CHAPTER1 Introduction
Contents
1.1 Motivation2
1.1.1 AdvantagesofMillimeter-WaveRadios2
1.1.2 Deep-SubmicronCMOS5
1.1.3 DigitallyIntensiveApproach7
1.2 DesignChallenges9
1.2.1 TowardAll-DigitalPLLinmm-WaveRegime9
1.2.2 WideTuningRangeandFineFrequencyResolution11
1.2.3 LinearWidebandFM12 References13
WirelesscommunicationhasevolvedremarkablysinceGuglielmoMarconi demonstratedthetransmissionandreceptionofMorse-codedmessages acrosstheAtlanticOceanintheearlytwentiethcentury.Sincethen,new wirelesscommunicationmethodsandserviceshavebeencontinuously adoptedthatrevolutionizeourlives.Today,cellular,mobile,andwireless local-areanetworks(WLANs),affordedbybreakthroughsinsemiconductor technologiesandtheircapabilityofmassproduction,areinuseworldwide. Theyenableustoshareimagesofourcherishedmomentswithfamilyand friendsanywhere,andatanytime.Thecurrenttrendstoward portablewirelessdeviceswithultra-high-speed(e.g.,gigabitpersecond) connectivitywillsoonallowustogoonlineviaournotebooks,cell phones,andtablets,simultaneouslyemailing,chattingwithfriends,web browsinganddownloadingmoviesandmusicinafractionofthetimeit takestoday.Thesedeviceswillhavetomeetaggressiveperformance specificationsinasufficientlysmallandlow-costproductatlowpower dissipation.Thishaspromptedfranticresearchintonewradiofrequency (RF)-integratedcircuits,systemarchitectures,anddesignapproaches. Thisbookexploresthefeasibility,advantages,design,andtestingof digitallyintensivefrequencysynthesisinthemillimeter-wave(mm-wave) frequencyrange.Anall-digitalphase-lockedloop(ADPLL)-basedtransmitter
demonstratorfabricatedinaproductionbulkCMOSprocessisdescribed, whichoperatesinthe60-GHzband,andachievesfractionalfrequency generationandwidebandfrequencymodulation(FM).Thisdigitally intensivedesignhasthepotentialforlowcostinvolumeproduction.It isalsoamenabletoscalinginfuturetechnologynodesasopposedto otheranalog-intensiveimplementations.Thesiliconareaandpower consumptionofsuchtransmittersmaybereducedfurtherinfutureby harnessingthepowerofdigitalsignalprocessing(DSP).
1.1MOTIVATION
Toachievegigabitpersecond(i.e.,Gb/s)transferrates,Wi-Fitechnology (IEEE802.11acinthe5-GHzband) [1] hasbeendevelopedinrecent years.MultistationWLANthroughputofatleast1Gb/s,andasinglelink throughputofatleast500Mb/sisspecified.ItemploysRFbandwidthsof upto160MHz,multiple-inputandmultiple-output(MIMO)array transmitter/receiverstreams(upto8),multi-userMIMO,andupto 256-QAM(quadratureamplitudemodulation)schemesinordertoachieve thatlevelofperformance.Themm-wavefrequencybands,bycontrast,are lesscrowdedthanthelow-gigahertzradiocommunicationbandsand,more attractively,havewiderlicense-freeRFbandwidthavailable(e.g.,7GHz bandwidthinthe60-GHzband).Thiswillenablethegigabit-per-second short-rangecommunicationforconsumermultimediaproductsandsupport thedevelopmentofemergingshort-rangewirelessnetworkinginmany importantareas,forexample,commerce,manufacturing,transport,etc., andthusprovidesignificantgrowthpotentialinnewinternetapplications inprice-sensitivecommunicationmarkets.
Inthefollowingsections,theadvantagesandchallengesofmm-wave transceiverdesigninCMOStechnologywillbeexamined.Thefocusis onmm-wavefrequencysynthesis.
