
SystemSpecifications
∙∙ OutputFrequencyRange:
1.8to2.5GHz
∙∙ OutputPower:
Stableoverwhole
frequencyrange
∙∙ OutputImpedance:
50
∙∙ TuningVoltage:
0to-30V
∙∙ BiasingVoltage:
20VDC
SystemLayout
Figure1:
SimplifiedSystemLayout
SubsystemDesign:
DistributedAmplifier:
TheconceptintheseniorcapstoneprojectistestingthefeasibilityofusingadistributedamplifiertoincreasethebandwidthatwhichtheVCOcantune.Distributedamplifiersareattractiveinaprojectsuchasthisbecausethemaintainhighgainwhileprovidingalargebandwidthofoperation.
Thedistributedamplifier(DA)worksbycascadingtransistorsconnectedtodrainandgatelinesmadeupofinductorsandcapacitors.Thecurrentwillinterfereinade-constructivewaytowardsthegateandcontructivelyinterfere,oradd,andproducepoweracrosstheoutputresistor.Byincreasingthenumberoftransistors,onecanincreasethecurrentthatisproduced,thusincreasingthepower.
BecausethemodelforatransmissioncanbeinterpretedasaT-modeloftwoinductors,eachofonehalfvalue,andacapacitor.Therefore,modelingthegateanddrainlineswithcomponentsgivestwoadvantages:
1.)1.) Firstitoffersmoreflexibilitybecauseonecantunethevaluesofthelinesaccordingtohis/herneeds.
2.)2.) Secondly,anythingthatismadeupofcomponentscanberedesignedasmicrostriplinesforeasyfabricationinsteadofsolderingtinycomponents.
TheDistributedAmplifierdiscussionwillbebrokendownintothestepstakentodesignandimplementtheDA.
∙∙ Componentsused
∙∙ K-derived
∙∙ M-derived
∙∙ Microstripimplementation
∙∙ Nonlinearmodel
ComponentsUsed
Thefirsttaskinthedesignofthedistributedamplifieristheresearchofthecomponentstobeusedinthedesign.AllofthecomponentsmustoperateovertheVCO’spassband.Sincethegate,drain,andterminationwillberealizedasmicrostriplines,thefocuswastowardfindingtheidealtransistortobecascaded,chokeinductors,andblockingcapacitors.
ThecapacitorsandinductorsbrandsandvalueswerealreadyknownsincesimilarprojectshavebeenconstructedintheRFlaboratoryatBradleyUniversity.A2400pFcapacitoristhestandardused(see
CapacitorSpecSheet,AppendixA).ThechokeinductorisalsoastandardfromCoilCraft(22nH).Theinductorandcapacitorareusedinthebiasingnetwork(InductorSpecSheet,AppendixA).
TheVCOwasintendedtotuneoverarangeof1to3GHz.Thereforethetransistorusedneedthesameoperatingfrequency.Aftermuchinvestigation,aCELbrandtransistorwaschosen.TheNE72218hastherequiredoperatingfrequency(seeNE72218SpecSheet,AppendixD).
Athighfrequencies,theinductoristheequivalenttoanopencircuit.ItwillnotbeandidealOCthough.ThereforethecapacitancewillbecomeashortcircuittoallowforaverylowresistanceroutetogroundfortheRFsignalsoitwillnotloadtheDCsourceused.
DCwillmaketheinductorashortcircuitsothecurrentwillbiasthetransistorandnotentertheRFsource.
K-DerivedDistributedAmplifier
ThechosenamplifiersmallsignalmodelisabletobesimulatedusingitsS-Parameters.Theseareavailableatmostproduct’shomepages(www.CEL.com).ThisisnecessarytoprovethattheactualsimulatedS-Parameterswillmatchwhatthespecsheetssayitwilldo.TheS-Parametersweredownloadedintoa*.s2pfileandturnedintoanS-ParameterblockonAgilentADS.Thetransistorwasablethentobesimulated.Theresultantoutputswereveryclosetothespecsheetsreadouts(SeeNE72218SpecSheet,AppendixD).
ThefirstmodelingofthedistributedamplifierhastobedoneinwhatiscalledK-Derivation.ThismodelincorporatesparasiticcapacitanceCpadintothedesignasaphysicalcapacitor.Thisisdonesothatvaluesofinputcapacitanceandoutputcapacitancecanbecalculated.
Cpad+Cout=Cin
Fromthecapacitancefound,theequivalentinductanceofthegatelineinductance,drainlineinductance,andthefrequencycut-offcanthenbederived.
(LC)=50becausethedesirednormalizationis50standard
Lg=(50)^2*Cin
Ld=(50)^2*(Cout+Cpad)
Itisnecessarytousetheaveragevalueoftheinputandoutputcapacitance.ThisisobtainedbycreatingthenecessaryformulaeinAgilentADSandsimulatingovertheoperatingfrequencies.Figure2andFigure3showtheADScomputationstoacquireCinandCoutaveragesfromtheY-Parameters.
Fromtheaveragevaluecomputations:
Cin=7.790e-13F
Cout=4.011e-13F
Cpad=3.779e-13F
Lg=Ld=1.9475nH
Figure2.Frequencyv.Y-Parameters
Figure3.Frequencyv.Capacitance
Itisnecessarytominimizephaseshiftingwithinthedevicesothephaseoftheoutputisasclosetotheinputaspossible.Capacitanceisknownforcausingphaseshift.Ifanequivalentinductanceisputintoplace,itwilleliminatetheeffect.Thereforewhenthegateanddrainterminationsarecreated,theoptimaldesignwouldtoincorporateam-derivedtermination.M-derivedterminationsmakeuseofequivalentinductance,notcapacitance.ItwillalsobeseenlaterthattheoptimalDAdesignwillalsobem-derived.Itisjustnecessaryfortheinitialdesigntobek-derivedbecauseitallowsthevaluesofthegateanddrainlineinductancetobecalculated.Figure4showstheADSterminations.
Figure4.M-DerivedTermination
Cterm=(m*C)2
Lterm1=(m*l)2
Lterm2=[(1–m^2)*L]2m
Wheremistheconstantofderivation.Theoptimalvalueformwasstatedas0.6.
Nowwiththevaluesofallcomponentsneededforthedesign,thequestionofgaincomestomind.Theprojectwouldneedmoderategain,maximallystable,overthepassband.Inordertoseetheresultantgain,say,withthreeamplifierscascaded,Agil
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