withResistorTailBiasing
SergioGagliologagliolo@dibe.unige.it
∗
GiacomoPruzzopruzzo@dibe.unige.it
†
DanieleD.Cavigliacaviglia@dibe.unige.it
‡
BiophysicalandElectronicEngineeringDepartment-DIBE
UniversityofGenoaViaOperaPia11aI-16145
Genoa,Italy
tel.:+390103532788fax.:+390103532777
ABSTRACT
TheVoltageControlledOscillator(VCO)isafundamentalblockinRFICarchitectures.Today’swirelesscommuni-cationapplicationsdorequireahighlevelofperformancesfromsuchacircuit,andspecificallyitsphasenoisefigureanditspowerconsumption.Infact,modernstandardsof-tendemandforphasenoiselevelbetterthan-95dBc/Hzat100KHzinthevastmajorityofcases,withsupplyvoltagesapproachingthe1Vrange.
Thispaperpresentsthedesignchallengesofacross-cou-pled1.8GHzCMOSVoltageControlledOscillatorwithatuningrangeof7%,andaphasenoisefigureof-113dBc/Hzatanoffsetfrequencyof100KHz.Itemploysaresistorforbiasing,avoidinginthiswaythecommontailcurrentsourcebasedonactivecircuitry(e.g.currentmirrorsinCMOSdesigns).Thischoicepreventsthe1/fdevicenoiseupconversion,leadingtoanimprovedspectralpurity.
Sincephasenoisealsovarieswiththereciprocalofthetailcurrent,atrade-offcanbeestablishedbetweennoiseperformancesandpowerconsumptionbysimplychangingthebiasingresistor.ThesamecircuittopologymaythusbeusefulforbuildingVCOswhoseapplicationsrangefromhighperformanceWirelessstandardswhereanextremelylowphasenoiseismandatory,tolow-costportablesystemswherethereducedpowerdrainisofmajorconcern.
taCommunicationsDevices—Transmitters;B.7.m[Inte-gratedCircuits]:[Miscellaneous];C.2.1[Computer-CommunicationNetworks]:NetworkArchitectureandDesign—Wirelesscommunication
GeneralTerms
DesignPerformanceTheory
Keywords
VoltageControlledOscillator(VCO),PhaseNoise,ResistorBiasing,CMOS,LowPower,LowVoltage,Wireless
1.INTRODUCTION
TheevergrowingdemandforWirelesssystemswithhigh-erspectralefficiencies,lowerpowerconsumptionsandevenlowercosts,pushesforanintenseactivityinthefieldofintegratedRFcircuits.
OneofthemainblocksinRFintegratedarchitecturesisthefrequencysynthesizer,usuallybasedonaVoltageCon-trolledOscillator(VCO)anditsassociatedanaloganddigi-talcircuitries.Theneedforthehighestlevelofperformancesfromtoday’sWirelessapplications,oftenrequiresaVCO’sphasenoisefigurebetterthan-95dBc/Hzat100KHzfromthecarrier.
DuetothegoodphasenoisebasicperformancesandeaseofimplementationintheCMOSprocess,LCoscillatorswithdifferentialandcross-coupledtopologyareamongthemorefrequentlyusedcircuitalprimitives[11].Ifasinglediffer-entialpairisusedtosustaintheoscillationandasuitabletankcircuitwithaQvaluehigherthan5isavailable,aphasenoisefigureaslowas-90dBc/Hzatanoffsetof100KHzfromacarrierintheGHzrangecaneasilybeobtained.Insuchstructures,however,notonlythe1/fnoiseofthecross-coupledpairdictatesforthefinalbehaviourduetotheupconversionmechanismofsuchanoisesource[10,3,6,1,7,2],butalsothetailcurrentsourceplaysanimportantroleinthespectralpurityaspects.Infact,currentmirrorsusedforbiasingpurposescanintroducenoisedegradation-closetothecarrier,soloweringtheattainableperformancesandthegeneraloverallspectralefficiency.
CategoriesandSubjectDescriptors
B.4.1[Input/OutputandDataCommunications]:Da-∗IEEEStudentMember†Ph.D,AffiliatedResearcher
‡IEEEMember,FullProfessor
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SBCCI’05,September4–7,2005,Florian´opolis,Brazil.Copyright2005ACM1-59593-174-0/05/0009...$5.00.