1.1.1AdvantagesofMillimeter-WaveRadios
Themm-wavefrequencybandisdefinedas30 300GHzwithawavelengthbetween1and10mmintheair [2].Therearevariousaspectsof mm-wavebandsthatmakeitattractiveforshort-rangeapplications.One majoradvantageisthebandwidthavailabletocarryinformation.Tokeep operatingcostslow,regulatorylicensedbandsshouldbeavoided,thus callingfortheexploitationoftheunlicensedortheindustrial,scientific, andmedical(ISM)radiobands. Figure1.1 plotstheavailablebandwidth 2
Figure1.1 BandwidthallocationfortheISMandunlicensedbandsbelow100GHz bytheFCC(intheUnitedStates) [3].
(indicatedinGHzatthetopofeachcolumn)forISMandunlicensed bandsbelow100GHzintheUnitedStates [3].Below25GHz,theRF spectrumiscongestedduetofrequencyslotsreservedformilitary,civil, andpersonalcommunicationservices.Forreference,mostcommercial productsoperateinbandsbelow10GHz,forexample,theglobalsystem formobilecommunicationsoperatesat900and1,800MHz(inEurope), and850and1,900MHz(intheUnitedStates),andultra-wideband (UWB)radiosarepermittedtooperatefrom3.1to10.6GHz [4].Less than1GHzofbandwidthintotalhasbeenallocatedforthelicense-free ISMbandsat2.45,5.8,and24GHz.Onthecontrary,thereis7GHzof bandwidthinthe60-GHzspectrumbandallocatedforlicense-freeuse, whichisthelargesteverallocatedbytheFederalCommunications Commission(FCC)intheUnitedStatesbelow100GHz.Withsuchwide bandwidthavailable,mm-wavewirelesslinkscanachievecapacitiesashigh as7Gb/sfullduplex,whichisunlikelytobematchedbyanyoftheRF wirelesstechnologiesatlowerfrequencies.TheFCChasalsorecently approvedanotherunlicensedband(92 95GHz)tomeetthegrowing demandforpoint-to-pointhigh-bandwidthcommunicationlinks [5]
Foragivenantennasize,thebeamwidthcanbemadefinerbyincreasing thefrequency.Anotherbenefitofthemm-waveradioisanarrowerbeam duetotheshorterwavelength(λ 5 c/fc ,where c isthespeedoflightand fc isthecarrierfrequency),whichallowsfordeploymentofmultiple, independentlinksincloseproximity.Themainlimitationofmm-wave radioisthephysicalrange.Duetoabsorptionbyatmosphericoxygenand
Figure1.2 Averageatmosphericattenuationofradiowavespropagatingthrough freespaceversusfrequency [6]
watervapor,signalstrengthdropsoffrapidlywithdistancecomparedto otherbands. Figure1.2 illustratesthegeneraltrendofincreasingthe attenuationofradiowaveswithfrequency(dueonlytoatmosphericlosses; freespacepathlossisnotaccountedfor) [6].Atmosphericabsorptionby oxygencausesmorethan15dB/kmofattenuation.Thelossofalink budgetat60GHzisthereforeunacceptableforlong-distancecommunication(e.g., . 1km),butcanbeusedtoanadvantageinshort-rangeindoor communicationsbecausethelimitedrangeandnarrowbeamwidthsprevent interferencebetweenneighboringlinks.Theseattributeshaveledtogreatly reducedregulatoryburdensformm-wavecommunications.
Duetoitspotentialforshort-range,gigabit-per-secondcommunications, severalstandardsinthe60-GHzbandhavebeenestablishedinrecent years.TheIEEE802.15.3cstandardwasapprovedin2009forwireless personal-areanetwork [7].AsimilarstandardforEurope(ECMA-387 [8]) waspublishedin2008.TheWirelessHDconsortiumhasreleasedaspecificationversion1.0aforregulatingthetransmissionofhigh-definitionvideoin thisunlicensedband [9].Mostrecently,theIEEE802.11adstandard(known asWiGig) [10] wasadoptedin2013.Itprovidesdataratesupto7Gb/s,or morethan10 3 themaximumspeedpreviouslysupportedbytheIEEE 802.11standard.IEEE802.11adalsoaddsa“fastsessiontransfer”feature,
whichenableswirelessdevicestoseamlesslytransitionbetweenthe60-GHz frequencybandandlegacybandsat2.4and5GHzinordertooptimizelink performanceandrangecriteria.
