GPSRFFRONT-ENDCONSIDERATIONS

Component selection guide

System circuit design

Layout and placement considerations

Mainlyfocusonsystemlevel

(BoardLevel)

Author : Criterion

TableofContentsGPS Signal……………………………………………………1

SNR V.S. C/N0………………………………………………..4

Noise Figure…………………………………………………..6

Pre-SAW………………………………………………………9

Linearity………………………………………………………21

LNA…………………………………………………………...26

Mixer………………………………………………………….37

VCO…………………………………………………………..42

Layout and placement consideration……………………...52

Author : Criterion

GPSSignal

GPSsignalisDirect-SequenceSpreadSpectrumfeaturingtheseadvantagesthat

allowsmanytransmitterstosharethesamefrequencyband,andhardtojam[94].

Furthermore,GPSadoptsCodeDivisionMultipleAccess(CDMA),andthe

frequencyspectrumofthesignalisspreadwithanoiselikecode(sequence).

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Spreadingcodeshaveverylowcross-correlationandareuniqueforeveryuser

(lowinterferencewithothersignals).Transmissionbandwidthismuchwider

thanwhichofinformation.Asshownbelow[94]:

TheL1signalisaBPSKsignalmodulatedinphasebytheC/A-codeandthe

informationofthenavigationmessage.TheL1frequency=1575.42MHz

=154x10.23MHz.AndthechippingrateofC/Acodeis1.023Mcps[49].

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Mostcommercialfront-endshaveahighgainexceeding80dB.Thehighgainis

achievedatacostofeitherahighnoiselevelorhighpowerconsumption[49].For

example,theRTR6285AofQualcommhasapproximately82dBgain.Because

GPShas43dBprocessinggain,theGPSbasebandsignalhas123dB(80+43=

123)gain[49].

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SNRV.S.C/N0

SNRisusuallyexpressedintermsofdecibels(dB).Itreferstotheratioofthe

signalpowerandnoisepowerinagivenbandwidth[57].

C/N0,incontrast,isusuallyexpressedindecibel-Hertz(dB-Hz)andreferstothe

ratioofthecarrierpowerandthenoisepowerperunitbandwidth.Thus,wecan

expressC/N0asfollows:

InrealGPSapplication,theC/N0isusually37~45dB-Hz.Duringdesignstages,

theconductedC/N0shouldbelargerthan40dB-Hzwith-130dBmGPSsignal.

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WeassumethatGPSreceiverfront-endbandwidthis4MHz,andtheSNRshould

be-29~-21dB.Thisislogicalforthereceiverfront-endnoisefloorisabout-110

dBm,andit’simpossibleforrealGPSsignaltobelargerthan-110dBm.Namely,

theGPSSNRmustbenegativeuntilitentersthereceiver’sbasebandprocessing

stages.

Asshownabove,wecanseethatbeforeintegration,theSNRisreallynegative.

ButtheSNRincreasesastheintegrationtimeincreases.TheSNRgaininthiscase

isalsoreferredtoasprocessinggainduetospreadspectrumfeature.

TheSNRismostusefulwhenconsideredwithinthebasebandprocessingblocks

ofaGPSreceiver.IndealingwithSNR,thebandwidthofinterestneedstobe

specified.Areceiver’sfront-endbandwidthdeterminestheSNRseenbytheinput

sideofthevariousbasebandprocessingstagesofthereceiver.Aswehaveseen,

theSNRinaGPSreceiverisdependentonthereceiver’sfront-endbandwidth.

ReferencingjusttheSNRvalueinaGPSreceiverdoesnotusuallymakesense

unlessonealsospecifiesthebandwidthandprocessingstagewithinthe

receiver[57].

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Author : Criterion

NoiseFigure

GPSsignalsaretransmittedbymediumpowersatellites,withapproximately

40dBm.WhentheyreachtheEarth,theyarenormallyreceivedbyantennaswith

aminimumpowerofapproximately–131dBm.Thatiswhyit’simpossiblefor

realGPSsignaltobelargerthan-110dBmasmentionedearlier.TheC/N0is

relatedtothemaximumbiterrorrate(BER)requiredforaGPSreceiveratthe

output,whichis10-5[49].Nodoubt,withhigherC/N0,comesbetterpositioning

accuracyandTTFF[3].

