基于FPGA的信号发生器 翻译.docx

基于FPGA的信号发生器 翻译.docx

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基于FPGA的信号发生器翻译

ReconfigurableReadback-SignalGeneratorBasedon

aField-ProgrammableGateArray

JinghuanChen,JaekyunMoon,andKiaBazargan,Member,IEEE

Abstract—Wehavedesignedareadback-signalgeneratortoprovidenoise-corruptedsignalstoareadchannelsimulator.ItisimplementedinaXilinxVirtex-Efield-programmablegatearray（FPGA）device.Thegeneratorsimulatesinhardwarethenoiseprocessesanddistortionsobservedinharddrives.Itusesembeddednonuniformrandomnumbergeneratorstosimulatetherandomcharacteristicsofvariousdisturbancesintheread/write

process.Thesignalgeneratorcansimulatereadbackpulses,intersymbolinterference,transitionnoise,electronicsnoise,headandmedianonlinearity,intertrackinterference,andwritetimingerroraccordingtothecharacteristicsspecifiedbytheuser.Asampleimplementationoperatesata70-MHzclockspeed.Thedesigncaneasilybescaledfordifferenterrorrates.ThegeneratorcanbereconfiguredinrealtimetogivetheuserflexibilityandincreasethecapacityoftheFPGAdevice.Thereadback-signalgeneratorcanbeintegratedintoanFPGAreadchannelsimulator

orserveasatestbenchfordata-recoverycircuits.IndexTerms—Fieldprogrammablegatearray（FPGA）,Gaussianpseudorandomnumbergenerator,magneticrecording,readchannel,readchannelmodeling,reconfigurablecomputing,transitionnoise.

I.INTRODUCTION

INTENSIVEsimulationisoftencarriedouttoinvestigateadvanced

signalprocessingtechniquesfordatastorageapplications.

Thesimulationisnormallyperformedinsoftwarethat

maytakeaverylongtime.Forinstance,thesimulationofa

10bit-errorrate（BER）,assuming10biterrorsareobserved,

cantakedaysrunningonapersonalcomputerequippedwith

a1-GHzPentiumIVprocessor.Ontheotherhand,thetarget

sectorretryrates[i.e.,unrecoverableerrorratesaftererror-correction-

code（ECC）]incommercialharddrivesnormallygo

below10fordesktopproductsand10forserver-class

products.Oneapproachtospeedupthesimulationprocessisto

implementthewholesimulatororpartofitinhardware.Because

thenoisecharacteristicsinharddrivesareunique,the

additivewhiteGaussiannoise（AWGN）assumption,whichis

widelyacceptedinstudyingtheperformanceofmanycommunication

systems,isnotreliable.Instead,adedicatedreadbacksignal

generatormustbeimplementedinfrontofdata-recovery

modulesinthesimulator.Thissignalgeneratorshallincorporate

allmajornoiseprocessesanddistortionsandbecapableof

ManuscriptreceivedDecember8,2003;revisedFebruary10,2004.

J.ChenwaswiththeDepartmentofElectricalandComputerEngineering,

UniversityofMinnesota,Minneapolis,MN55455USA.Heisnowwith

MaxtorCorporation,Shrewsbury,MA01545USA（e-mail:

jinghuan_chen@

）.

J.MoonandK.BazarganarewiththeDepartmentofElectricalandComputer

Engineering,UniversityofMinnesota,Minneapolis,MN55455USA（e-mail:

moon@ece.umn.edu;kia@ece.umn.edu）.

DigitalObjectIdentifier10.1109/TMAG.2004.826913

generatingverylowprobabilityeventsaccordingtotheuser-defined

statistics.Itshallalsohavethereconfigurationcapability

thatallowstheusertochoosefromvariousnoiseanddistortion

combinations.Field-programmablegatearray（FPGA）becomes

thechoiceofimplementationplatformbecauseofitsadvantage

oflow-costandreconfigurability.TheFPGA-basedsignalgenerator

proposedinthispapercanalsoserveasatestbenchfor

data-recoverycircuits（usuallycalledthe“readchannel”）since

itiscompletelytunableincontrasttotypicaltestspindlesetups

andisabletoprovideacompletesetoftestconditions.

Aseriesofnoiseprocessesanddistortionsinharddrives

degradestheperformanceofreadchannels.Statisticalmodels

ofthesenoiseprocesses,definitionofsignal-to-noiseratio

（SNR）,andanalysisofquantizationeffectareprovidedinSection

II.InSectionIII,thedesignandperformanceofembedded

randomnumbergeneratorsareexplained.Thesegenerators

producerandomnumbersaccordingtothestatisticsspecified

bytheuser.InSectionIV,wepresentthearchitectureofthe

designandbrieflydiscusstime-duplexingandJBits-aided

reconfiguration.InSectionV,wedemonstratethenoiseand

nonlinearitystatisticsinthegeneratedsignal.Conclusionsare

giveninSectionVI.

II.MODELSANDALGORITHMS

Inharddrives,eachbinarydatumisstoredinaverysmall

area,calledabitcell,onthemagneticsurfaceofadisk.A

writeheadmagnetizeseachbitcelltooneoftwodirections

thatrepresent0or1.Areadheadpicksupthemagnetization

fluxemittedfromtheboundariesofbitcellsandgeneratesreadback

pulses.Thesepulseswillbepassedthroughapreamplifier,

analog/digitalfilters,anddata-recoverymodulesandconverted

backtobinarydata.