Inadditiontothegigabit-per-secondcommunication,the60-GHz unlicensedbandalsoholdspromiseforintegratingwirelesssensors. Frequency-modulatedcontinuous-wave(FMCW)radarsmaybeutilized forpresencedetectionandrangingat60-GHzapplications,where high-frequencyresolutionisrequired [11].Thisisalsotheintended applicationfortherealizedADPLLfrequencysynthesizerthatisfully describedinthisbook.AsanexampleofsuchFMCWapplicationisa gesturerecognitionsystemforcars,wherethedrivergestures(e.g., noddingthehead)withouttakingtheeyesofftheroadwheninterfacing withapplicationssuchasnavigation,phone,HVAC(heating,ventilating, andairconditioning)controls,etc.Thetargeteddetectionrangeis from0.3to10mandtherangeresolutionisbelow5cm.Alow-cost implementationofshort-rangeradarsystemswillenablenumerous applicationsinsecurity,searchandrescue,imaging,logistics,quality control,tonamejustafew.
Figure1.3 illustratestheoperatingprincipleofanFMCWradar transceiver.Thecarriersignalismodulatedasshownin Figure1.3b, resultinginasignalwhoseinstantaneousfrequencyvarieslinearlywith time,i.e.,alinearchirp [12].Thislinearchirpistransmittedtowarda target,andthereceivedechoisconvolvedwithaportionofthetransmittedsignaltodeterminetheround-trippropagationtime, τ .InanFMCW radar,theachievablerangeresolution(Δr)isdeterminedby
where c isthespeedoflight,andBW(Figure1.3b)isthemodulation rangeofthetransmitsignal [12].Whenthefull7GHzofbandwidthat 60GHzisutilized,arangeresolutionasfineas2cmcanbeachieved.
1.1.2Deep-SubmicronCMOS
Silicontechnologies(e.g.,CMOSandSiGe-BiCMOS)aremainstreamintegratedcircuit(IC)processesdrivenlargelybymassproductionofICsusedin digitalcomputers(e.g.,desktopsandnotebooks)andotherelectronicdevices (e.g.,cellphones,gameconsoles,andtablets).Thedemandforahigherintegrationlevelandlowercostinvolumeproductionhasdrivenmm-wave electronicsdevelopmentinsiliconCMOStechnology.With65-nmbulk
Figure1.3 (a)SimplifiedblockdiagramofanFMCWradartransceiver;(b)transmittedandreceivedlinearFMCWsignal.
CMOStechnologiesinproductionofferingpeaktransitfrequency(fT)and maximumfrequencyofoscillation(fmax)closeto200GHz(simulatedusing baselinetransistors) [13],severalexperimental60-GHztransceivershave beenreportedthatachievedataratesabove4Gb/sacrossa2-mlink [14 16].TheseprototypesdemonstratethepotentialtouseCMOSfor RF/basebandco-integration,andrevealthedesignchallengesandopportunitiesforimprovedRFperformance(e.g.,higherRFoutputpower,lower oscillatorphasenoise(PN),etc.)andlowerpowerconsumption. Nevertheless,III-Vprocessesstillhaveanicheinpoweramplifiersand antennaswitchesformobilephones/basestations,andultra-high-frequency, high-powerelectronicsformilitaryandspaceapplications.
Abetterunderstandingofthepropertiesofdeep-submicronCMOS technologiesiscrucialinordertoimplementhigh-performance mm-waveICs.Severalkeypropertiesaresummarizedhere,whichshould betakenintoaccountwhendecidingonthepreferredarchitecturesand designapproaches.