Inthispaper,thepossibleuseofatailresistorasabi-asingdeviceforlow-voltagelow-poweroscillatorshasbeencarefullyinvestigatedandthetrade-offbetweenthecurrentconsumptionandthephasenoiseperformanceisreported.Usingthetailresistorbiasing,phasenoisefiguresbetterthan-113dBc/Hzatanoffsetof100KHzhavebeenreachedfromVCOsworkingfrom1.8GHzto2.5GHz,withato-talpowerconsumptionaround10mW.Thelatterfigurecanbefurtherreducedforportableapplicationsstillretain-ingaphasenoisefigureacceptableformanylow-gradelow-coststandardssuchasBluetooth(IEEE802.15.1)orZigbee(IEEE802.15.4);insuchcases,retainingthesametopologywhilejustscalingupthetailbiasingresistor,apowercon-sumptionof600μWstillallowstoobtainaphasenoiseof-87dBc/Hzat100KHz.
2.BASICCIRCUITDESCRIPTION
Thebasiccross-coupleddifferentialLCoscillatorisshowninFig.1.
VddVddLVCOUT1VOUT2
IbiasM1AM2M3M4Figure1:ThedifferentialLCoscillatorwithcon-stantcurrenttailgenerator
ThedifferentialpairM1,M2iscommonlybiasedwiththeaidofthecurrentmirrorM3,M4,keepingthesourcenodeAvirtuallyinsulatedfromground(atleastforthesig-nal),andallowingthedifferentialbehaviouroftheswitchingtransistors.TheCMOSimplementationishighlyattractivesincenocouplingdevicesatallarerequiredtoconnectthetwodrainstotheoppositegates,thussimplifyingthecircuittopologyincomparisontothebipolarversion.Insodoingbiasingisentirelycontrolledbythecurrentsourcesasstatedbefore,aswellasbythevoltagesupplyVdd.Thelatterisusuallyfixed,butcanbecontrolledbyasuitableservo-loopinordertoachievealevelledoutput.
Theoutputvoltageisavailableateachdrainandinordertokeepthebestwaveformsymmetryandthebestefficien-cy(alongwithcircuitbalancing),itisusuallydrawninadifferentialwayviasuitablefollowers.Theuseofabufferstageismandatorytoreducethetank’sloadasmuchaspossible,whilereducingthefrequencypullinginoscillatorswherefrequencytuningisaccomplishedbyvaractordiodesplacedacrosstheresonatorinductor.
ItiswellknownthatthePhaseNoiseL(ω)ofsucharchi-tecturescloselyfollowstheso-calledLeesonequation.[10,6,7].Amodifiedformofsuchanexpressioncanbewrittenasfollows:
L(ω)=F·4kTR„ω0
«2
V22QΔω
(1)
whereVistheRMSvoltageacrosstheLCresonator,Δωisthefrequencydisplacementfromthefundamentalfrequen-cyandQisthequalityfactorofthetankcircuit.
IntheoriginalLeeson’sformulation,Fisanalmostem-piricalfactorthatisusuallyreferredastheOscillator’sNoiseFactor.ForaCMOSoscillator,theFtermcanbewrittenasaweightedsumofaconstantunityvalueplustwonoisefactors,onedependingonthedifferentialpairnoisecontri-butionwhiletheotherproportionaltothecurrentsourcenoise.Inarecentpaper[5]thenoisetermFhasbeenex-pressedas:
F=1+
K1γIbiasR
V
+K2γgmbiasR
(2)
whereK1andK2areconstants,γisaFETnoisefactor
(equalto2/3forlongchanneldevices),Ristheresonatorequivalentresistanceandgmbiasreferstothecurrentsourcetransistortransconductance.Eq.1holdswellifweconsiderΔω0largerthanthe1/fcornerfrequencyofthechosende-vices(thismeansthatitonlyappliesatnotlessthanafewKHzawayfromthecarrier).Itclearlystatesthatthenoisedependsonthereciprocalofthesquaredsignalamplitude,alongwiththeQqualityfactoroftheLCresonator.ThisisthereasonwhytheusualdesignpracticestrytomaximizeboththeavailablevoltageamplitudeandtheresonatorQinordertoachievethebestnoiseperformances.Insodoing,alimitishoweverreachedastheinstantaneousdrainvoltageapproachesthebreakdownvalue,associatedtothechosentechnology.Foratypical0.18μmprocess,thebreakdownvoltageofslightlylessthan4Vlimitstheoscillatingvoltageatnomorethan4−4.5VppacrosstheLCresonatorwithapowersupplyof1.8V.Duetothecontinuedownscalingofdimensions,thismarginwilldecreasefurthermoreinthefuture.