Thenoisyenvironmentinharddrivesisuniqueandharsh.

Thesenoiseprocessesanddistortionscanbecategorizedinto

intersymbolinterference,transitionnoise,electronicsnoise,

head/medianonlinearity,timingerror,andintertrackinterference.

Noiseprocessesinhard-driverecordingsystemshave

beenmodeledinseveralways.Moon[1]usesTaylorseries

toapproximatetransitionjitterandwidthvariation.Caroselli

andWolf[2]simplifythestructureofrecordedtracksbyusing

amicro-trackmodel.Inourimplementation,wemimicthe

physicalrealitybyactuallyshiftingthepulsepositionand

changingthepulseshapeinruntime.Thebehaviorofthe

variousnoiseprocessesanddistortionsfollowsthestatistics

laiddownbytheuser.Itshouldbenotedthatthestatisticsare

definedbytheuseranddonothavetobeGaussian,ashasbeen

assumedinmanypreviousmodelingworks.

0018-9464/04$20.00©2004IEEE

CHENetal.:

RECONFIGURABLEREADBACK-SIGNALGENERATOR1745

A.StatisticalModelingofNoisesandDistortions

1）IntersymbolInterference（ISI）:

Sincethereadheadsare

band-limitedsensors,readbacksignalswillexperienceISIwhen

therecordingsymbolrateexceedstheheadbandwidth

.Asequenceofreadbackpulsesoverlapandaredistorted

whenISIextendsoveranumberofsymbols.Thenoiseless

readbacksignalisgivenby

（1）

（2）

whereistheisolatedpulseandisthedifferenceoftwo

consecutivedatabit

（2）.Thesequencetakes

valueinandanonzeroindicatestheexistenceof

atransition.ISIhappenswhenthelengthofpulseis

longerthanthesymbolperiod.

2）TransitionandElectronicsNoise:

Thezigzagboundary

betweentwoopposingmagnetizationscausesthepositionofa

readbackpulsetoshiftintime（transitionjitter）anditsshape

todeform（widthvariation）[3].Thenoiseintroducedbythis

sortofphenomenoniscalledtransitionnoise（TN）andisdata

dependentinthesensethatnoisearisesonlyinthepresenceof

transitionsthatmakeupadatapattern.Bothofthesenoiseprocesses

canbemodeledusingrandomvariables.Werewrite

（1）

toreflectthetransitionnoise.Nowtheisolatedpulseisnot

deterministicbutarandomprocesswherethesamplespaceisa

seriesofpulsesthathavedifferentpulsewidthandamplitude.

Wenotethatawiderpulsehasreducedamplitudesoastomake

theareaunderthepulseremainsconstant.Thisisnecessaryif

thepulsewideningisduetothebroadeningofthemagnetictransition.

Positionjitterisarandomvariablefallingintherange

of,whereweassumeisthelimitofjitteroneach

side.

Electronicsnoisenormallyismodeledasanadditive

whiteGaussiannoise（AWGN）band-limitedbyW.Itspower

spectraldensityis.Nowthenoise-corruptedreadsignal

iswrittenas

（3）

3）HeadandMediaNonlinearities:

Magnetoresistance

（MR）readheadsarenotlinearsensors.Whenincorrectly

biased,theycancauseanunsymmetricalheadsensitivity

function[4].Thiseffectiscalledtheheadnonlinearity（HNL）.

Itamplifiesthereadbacksignalinanonlinearfashionacross

therangeofamplitudeandresultsindisparitybetweenpositive

andnegativepulses.Itcanbecharacterizedbyanonlinear

function,whichcanbemeasuredfromareadhead.

Twomoretypesofmedianonlinearityalsooccurduringthe

writeprocess:

nonlineartransitionshift（NLTS）andpartial

erasure（PE）.Theygenerallycreateadditionaltransition

positionshiftandamplitudeloss.Simplisticassumptionshave

beenmadeonthedatadependencein[5]and[6].Weconsider

onlythefirst-orderPEeffectandcombinethemasfollows:

（4）

Fig.1.Generationofwritetimingerror.

whereisthePEparameterandistheamountofnonlinear

transitionshift.TheNLTScomponentreducestozeroeffectively

unlesstwoconsecutivetransitionsoccur,i.e.,.

4）WriteTimingError:

Aslowlyvaryingwritetimingerror

（WERR）isaddedtomodelaccumulatedphasejitterinthewriteclock

synthesizercircuit.Itcanbesimplifiedandmodeledasa

slow-paced“randomwalk.”AsshowninFig.1,onewriteerror

isuniformlygeneratedfromineveryclockcycles.

Theeffectisintegratedsoitwillinfluencethewriteposition

ofalllaterpulses.Theaverageofthedriftiszeroinavery

longrunandthepaceiscontrolledby.

5）IntertrackInterference:

Areadheaddoesnotalwaysposition

itselfwith100%accuracyonthetopofatrack.Itconsequently

sensesthesignalfromtheneighboringtrackandissubject

tothecrosstalk,whichisalsocalledintertrackinterference

（ITI）.ITIisbroughtintothepicturebyaddinguptwoweighted

outputsignalsfromtwoindependentgenerators.

B.SNRandStatisticsofNoiseProcesses

Theperformanceofdata-recoveryschemesisusuallycompared

acrossarangeofSNR.ArelationshipbetweenSNRand

statisticsofgeneratedrandomnoiseshallbeestablishedsothat

theeffectivenessof