•Lowsupplyvoltage:comparedtoIII-Vandbipolartechnology,which relyonalargersupplyvoltage(3.3and2.5V),deep-submicron CMOSfeaturesanominalsupplyvoltageof B1V.
•Duetotheaforementionedlowsupplyvoltageofdeep-submicron CMOSandrelativelyhighthresholdvoltage(0.5Vandoftenhigher, duetothebodyeffect),theavailablevoltageheadroomisquitesmall. Thus,themarginbetweentechnologyperformanceanddesign requirementsappearslargerinthetimedomainthaninthevoltage domain [17]
•ExcellentswitchingcharacteristicsofMOStransistors—bothrise andfalltimesareontheorderoftensofpicosecondsorlessfor deep-submicronCMOStechnologies.
•Rapidpaceofprocessscaling—eachnewdigitalCMOSprocessnode occursroughlyevery18months,resultinginanincreaseinthedigital gatedensitybyafactorof2(knownasMoore’slaw [18]).
•Multiplemetallayersarecommonlyavailableforinterconnectioninlargescaledigitalcircuitry,whichalsoprovidehigh-densitymetal-oxide-metal capacitors.
1.1.3DigitallyIntensiveApproach
Duetotheaforementionedpropertiesofdeep-submicronCMOS technologies,especiallythestrengthincircuitspeedanddensity,digitally assistedanddigitallyintensiveRFsystemsarebecomingattractivefor mm-wavetransceiverICs.WhenthedesignedRFsystememploysdigital logicandsignalprocessingextensivelytoobtainbetterRFperformance, itiscalled digitallyintensive.Comparedto analog-intensive architectures,the numberofpurelyanalogcircuitfunctionsinadigitallyintensive mm-wavetransceiverisreduced,whichresultsinadvantagesthat conventionaldigitaldesignflowshaveoveranalogdesignmethodologies. Amongthemare:reduceddesigncycletimesusingautomateddigital implementationtoolsandflow,easeoftestabilityviabuilt-inselftest, on-chipDSP,high-densitymemory,andautomaticfunctionaltesting withgoodfaultcoverage.Moreover,digitallyintensivearchitectureshave lowersensitivitytoprocess/deviceparametervariabilitycomparedtothe analogintensivesystems.Inaddition,digitalcircuitsprovidereconfigurabilitytocontroltheoperationmodeandimprovesystemperformance viapowerfulon-chipcalibrationtechniques,whichmayreducesilicon areaandpowerdissipationoftheSoC.
Notethatthe“digitallyintensive”termdoesn’timplythatanalog/RF designtechniquesarenotimportant.Onthecontrary,theyareascrucial asbefore.Theoverallsystemperformanceisusuallydominatedbyafew keyanalogcircuitblocks.Theessenceofthedigitallyintensiveapproach istomaketheinputs/outputs(IOs)oftheRF/analogbuildingblocks digitalsothatthesystemcanbemodeledandanalyzedusingthedigital designflow,withitsmanyadvantagesfordesignthroughputandyield. Consequently,itrequiresRF/analogdesignerstobeconversantwith digitalcircuitsandsystemdesign,toanalyzethesystemfrombothanalog anddigitalperspectives,andtocollaboratewithdigitaldesigners.
AgoodexampleofadigitallyintensivearchitectureisADPLL synthesizershownin Figure1.4.Itcontainsadigitallycontrolledoscillator, whichmayoscillateinthegigahertzrangeandiscontrolledbyadigital oscillatortuningword(OTW).ThegigahertzDCOoutput(usually followedbyaninvert-basedbuffer)hassharprising/fallingedgeswhen implementedindeep-submicronCMOStechnologieswith fT above 100GHz,thusbehavinglikeadigitalclock.Thetime-to-digitalconverter (TDC)measuresandquantizesthetimedifferencebetweenthereference andDCOclocktransitionedges.Thenthedigitizedphaseerrorisfiltered byadigitalloopfilter(LF)andeventuallyconvertedtotheOTWinorder totunetheoscillatortothedesiredfrequency.AlthoughtheDCOand TDCarebothanaloginnature,allbuildingblocksintheADPLLare definedasdigitalattheI/Olevel,andthereforetheloopcontrolcircuitry isimplementedinafullydigitalmanner,asillustratedin Figure1.4.