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Also,asshownbelow,thehigherC/N0is,thehigherdetectionprobabilityand

thebettersensitivitywillbe.

WithlowerNF(NoiseFigure),comeshigherC/N0.Asshownbelow[58]:

That’swhythisprocesswillrequirealowerNF.EachdBdecreaseinnoisefigure

helpsimprovethesensitivitybyadB[33].Ingeneral,thesensitivitycanbe

-158dBm~-160dBmwith2.5dBNFofoverallreceiver[4].

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AnotherreasonforrequiringalowerNFliesintheincreasinguseofGPS

receiversinurbanenvironments.Thereceivedsignalpowerisreducedinplaces

withhighbuildingsandnarrowstreets.Inthiscase,thereceiverisunableto

detectthesatellitesignalorprovidesanimpreciseposition.Moreover,another

factortoconsiderinurbanenvironmentsisthepresenceofinterferencesignals

thatthereceivercancapture,increasingCNRdegradation,andallthe

performance,suchasTTFF,positioningaccuracy,andsensitivity,willaggravate

aswell[49].

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Pre-SAW

Asshownbelow[31],

TheseoutbandblockersmayleakintotheGPSreceiver’spathandhaveagigantic

impactonthereceiver’ssensitivitybyoverloadingthereceiver’sLNAor

backend[31].Thestrongeroutbandblockeris,themoredegradationofSNRwill

be[41].

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Asshownbelow,iftheseblockersareclosetoLNA’sP1dB,thegainwilldecline

duetosaturation:

Asshownbelow,whentheblockerislargerthan-10dBm,thegainwilldrop

dramatically,whereastheNFwillincreasesignificantly[39].

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Moreover,iftheseblockersarelargerthanLNAmaximuminputlevel,thegain

willbediminishedtozero,andthiscircumstancemakesLNAhaslargeinsertion

lossmerely.Asaresult,insteadofbeingamplifiedbyLNA,theGPSsignalwill

besubmergedinLNA’snoisefloor[31].

Thiscreatesabigchallengetothehandsetdesigners.Thedesignersneedto

maintainthesensitivityoftheGPSreceiverfortheweakincomingGPSsignal

whiletherearestrongblockersfromthetransmittingvoiceordata.Thisrequires

aGPSreceiverfrontendwithverygoodblockingtothesestrongblockers[31].

Inotherwords,aLNAwithextremelyexcellentlinearityisabletoavoidbeing

overloadedbythesestrongblockers.

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Author : Criterion

Intermsofdynamicrange,thelowlimitissensitivity.Inordertoacquirethe

extremelyweakGPSsignal,thesensitivityshouldbelowenough.But,dynamic

rangeisfinite,thisfeatureindicatesthattheLNA’sP1dBwillnotbelarge.Thatis

tosay,it’simpossibleforGPSLNAtopossessextremelyexcellentlinearity.

Typically,theDCS1800(1710MHz)PAoutputistakenastheprimaryconcernof

theoutbandblockers,whichiscertainlymuchhigherthanGPSsignal,directly

saturatingtheLNAwithoutexternalSAWfilters,thentheSNRdegrades[41,44].

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Asshownbelow,ALM-1912haspre-SAW,butALM_1612doesn’t.Ifwefixonthe

blockerstrength(-10dBm),itcanbeseentheNFofALM_1612increasesmuch

morethanwhichofALM-1912.That’swhyaddingpre-SAWpriortoLNA

forrejectionofthejammerisnecessarytoachieveacceptablesensitivitywith

strongoutbandblocker[55].

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Furthermore,weneedtoputDC-blockatLNAinputtoavoidmakingDCOffset

feedintoLNA.

