DistributedAmplifierBasedVoltageControlled Oscillator.docx

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