ThisADPLLarchitecturehasbeenusedinmassproductionfor RFconnectivityand2G/3Gmobilecommunications [17].Withthe improvedRFcapabilityof65-nmCMOStechnology,digitallyintensive frequencysynthesiscouldbeexploredinthemm-waverange,which isover10 3 thepreviouslyprovenfrequencyrange.Suchmm-wave
Figure1.4 SimplifiedblockdiagramofanADPLL.
ADPLLincreasesthereconfigurabilityoffrequencygenerationina mm-wavetransceiver.Moreover,FMcanbeincorporatedtheretoforma digitaltransmitterwiththepotentialforsuperiormodulationqualityand lowercostinmassproduction.
1.2DESIGNCHALLENGES
Torealizelow-cost,yethigh-performancemm-wavetransceiversinCMOS technology,newconceptsinICimplementationsforultra-widebandsignal generationandmm-wavefront-endsarenecessary.Thisbookfocuseson frequencysynthesis,whichiscriticaltomanymoderncommunication systems.Duetothehighoperatingfrequency,finefrequencyresolution,and widebandlinearFMrequirements,afullyintegratedmm-wavefrequency synthesizerhasvariousdesignchallengesthatarediscussedinthefollowing sections.
1.2.1TowardAll-DigitalPLLinmm-WaveRegime
Beforethisvery60-GHzall-digitalphase-lockedloop(PLL)thatwillbe elaboratedinthisbook,therehadbeennootherreportedsuccessful fractional-Nsynthesizerimplementationsabove10GHzforwireless applicationsthatwouldemployanall-digitalapproach.Therewere, however,tworeportsofdigitalPLLsynthesizers [19,20] operatingat20 and40GHz,respectively,usedforhigh-speedserialwirelineapplications, andnumeroussuccessfulADPLLdemonstratorsoperatingbelow10GHz forvariouswirelessapplications,forexample,bluetooth,cellular,WLAN, WiMAX,etc. [21 24].Forlow-gigahertzapplications,anLCoscillator isnormallyusedtosatisfystringentPNrequirements,inwhichthe tuningoftheoscillationfrequencyisachievedviadigitalcontrolofan arrayofMOSvaractorsthatoperateinflatregionsoftheir C V curve.It iswell-knownthatthePNofanLCoscillatorintheupconverted thermalnoiseor 1/f 2 regime(i.e.,outsidetheloopbandwidthofaPLL synthesizer)isinverselyproportionaltothesquareofthetankquality factor Q [25].Thetank Q-factorbelow10GHzisdominatedbythe Q-factoroftheinductor,whilethevaractor Q-factorisnormallymuch higher(e.g., B100at2GHz)ina65-nmCMOStechnology.
However,thisisoppositetothesituationatmm-wavefrequencies. Figure1.5 plotsthe Q-factorandcapacitance(Cv)versusbiasvoltagefor n 1 /n-wellandp 1 /p-wellaccumulationmodevaractorsina65-nm CMOStechnologyatminimumgatelength [26]. Q-factorvarieswith
Figure1.5 Capacitanceand Q-factorat60GHzversusgatevoltageforminimum length,thin-oxideaccumulationmodevaractors(65-nmCMOS) [26]
bias(duetochanging Cv)andisapproximately20or5intheflatregions. Whiletheinductor Q-factor(QL)increaseswithincreasingfrequency, the Q-factorofcapacitorsandvaractors(QC)isinverselyproportionalto frequency.Therefore, Q ofthetankcapacitance(varactorplusparasitics) becomestheprimaryfactorlimitingthequalityoftunableon-chip resonators,andthePNperformanceofmm-waveoscillators.Power consumptionoftheoscillatormustthereforebeincreasedinorderto maintainsignalswingandcompensateforgreaterlossesintheLCtank.