Nevertheless,theDCblockatinputisoptionalasitisusuallyprovidedbythe

pre-filterbeforetheLNAinmanyGPSapplications[75].

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AccordingtoFriisformula:

Theimportantvariablescontributingtosystemsensitivityarethepre-SAW’s

IL(insertionloss)andLNA’sNF,whileLNAsubsequentblockshaveminimal

impact[28]:

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Let’sillustratetheideafurther[55].

Asshownabove,therearethreetypesofGPSmodules:

Type1:pre-SAW+LNA+post-SAW

Type2:LNA+post-SAW

Type3:pre-SAW+LNA

Comparedtotype1andtype3,type2haslowestNFforithasnopre-SAW.

Besides,comparedtotype3,type1haspost-SAW.ButNFoftype1ishigherthan

type3merely0.04dB.ItprovesagainthatLNAsubsequentblockssuchas

post-SAWhaveminimalimpact[28]:

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Thus,thepre-SAWmusthavelowILtoposseslowerNF.Thepost-SAWshouldbe

selectedtoemphasizeitsoutbandrejectionoverIL,sinceitsprimaryfunctionis

toblockanyblockeramplifiedbyLNA[28].Asshownbelow:

Bytheway,type2hasnopre-SAWbutitslinearityisnotworsethantype1and

type3.That’stosay,insteadofimprovingLNA’slinearity,pre-SAWcansimply

helprelaxLNA’slinearityrequirement.

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Author : Criterion

As shown above, the input impedance of pre-SAW, including matching1 and

matching2, must be 50Ω to avoid degrading SAW filter performance due to

mismatched impedance. Although matching1 and matching2 belong to LNA

source-pull as well, matching3 influences LNA more than matching1 and

matching2duetothefactitisclosertoLNA.Thus,intermsofsource-pull,wejust

needtotunematching3tolowestNFlocationonSmithChart.Asforplacement,

pleaseplacepre-SAWasclosetotheLNAaspossible,leavingonlyenoughroom

toplace thematching componentsbetween them[54].Otherwise, the outband

blockermaystillfeedintoLNA.

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As shown above, the Pin2/3/5 are GND pins[101]. For a good outband noise

rejection,alowcrosstalkisnecessary.Lowcrosstalkcanberealizedwithagood

RF layout. The major crosstalk mechanism is caused by the “ground-loop”

problem.Grounding loops are created if input-and output transducerGND are

connectedonthetop-sideofthePCBandfedtothesystemgroundingplanebya

commonviahole.Toavoidthecommongroundpath,thegroundpinoftheinput

and output transducer should be isolated from the top-side grounding plane.

Otherwise, the outband noise rejection degrades. In this PCB layout, the

groundingloopsareminimizedtorealizegoodultimaterejection[100].Asshown

below:

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Besides,the inputandoutputgroundingpinsare isolatedandconnectedtothe

common ground by enough separated via holes. Plentiful GND via holes will

provide a more effective ground and better outband noise rejection for the

pre-SAW[55].

Besides50Ωimpedance,thevariationinpre-SAWresponseisalsodominatedby

temperaturedrift,resultinginunacceptableinterferenceandhighIL[102].

Asaresult,numerousGNDviaholeswillalsohelpmitigatethethermaleffects.

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Author : Criterion

Linearity

Asmentionedearlier,linearityrequirementsareimposedbyreceiverbehaviorto

externalinterferences.Thelinearityspecificationsaredictatedbytherequired

system’sabilitytoperforminthepresenceofexternalinterferingsignals[49].

However,theGPSreceiverdoesnothaveadjacentoralternatechannel

interferersandforthatreasononlyout-of-banddesensitizationperformanceis

ofinterest[46].Thus,oneofthemostimportantcharacteristicsofaGPSreceiver

inamobileterminalishowgoodtheoff-bandlinearityis[46].