FromEq.1aphasenoisefigureisalsofoundthatdependsonthereciprocalofthebiascurrentvaluesquared,being:
L(ω)∝I2
−1
bias
(3)
soagoodnoiseperformancedictatesforcurrentsinthemArange.Irrespectiveoftheotherfactors,thistrade-offbetweenpowerconsumptionandphasenoiseholdswellforthestatedtopologyandenablethedesignertofurtherre-ducephaseuncertaintyincreasingthebiascurrentuntilanoptimumpointisusuallyreached.
WitharesonatorQrangingfrom5to10[8,9,2],theCMOSVCOsphasenoiseperformancesreportedinthelit-eraturearecloseto-95dBc/Hzat100KHz.Consequently,withthesametechnology,afurtherimprovementinspectralpuritycanbereachedonlythoughabettercontrolovertheFfactorifphasenoisehastodropunderthe-100dBc/Hzthresholdat100KHz.
Besidethecorrectdesignofthecross-coupledoscillatorcell,thecurrentsourcecanseriouslyimpairthephasenoisespecificationoftheentirecircuit.IncaseofaNMOScurrentmirrorbasedsource,ahighcurrentgain(mirroringratio)isusuallychosentolowertheoverallpowerconsumption,butEq.2demandsfordeviceswiththelowestgm(lowW/Lratio).
Evenifasuitablecompromiseischosen,thesmallestprac-ticaltransistorstillexhibitsanexcessivedraintogateca-pacitancethatinjectsinthegatelinethedouble-frequencysignalpresentatthesourcenodeA.Abypasscapacitorinthegatelineprovestobeuselesshereduetotheinjectionof
theabovesignalinthecommonreference,furtherimpairingtheACisolationpropertiesofthecurrentgeneratoritselfduetothecapacitivepathfromnodeAtoground.
TheinsertionofasmallresistorinserieswiththesourceofM4mayalsogiveanaid,providingsomelocaldegenerationandthusloweringthegm,butevensothegaininphasenoiseislimitedtoafewdB.Someauthors[1,4]suggestedtheusageofahighvalueinductorinserieswiththecurrentsourceinordertoisolatethetwocircuits,thusobtainingacompositecurrentgeneratorthatinsuresbothgoodACandDCperformances.Insuchanapproach,however,the1/fnoiseofthetailtransistorremainsunfilteredatthesourcenode,whilethewastingofareaforthenewinductormaybesensible.Moreover,incaseofhighdensityRFdesigns,themagneticcouplingwiththisnewinductor(anditsfieldat2ω0)canleadtoinstabilityandspuriousbeatings.
Afeasiblesolutionistodispenseoftheactivecurrentgen-eratoranditsnoise,thusreducingtheFfactorexpression2tothefirsttwotermsonly.
Toavoidthenoisecontributionoftheactivecurrentsourceatallwhiledispensingofanewcoil,alowvalueresistorhasbeenproventobeapracticalbiasingmethodforacross-coupledoscillators.Thelowthermalnoiseintroducedbyaresistancelowerthan1,000Ohmshasnearlynoinfluenceintheoscillator’sspectralpurity.
Theproposedcircuitforalowphase,low-powernoiseCMOSoscillatorthusbecomes(Fig.2):
VddLVOUT1CVOUT2
M1AM2RIBiasFigure2:TheCMOScross-coupledLCoscillatorwiththetailresistorR
Thedifferentialoperationisnowdeteriorated,andtheintrinsicsupplynoiserejectionofthebasicarchitectureispartiallylost.Moreprecisely,dependingonthebiascurrentandhencetheRvalue(ameanvoltageof200mVatnodeAwillworkinthemostcases,leadingtoresistancevaluerangingfrom20Ωto1500Ω),thechosentopologywillruntheoscillatorpairM1,M2fromacross-coupledoperationtoadifferentialone.Thelowestthebiasingresistor,thelowestthesupplyrejectionfortheremovingofcurrentmirroritself.Itisworthnotingthatitispossibletoprovideasufficientlynoisefreesupplythroughawidep-channelseriesregulatorandasufficientlylargebypasscapacitorattheVddline.Thisway,thephasenoiseperformanceshowsadefiniteadvanceovertheactivecurrentsourceofFig.1.Thisbecomesmuchmoreevidentintheoscillatorcircuitvarietywherethetankcapacitanceisafixedhighqualityone(asinthecaseofMIMRFcapacitors)andnocontributiontototalphaseun-certaintycomesfromVaractor’samplitudetocapacitance
variations.InawelldesignedVCOwithatuningrangeof5-10%,however,theimprovementisquiteremarkableoverthecommonarrangementofthecircuitinFig.2,especiallyatverylowvoltageoperations.