Inaddition,afrequencydividerchainisnecessarytobringthecarrier frequencydowntoafewGHzforfurtherprocessingbythedigitalphase detector.Therearestrongtrade-offsbetweenpower,chiparea(inductors), maximumoperatingfrequency,andoperatingrangeinthedividerchain design,whichnormallydissipatesmorethan50%ofthetotalpowerina mm-waveanalogPLL [27].Moreover,thedivider’soperatingrangeshould bealignedwiththeDCOfrequencytuningrangeinthepresenceof process,voltage,andtemperature(PVT)variations.Thedividerchain introducesextradelayintheloopandmayaffectthestabilityofthePLL. Allthesebringextrachallengestothedesignofmm-waveADPLLs.
AlthoughthedigitallyintensivenatureoftheADPLLpermitsthefast system-levelsimulationandverificationbyanevent-drivensimulator,the transistorsizingandphysicallayoutofthekey“analog-nature”building blocks,suchasDCO,dividerchain,andTDChavetobe“handcrafted” accordingtothedesignspecifications,andthenmodeledatthebehavioral levelfortheclosed-loopsimulations.Comparedtodesignforlow-GHz
applications,theinterconnectionsbetweenmm-wavefrequencybuilding blocksaffectsthesystemperformanceduetoparasiticcapacitance,losses, andunwantedcapacitiveandmagneticcouplingeffects.Therefore,intensiveelectromagneticsimulationisalsorequiredforasuccessfulADPLL designinthemm-waveregime.
1.2.2WideTuningRangeandFineFrequencyResolution
Asmentionedearlier,onemajorbenefitofthe60-GHzbandisthe7-GHz worthofunlicensedbandwidth.Whenthe7-GHzbandwidthisfully employed,the60-GHzFMCWradar,asshownin Figure1.3,cantheoreticallyachievearangeresolutionasfineas2cm.SincePVTvariationsmust beaccommodatedbythetuningrangeoftheoscillator,awiderthan9-GHz tuningrangeisdesiredtoensurefullcoverageoftheentire60-GHzband. However,atuningrangelessthan5%istypicallyexpectedforanLC voltage-controlledoscillator(VCO)operatingatthesefrequencies [26]
Thetank Q-factorandfractionaltuningbandwidthfortanks optimizedateachfrequencyareplottedin Figure1.6 (schematicshown inset),usingsimulationsinthesame65-nmRF-CMOStechnologyas Figure1.5[26].Inordertoconstructthetuningrangecurve,wefirst
Figure1.6 Optimumtank Q-factorandfractionaltuningrangeforresonatorsin 65-nmRF-CMOSfromsimulation(basedonsimulationswith Cfixed 5 20fF) [26]
selectaninductorthathasthehighestpeak Q-factorwhendriven differentiallyateachfrequency(f0).Thefixedandvariablevaractor capacitances(Cfixed 5 20fFandn 1 /n-wellthickoxidevaractorwith L 5 0.4 μm)arethenaddedtosettheresonanceat f0.Thefixedportion ofthetankcapacitance(Cfixed)accountsforwiringinterconnectsand transistorparasitics.Forexample,thetunablecapacitance ΔC willbe 21fFofthe70.4-fFtotaltankcapacitance(C0),ifa100-pHtank inductor(L0)resonantat60GHzisassumed.Itcanbeseenthatthe oscillatortuningrangedropsto B5%athigherfrequencies.Tuningrange maybeimprovedbysacrificingthetank Q-factor,i.e.,usingsmaller inductorvaluesandlargervaractors.However,thepowerconsumptionof theoscillatororthesizeofcoretransistorneedtobeincreasedforPN performance,which,inthelattercase,canintroducemore Cfixed,thus limitingtheachievabletuningrangebythistradeoff.