But,asshownabove,signalsfromclosebands,oreveninter-modulation(IMD)

productsofothersignalsfromotherbandsduetoLNA’snonlinearity,could

createasignalinthesamebandastheGPSsignal.Andthesein-bandinterferers

can’tbefilteredbyanySAWfilterposteriortoLNA.Forexample,theIMD3

(1570MHz)consistingofDCS1800andPCS1900fallsintoGPSband,becomingan

in-bandjammerandcannotbefilteredbypost-SAW[39,42].

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Althoughthesein-bandinterfererswillnotsaturateLNAsubsequentblocks

necessarily.Nevertheless,infact,jammersmerelystrongerthan-110dBmcan

causeproblemsinGPSperformancesuchasTTFFandaccuracy,evenifthereis

processinggainforGPSbasebandsignal[3].Becausereceivercanpickfalse

jammerpeakinsteadofrealGPSsignalpeakincaseofmultitonejammers,or

consumemoretimetofindrealGPSsignal[3].Asshownbelow:

Asaresult,mixeristhebiggestlinearitydeciderinreceiverchain,butifLNA’s

linearityisnotgoodenough,thoseIMDproductsasaresultofLNA’snonlinearity

can’tbefilteredandwillaggravatesensitivityaswell,eventhoughthemixerhas

excellentlinearity.Namely,LNA’slinearityisequallyasimportantasmixer’s.

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Author : Criterion

However,asmentionedearlier,theLNA’slinearityisnotgoodenoughdueto

finitedynamicrange,butpre-SAWcanhelprelaxLNAlinearityrequirement.

Thus,priortoLNA,weneedtoeliminatetheoutbandblockersasmuchas

possibleinadvancetoreducetheIMDproductsduetoLNA’snonlinearityfor

weakerblockersresultinweakerIMDproducts.Itprovesagainthatpre-SAWis

necessary.

AccordingtoFriisformula,forachievingthelownoiserequirements,quitelarge

gainsareused,especiallyintheLNA,todetectlowerpowersignals,thereby

improvingthesensitivityofthereceiver[49].Butlargegaincandeterioratethe

receiverpass-bandlinearityaswell.Asshownbelow,withlargergain,comes

lowerNFatfirst.ButNFremainsnearlyconstantwhengainislargerthan12dB.

Incontrast,theIIP3declinescontinuallyasgainincreases.

Consequently,acompromiseforthegainandlinearityperformanceis

needed[46].LargergainisnoguaranteeoflowerNF,andyetitisguaranteedthat

worselinearity.

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Sincethosein-bandinterferersarealmostIMDproducts,indicatingthe

worst-casescenariosforthelinearityrequirementswithrespecttoIIP3and

IIP2[46].Thoseinterferenceswillhaveasignificantinfluenceonthe

performanceofthesignalprocessing,astheywillnotbefilteredintheRF

front-end.Thus,thelinearityoftheGPSreceiverisredefinedasthelimitofthe

highestinterferencepowerthatthereceivercanhandlebeforeitbeginsto

performincorrectly.That’stosay,thehigherlinearityis,thebetterimmunityto

interferenceswillbe.Withbetterimmunitytointerferences,comesbetter

sensitivity.

Thereareseveralwaystomeetthelinearityrequirementinthereceiver.Oneway

wouldbebycarefulsystemdesignandpartitioningtheblockgainsappropriately

[46].AnadditionalLNA,whichdeterioratesthereceiverlinearitywiththeadded

gain,wouldthenbestillneededtocomplywiththenoisefigurerequirement.

ItindicatesagainthattheLNAgainisrelatedtooverallreceiverlinearityand

sensitivity.TheLNAgainshouldnotbetoohightomeetthelinearity

requirement[46].Asmentionedearlier,withpoorlinearity,comespoor

sensitivityaswell.

Theotherwaywouldbetointensifythelinearityofthemixer,whichcanbe

passivetype.Asmentionedearlier,mixeristhebiggestlinearitydeciderin

receiverchain.Passivemixersdonotconsumestaticpower,thenthey

introducenoFlickernoise(or1/fnoise)andDCOffset.Theyalsoshowagood

nonlinearitybehavior.