3.DESIGNEXAMPLE
ThecompleteoscillatordiagramwithresistorbiasingisthusshowninFig.3.Thecircuithasbeendesignedandsim-ulatedusingtheBSIM3v3RFparametersforthe0.18μm,6metallevels,mixed-modeprocessavailablefromTSMC.
VddLVC1OUT1VCOUT22MOSMOSvar1128128var20.180.18MSupply1AM2BypassRIBiasVtuneR = 7kFigure3:TheMOSlowphasenoiseLCoscillatorwithresistortailgenerator
Theresonatorexhibitsa5.28nH,high-Q(10-11)inductorandtwofixedMIMcapacitorsC20.5μm).Theinductoris1,Cacentre2of400fFeach(20.5μmxtappedoctagonalspiralwithadiameterofabout80μm.Thetwocapacitorsareneededtomaintainbalancingoflossesonthesubstrateandtheyalsoactlimitingthetuningrangeforanimprovedoverallnoise;infact,areducedKVCOisoftenadesirablefeatureinlownoiseoscillators.
WiththegivenvalueofthevaractorsMOSVar1andMOSVar2,a10%maximumtuningrangehasbeenreached;ifneeded,a25%tuningrangemaybeprovidedbyslightlyloweringC1,C2anddoublingthevaractor’sunits,butinthatcasethephasenoiseshowsanincreaseofabout2dB.A60μmx20μmbypasssupplycapacitorof25pFkeepstheoscillator’scurrentwithintheloopwhileminimizingtheinjectingofthedoublefrequencycurrentofnodeAintothesupplyandgroundlines.Vddisfixedat1.6V.
4.SIMULATIONRESULTS
UsingthetopologyshowninFig.3withdifferenttailbiasresistors,severalphasenoisesimulationshavebeencarriedoutinordertoinvestigatethespectralpurityperformances.TheCadenceSpectreRFsimulationenvironmenthasbeenadopted.Itusesaperiodicsteady-stateanalysistodeter-minetheresponseofautonomouscircuits;oncetheperiodicsteady-statesolutionisdetermined,thephasenoiseiscom-putedthroughlinearizationofeverytimestepintheperiod.Nonlinearmodelingofthenoiseprocessesissupported,in-cludingtheflickernoiseupconversion.
Figure4:PhasenoiseplotofaCMOScross-coupledoscillatorwithtailresistorincomparisonwithcur-rentmirrorbiasing.BestoverallperformancesareobtainedwithR=85Ω,Ibias=6.5mA
ThephasenoisesimulationofFig.4reportsafigureof-113dBc/Hzat100KHzforthe1.8GHzoscillatorwithatailcurrentresistorof85Ωincomparisonwithanactive(1:4currentmirror)commonsolution;currentis6.4mAat1.6Vsupply.Totalpowerconsumptionis11mW.
Furtherpushingthebiascurrenttothe18mAasinFig.5enablesafurtherdecreaseof4dBat10KHzand2dBat100KHz;sincetheoptimumoperatingpointforthecross-coupledpairisfoundatlowercurrentandtheincreasedtankvoltageexcursionnowdeeplyexcitesvaractorsnonlinearities(whiletheRresistorismadesolittlethatthedifferentialbehaviourisnolongervalid),thetrade-offbetweenbiasingcurrentislessthanpredictedfromEq.2.Ontheotherhand,ifRissetat1350Ω,phasenoiseat100KHzstillcomparefavourablywithmostcommonCMOSVCOs,atareducedcost,areaandinputpower.