Inadditiontotheaforementioneddifficultiesinachievingawide tuningrangeforamm-waveLCoscillator,itisalsochallengingtorealize finefrequencytuningunderdigitalcontrol.Amm-waveDCOisthe heartofamm-waveADPLL.Itprovidesthemeanstoconvertdigital controlwordsintooutputfrequencies.Thelackofahigh-resolution DCOhashinderedtheADPLLfromreachingmm-wavefrequenciesin thepast.Theminimum-sizedNMOSvaractorina65-nmCMOS processgeneratesa ΔC of B40aF,whichresultsinafrequency resolutionof B17MHzfora60-GHzcarrier(i.e.,
,assuming
analysisinRef. [17],thiscorrespondstoaquantizationnoise of 62.2dBc/Hzat1-MHzoffset(referenceclockis40MHz),whichis 28dBhigherthanthenaturalPNofa60-GHzDCO(e.g., 90dBc/Hz at1-MHzoffset).Moreover,minimum-sizeddevicesdonottracklarger deviceswell,resultinginamismatcheffectinsidethetuningbankarray. Therefore,newdigitalfine-tuningtechniquesneedtobedevelopedfor mm-waveDCOstoachievearawfrequencyresolutionontheorderof 1MHz. ΣΔ ditheringoftheleastsignificantbitsintheDCOtuning bankcanbeemployedtoimprovethefrequencyresolutionfurther [17]
1.2.3LinearWidebandFM
Tomaximallyexploittheavailablebandwidth(e.g.,7GHzinthe 60-GHzband)allocatedatmm-wavefrequenciesforhighdatarate communicationsandhigh-precisionradars,mm-wavetransceiversshould
providelinearwidebandFMcapability.Forexample,ina60-GHz FMCWradartransceivershownin Figure1.3,thefrequencyofthe transmitsignalislinearlyrampingupanddownacross5-GHzrange. Theradarrangeresolutionisdeterminedby Eq.(1.1) anddegradeswith thesweepnonlinearity [12].However,inpractice,theDCOtuningmust besegmentedintocoarse-andfine-tuningbanks,eachwithdifferent tuningstep, KDCO (definedasfrequencychangeperbit)torealizehigh resolutionandawidetuningrangesimultaneously.Consequently,the widebandtriangularmodulation,asshownin Figure1.3a hastotraverse throughvarioustuningbanksandrelyonlinearized KDCO acrossmultiple banks.Moreover,thetuningstepmismatchesinsideeachbankalso introducenonlinearitiesintheFM.Dummystructurescanbeaddedin thephysicallayouttoimprovethematchingperformancebuttheymay notbepossibleatmm-wavefrequenciesduetotheincreasedparasitics andreducedoveralltuningrange.Alternatively,digitalcalibrationand compensationtechniquesshouldbedevelopedandappliedtoimplement thewidebandtriangularmodulation.Inaddition,sincethe Q-factorof thetankalsovariesfrom20to5acrossthemodulationrange (Figure1.5),theoutputswingoftheoscillatormayvarybymorethan 3dB.Theoutputbuffermustcompensateforthesignalpowerfluctuation acrossthemodulationrangeandproduceachirpwithaflatoutputpower (e.g., 6 dB)attheFMCWtransmitteroutput.
Toaddresstheaforementionedconcernsandissuesforhighperformancefrequencysynthesisatmm-wavefrequencies,somenew circuitsandsystemarchitecturesarrangementshavetobediscovered.In thisbook,alternativedesignapproachesandarchitectureformm-wave PLLsareexplored.Weusea60-GHzall-digitalPLLforFMCWradar applicationasadesignexample.ThePLLarchitecture,mm-wavecircuit design,andtheDSPtechniquesdevelopedinthisparticularworkcanbe universallyappliedtoothermm-waveapplicationsthatfocusonhigh performanceandlowcost.
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