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Author : Criterion

Incontrary,activemixersgenerateconversiongain(typically10dB)andshowa

reasonablenonlinearitybehaviorbuttheyintroduceFlickernoisebecauseofthe

biasingcurrentoftheCMOSdevices,therebyaggravatingsensitivity[40].

Asmentionedabove,ahigh-gainLNAwillhelpreduceNFbyminimizingmixer

contribution,andyetattheexpenseofhigherpowerconsumptioninthisblock

andworselinearityinthewholereceiver.TakeQualcommRTR6285Afor

example,thereceiverhashighlinearitymodeandlowpowermode.Asa

consequence,ifwewanttointensifytheimmunitytointerferencesfurther,we

arecapableofchoosinghighlinearitymodeattheexpenseofhigherpower

consumptioninthisblock[39].

Asmentionedearlier,theoutbandrejectionofpre-SAWisimportantaswell.In

general,FBARcanhavehighoutbandrejectionandlowILsimultaneously,and

yetattheexpenseofcost[28].

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LNA

Asmentionedearlier,inurbanenvironments,thereceivedsignalpoweris

reducedinplaceswithhighbuildingsandnarrowstreets.Asmentionedearlier,

GPSsignalisnormallyreceivedbyantennaswithaminimumpowerof

approximately–131dBm[55].But,inthiscase,thereceivedGPSsignalby

antennasmaybecome-150dBm[56],andthereceiverisunabletodetectthe

satellitesignalorprovidesanimpreciseposition[49,53].Thus,weneedexternal

LNAtoimprovesensitivity,TTFF,andpositionaccuracy[33].

EventhoughtheRFamplifierinputsignalissingle-ended,adifferentialstructure

hasbeenselectedbasedonitsadvantagesthathavebetterimmunitytosecond

orderdistortion(IMDandharmonics)andcommonmodenoise[41,49].The

advantageofthisarchitectureisthehigh-linearityforblockerrejectionfrom

otherinterferences[41].Furthermore,CDMAoperationrequireshighselectivity

torejectTxleakage,therebysuppressingcross-modulation.Thedifferential

configurationLNAisbeneficial[17].

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Asshownbelow,thebalancedsignalisgeneratedoff-chipbyabalun[46].The

LNAoutputdrivesdouble-balancedpassiveI/Qmixers.Themixersaredrivenby

LOsignalwith25%dutycycle[41].

But,adifferentialstructurehasdisadvantagesaswell.Themajordisadvantageis

thatbalunaddsextraloss,therebydeterioratingNF.BalancedtypeSAWfilterhas

largerILthanunbalancedtypeasaconsequenceofintegratedbalun[13].

TheprosandconsofdifferentialLNAaresummarizedasbelow[49]:

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Moreover,totunethedifferentialmatchingismorecomplexthansinglematching.

Thedetaileddifferentialmatchingtuningprocedureisdescribedin[17].Bythe

way,theL2andL3shownaboveshouldbeplacedinwayslimitingmutual

coupling.DonotlocateL2andL3tooclosetoshieldwalls(thismightcauseEM

couplingandinductorde-Q)[51].

Sincethemajorinterferenceandblockersignalsareout-of-bandsignals,some

additionalfilteringcanbeachievedbyhigh-Qinductorsinthematching[46].Due

tothefeaturesthathigh-Qinductorshavelowlossandnarrowbandwidth,these

advantagescanhelplowertheNFandimproveoutbandnoiserejectionfurther.

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Ingeneral,thewire-woundtypeinductorhashigherQvaluethanmulti-layer

type.

AsfortheLNAinputmatching,asmentionedearlier,thepre-SAWinput

impedancemustbe50ΩtoavoiddegradingSAWfilterperformancedueto

mismatchedimpedance.But,thepre-SAWoutputmatchingshouldbedesigned

toachievetheminimumNF(notnecessarily50Ohm).

Asshownabove,matching1affectsLNAinputimpedanceaswell.Nevertheless,

comparedtomatching1,matching2affectsmore.Thus,matching1mustbe50Ω,

toachievethebestpre-SAWperformance,minimumNFisjustrealizedby

matching2(non-50Ω).

