开关电容滤波器SCF的应用

A CMOS direct sampling mixer using Switched Capacitor Filter technique for software-defined radio

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ACMOSDirectSamplingMixerUsingSwitchedCapacitorFilterTechnique

forSoftware-De nedRadio

HongPhucNinh,TakashiMoue,TakashiKurashina,KenichiOkada,andAkiraMatsuzawa

DepartmentofPhysicalElectronics,TokyoInstituteofTechnology2-12-1-S3-27Ookayama,Meguro-ku,Tokyo152-8552Japan.Tel/Fax:+81-3-5734-3764,E-mail:phuc@ssc.pe.titech.ac.jp

Abstract—Thispaperproposesanoveldirectsamplingmixer(DSM)usingSwitchedCapacitorFilter(SCF)formulti-bandreceivers.TheproposedDSMhasahighergain,more exibilityandlower ickernoisethanthatofconventionalcircuits.ThemixerforDigitalTerrestrialTelevision(ISDB-T)1-segmentwasfabricatedina0.18μmCMOSprocess,andmeasuredresultsarepresentedforasamplingfrequencyof800MHz.Theex-perimentalresultsexhibit430kHzsignalbandwithwith27.3dBattenuationofadjacentinterfererassumingat3MHzo set.

I.Introduction

Nowadays,dozensofwirelesscommunicationstandards,e.g.GSM,UMTS,WLAN,Bluetooth,GPS,DTV,andRFID,havebeenusedinmobileterminals.Thusitindicatesaneedfora exiblemobileterminalthatcansupportmanywire-lesscommunicationstandards.However,thepresentmulti-standardterminalconsistsofseveralLNAs,VCOs,Mixers.Asaresult,itbecomesmorecomplicatedandneedslargerareaandpowercomsumption.Amulti-standardRFfront-endimplementedinasinglechipisrequiredforsmallersize,lowerpower[1].Software-de nedradio(SDR)isfocusedtosatisfytheserequirements.TheadoptionofMuli-tapDirectSamplingMixer(MTDSM),proposedrecentlybyR.B.Staszewski,etal[2],isoneofpromisingapproachesforSDR.However,theconventionalMTDSMconsistsofpassivecomponents,thuslargegainlossandloworderofthe lterareunavoidableproblems.Moreover,itisalittledi culttobeappliedtothewidebandwirelessapplications,e.g.WLAN,UWB,etc,duetonarrowpass-bandcharacteristicsofMTDSM.Inadditionthe ickernoiseisalsoatoughproblemoftheconventionalMTDSMduetothepassbandcharacteristics.Inthispaper,weproposeaMTDSMusingSwitchedCapacitorFilter(SCF),whichrealizesmoregainand icker-noisesupression.More-over,SCFintheproposedMTDSMcontributestoprovidehigher-order ltering,whichcanrealizegain atnessoverthein-band.

II.ProposedDSMArchitecture

Fig.1showstheproposedsamplingmixerarchitecture,whereRFinputsignalissampled, lteredanddecimated.Thismixerisbasedoncurrent-modedirectsampling,thusatransconductanceampli er(TA)isusedtoconvertthereceived

Fig.1.Theproposeddirectsamplingmixer.

RFvoltageintocurrentdomain.ThecurrentgetssampledbythelocaloscillatorandintegratedintothesamplingcapacitorCS.Duringthephaseφ11(orφ12),thesampledRFinputsareaccumulatedononeCS,whiletheaccumulatedchargeisreadouttothenextstagebyanotherCSinthesametime.Thus,itperformsaMoving-Average(MA)Filtering.Moreover,onecapacitorCRsharesthechargewithCH,a”history”capacitor,andanotherCRisreadout.ThetotalchargestoredinCHandCRatphaseφ11(orφ12)isequaltothesumoftheremainingchargeinthe”history”capacitorCHinthepreviousoneandtheaccumulatedchargeofthesamplingcapacitor.ThisoperationintroducesanIIRFiltering.ThesecondMAFilteringandIIRFilteringisrealizedwiththesamemethodaccordingtotheoppositeoperatingofphaseφ21andphaseφ22inthenextstage.

Intheconventionalsamplingmixer,DCandtheharmonicfrequencyatmultipleoflocaloscillatorwillalsobealiasedinwiththesampledsignal.ThustheDCcomponentbecomelargebecauseofthea ectofDCo setand1/fnoise.AsshowninFig.1,RFinputsignal,RF+andRF arealternatelyinputtothe ltercoreintheSCFmixer.Accordingtothat,thefrequencyresponseoftheproposedmixerisshiftedbyπand

978-1-4244-1922-7/08/$25.00 ©2008 IEEE

103

A CMOS direct sampling mixer using Switched Capacitor Filter technique for software-defined radio

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Voltage Gain (Simulation) [dB]

Voltage Gain [dB]

Frequency [MHz]

FrequencyresponseofSCFmixer.

Fig.5.

20

10010-20-30Frequency [Hz]

Fig.2.

Down-convertedfrequencyresponseofimplementedSCFmixer.

TABLEI

DSMperformancesummary.

NF [dB]15

10k

Frequency [Hz]

Thenoise gureofSCFmixer.

DDFig.3.

Fig.4.ChipmicrographofSCFmixer.

chipmicrographoftheimplementedcircuit.Thecoresizeis1150μm×750μm.

TwoSignalSourceAnalyzers(Rohde&SchwarzSML02)andSpectrumAnalyzer(AdvantestR3267)wereusedformea-surement.Fig.5showsdown-convertedfrequencyresponseofthemixer.BecauseofthematchingproblemoftheTAandtheoutputbu er,theconversiongain(powergain)oftheimplementedSCFmixerissmall,about12.4dBat400.1MHzinput.However,themeasuredresultshavethesametendencywiththesimulation,asshowninFig.5.TableIsummarizesthemeasuredresults.

IV.Conclusion

ThispaperhasproposedanoveldirectsamplingmixerusingSCFwith1/fnoisesuppressionforsoftware-de nedradio.TheproposedDSMisimplementedin0.18μmCMOSpro-cess.Measurementresultsshowthatthecircuitworkscorrectlyandtheexperimentalresultsexhibit430kHzsignalbandwithwith27.3dBattenuationofadjacentinterfererassumingat3MHzo set.

Acknowledgment

ThisworkissupportedbyVLSIDesignandEducationCenter(VDEC),theUniversityofTokyoincollaborationwithCadenceDesignSystems,Inc.

thealiasedsignalwillnotbealiasedtotheDCcomponent,thussmallerDCcomponentcanbearchieved.Fig.2and3showthefrequencyresponseandnoise gureoftheproposedmixerrespectively,wheretheTA,OPAmpsandswitchesweredesignedinMOSdevicebutidealdigitalclockingsignals.Thesimulationswereperformedbyusing50Ωinputsignalsourcesand50Ωterminations.AsshowninFig.2,thepeakvoltagegainis32.3dB,andinFig.3,NFisabout17dBat430kHz.

III.MeasurementResult

TheproposedSCFdirectsamplingmixerhasbeendesignedforoperationat800MHzsamplingfrequencyand430kHzbandwith.ALNTA,wheregmis10mSatsamplingfrequency,isalsoincluded.Inaddition,twoLNTA+DSMforIQmodearealsodeveloped.ThemixersandLNTAshavebeenim-plementedinTSMC0.18μmCMOSprocess.Fig.4showsa

References

[1]H.Eul,“ICsformobilemultimediacommunications,”Proc.ofIEEEInternational

Solid-StateCircuitsConf.,Feb.2006,pp.31–35.

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