Figure5:PhasenoiseplotofCMOScross-coupledoscillatorwithtailresistorbiasing.F0=1.8GHz,low-powerlow-currentversionwithR=1350Ω;highcurrentversionwithR=25Ω,Ibias=18mA
Thephasenoisebehaviourversusthetailcurrentatsev-
Phase Noise Vs. Current-87.5-90-92.5zH-95/cBd-97.5 esio-100N es-102.5ahP-105-107.5-110-112.5-1150.41.42.43.44.45.46.47.48.49.410.4Tail Current mAFigure6:PhasenoiseversustailcurrentbiasIbiasatΔf=100KHz,f0=1.8GHz.Tailbiasresistorrangesfrom1350Ωto25Ω
eraldifferentpointshasbeenreportedonFig.6;thebestoverallperformancesarereachedatabout6.4-6.5mA,be-forethephasenoisecharacteristicflattensout.Thetailbiasresistorrangesfrom1350Ωfor400μAto85Ωfor6.4mA.ItisworthtonotethatincreasingtheRvaluethebehaviourofthetailresistorapproximatesacurrentsource;insodo-ingthecircuitexhibitslessvariationswithsupplychangesanditsnoiseperformancesonlyslightlychangewithsupplyvoltageintherange1.2-1.6V.
At1.2Vthetotalpowercanbefurtherreducedat500μWwithnegligiblelossofperformancesat100Khz.
Tuningrangeandfrequencyvs.Vtunecontrolvoltagelin-earityareshowninFig.7.
Figure7:Frequencyvs.Vtunecontrolvoltage
5.CONCLUSION
Alow-power,lowphasenoisedesignofafullyintegrated1.8GHzCMOScross-coupledLCVCOispresented.NMOS
inn-wellvaractorsareusedtorealizethetunablecapaci-tance,whileatailresistorisusedtobiasthecross-coupledpair.Inthiswaythe1/fdevicenoisetypicalofMOScur-rentmirrorsisnolongerpresent,thusenablingustolowerthephasenoiseclosetothecarrier.
Simulationresultshaveshownexcellentphasenoiseper-formanceoverotherrecentlyresultsreportedinthelitera-ture.
Acknowledgment
TheauthorsacknowledgewithgratitudeSeleniaComuni-cationSpaandAccentSrlforprovidingsupportforthisactivity.
6.REFERENCES
[1]P.AndreaniandH.Sjoland.Tailcurrentnoise
suppressioninRFCMOSVCOs.IEEEJ.Solid-StateCircuits37(3):342–348,March2002.
[2]A.HajimiriandD.Ham.Conceptsandmethodsin
optimizationofintegratedLCVCOs.IEEEJ.Solid-StateCircuits36(6):896–909,June2001.[3]A.HajimiriandT.Lee.DesignissuesinCMOS
differentialLCoscillators.IEEEJ.Solid-StateCircuits34(5):717–724,May1999.
[4]E.Hegazi,H.Sjoland,andA.Abidi.Afiltering
techniquetolowerLCoscillatorphasenoise.IEEEJ.Solid-StateCircuits36(12):1921–1930,Dec2001.
[5]P.W.Lai,L.Dobos,andS.Long.A2.4GHzSiGe
lowphase-noiseVCOusingonchiptappedinductor.InEuropeanSolid-StateCircuitsConference,pages505–508,2003.
[6]T.H.Lee.TheDesignofCMOSRadio-frequency
IntegratedCircuits.CambridgeUniversityPress,seccondedition,2004.
[7]M.McCorquodale,M.K.Ding,andR.Brown.Study
andsimulationofCMOSLCoscillatorphasenoiseandjitter.InInternationalSymposiumoncircuitsandsystems,pages665–568,2003.
[8]A.NiknejadandR.Meyer.Analysis,design,and
optimizationofspiralinductorsandtransformersforSiRFICs.IEEEJ.Solid-StateCircuits33(10):1470–1481,Oct1998.
[9]A.NiknejadandR.Meyer.Design,Simulationand
ApplicationsofInductorsandTransformersforSiRFICs.KluwerAcademicPublishers,firstedition,2000.[10]J.RaelandA.Abidi.Physicalprocessesofphasenoise
indifferentialLCoscillators.InProceedingofCustomsIntegratedCircuitsConference,pages569–572,2000.[11]B.Razavi.RFMicroelectronics.PrenticeHall
CommunicationsEngineeringandEmerging
TechnologySeries.PrenticeHallPTR,UpperSaddleRiver,NJ,firstedition,1998.
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