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Certainly,matching2ispre-SAWoutputmatchingaswell,achangeoftheoutput

impedance(non-50Ω)ofthepre-SAWwillalsoinfluencethefrequencyresponse.

Asaconsequence,yououghttodotherealsensitivitymeasurementtodecidethe

matching2,minimumNF(non-50Ω)or50Ω.

TheLNAlandpatternisshownasabove[74],whichshouldbegroundedthrough

isolatedarea[55].Especially,pre-SAWandLNAgroundsareseparatedtoavoid

anyunwantedparasiticeffectfromdeterioratingLNARFperformance.Besides,

putGNDviasasmanyaspossibletospreadtheheattopreventfromdegrading

LNAperformanceduetohightemperature[54].Asshownbelow:

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Let’stakeAVAGOALM-1912forexample,theschematicisshownabove,L2isa

RFchokeisolatingtheGPSsignalfromtheDCsupply[55].Moreover,theL2

biasinginductortogetherwiththeC2bypasscapacitorsetstheoutputmatching.

Asaconsequence,L2hasinfluenceonLNA’sperformanceaswell.

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Inputandoutputreturnloss:

Gain:

NF:

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Linearity:

Assummarizedbelow:

Asaresult,inadditiontomatching,youcanalsoadjusttheL2valuetoachieve

thebestperformance.

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Ingereral,withhigherVdd,comesbetterlinearity.Asshownbelow[55]:

IfwefixontheVdd(2.2V),itcanbeseenthatlinearityvariesfordifferentIdd.

Thus,weareabletoadjustR2valuefordesiredIddcurrenttoachievebest

linearity[55].

Inadditiontolinearity,otherperformancecanalsobeobtainedbyincreasingthe

supplyvoltage[74].

Consequently,weshouldpreventsupplyvoltagefromIRdroptopreventfrom

deterioratingperformance.

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Asshowninthenoisecirclebelow:

ThehighgainzoneishighNFzoneaswellonSmithChart.Namely,intermsof

source-pull,highgainalsoleadstohighNF.Thereisalwaysatrade-offbetween

gainandNF[49].Asmentionedearlier,choosingthegainlevelwithinthevarious

blocksofthereceiverisalwaysatrade-off.Ahigh-gainLNAwillhelpreduceNF

byminimizingmixercontribution.Thatistosay,evenifweuseapassivemixer,

whichdoesn'tinfluencethecascadeNF.Butahigh-gainLNAincreasespower

consumptionandnoiseinthisblock[49].Alow-gainLNAmayimprovelinearity

andpowerconsumption,butwouldrequirealow-noisemixer.Suchamixer

wouldconsumealotofpower.Ifyouuseapassivemixerwithalow-gainLNA,

thereceiveroverallNFwillbehigh.Inotherwords,alow-gainLNAcombined

withalow-noisemixermaynotofferasignificantadvantageintotalpower

consumptionoverahigh-gainLNAcombinedwithamixerwithahigherNF.

Becausemixerplaysthemajorroleinpowerconsumption.Therefore,areceiver

configurationwithaccurategainhasbeenchosen.

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Asmentionedearlier,poorlinearityresultsinpoorsensitivity,therebyincreasing

theTTFF.Asaresult,weoughttoselectaLNAwithrelativelygoodlinearity.With

thesamelevelblocker,thebetterLNA’slinearityis,theshorterTTFFwillbe[25].

Ingeneral,theLNAP1dBshouldbeatleast-5dBm,andIIP3shouldbeatleast5

dBm.

.

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Mixer

Anactivemixerhasbeenselectedbecauseitpresentsahighergainand

lowernoisethanthepassiveones[49].But,forthedirect-conversion

receiver,thetypicalchallengesareDCoffsets,mixersecond-ordernonlinear

effects,andflickernoise.BecausetheseinterferenceisnearthedesiredGPS

signaldown-converteddirectlytobaseband[39,46].

Flickernoiseisevenlargerthanthedown-convertedRxsignal[69].Activemixers

sufferfromhigh1/fnoiseandpoorlinearity,especiallywhenthesupplyvoltage

islow.Incontrast,acurrentdrivenpassivemixercanproviderelativelygood

linearityandinherentlow1/fnoiseperformanceduetotheabsenceofDC

current[38,69].AlthoughapassivemixerhaslargerNFthanactiveones,theNF

ofLNAsubsequentblockscontributetosensitivityslightly.Thus,ingeneral,

passiveCMOSmixersareconsideredastheappropriatechoicefor

direct-conversionreceiversfortheydonotcontributeto1/fnoise[69].

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Takingpreviouspre-SAWconsiderationsintoaccount,theinputimpedanceof

themixerhasbeensettooptimizethepowerconsumption,gain,NF,and

linearityoftheentireRFmixer[49].

Nevertheless,asshownabove,themodernGPSreceiveralmosthasnoexternal

matchingcomponentsbetweenLNAandmixer.

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But,somereceiversmayaddexternalSAWfilterbetweenLNAandmixer.It

indicatesthattherewillbeexternalmatchingcomponentsatmixerinput.As

mentionedearlier,theinputimpedanceoftheexternalSAWfiltermustbe50Ωto

avoiddegradingSAWfilterperformanceduetomismatchedimpedance.Similarly,

theinputimpedanceofmixermustbealso50Ωtopreventfromdeteriorating

mixerperformance(powerconsumption,gain,NF,andlinearity)dueto

mismatchedimpedance.

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Inordertogetlowconversionlossfrommixers,typicallyahighLOpoweris

needed[49,69].

Asshownabove,thehigherLOpoweris,thehighergain(thelowerconversion

loss)ofmixerwillbe.Especiallyforapassivemixer,whichactslikeswitches

controlledbythesignalfromLO[38].ThehigherLOpowerisessentialtoachieve

thelowerconversionloss,therebybeingabletodrivetheADC[40].

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ButstrongLOpowermayresultinsignificantLOleakageduetothefinitemixer

porttoportisolation.LOleakagecausesself-mixing,therebygeneratingastatic

DClevelaggravatingsensitivity[78].Asshownbelow:

Therefore,asaresultofhighLOpowervalue,theisolationbetweenLOandmixer

mustbehighenoughtoavoidLOleakage[49].Besides,inordertoavoid

degradingtheperformanceoftheRFmixer,theLOpowerlevelmustguarantee

theappropriateswitchingperformanceofthemixercore.TheLOpowerlevel

shouldbebetween–10dBmand–3dBm.

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VCO

Voltagecontrolledoscillator,asthenameimplies,theoutputfrequencyis

dependentoncontrolvoltage[49]:

Asaconsequence,VCOsareverysensitivetonoiseonthesupplysources[45].To

achieveaccuratefrequency,theprecisetuningvoltageisessential[23].

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Asshownabove,theidealspectrumofoscillatorshouldhasno“skirt”.

Nevertheless,inrealworld,it’simpossible.Ifthewaveformofoscillatorsignalin

timedomainhastimingerror,i.e.,phaseerrororjitter,therewillbephasenoise

resultinginlarger“skirt”,therebyaggravatingsensitivityinspectrum.Besides

phaseerror,phasenoiseisgeneratedbypowersupplyinputaswell[45].

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So,weshouldcarethesepowertracesrelatedtoVCO,PLL,LO,andsynthesizer

verymuch.

Ingeneral,therearethreeproblemsinpowersupply:highfrequencynoise,

ripple,andIRdrop.Asthefollowingformula[48]:

IDDistotalpowersupplycurrent.Accordingtoohm'slaw:I=V/R.

IftheIRdropistoomuch,e.g.powerlayouttraceistoolongortoonarrow,the

currentdeclinesastheresistanceincreases,thenthephasenoiseraises.As

shownabove,themaximumIRdropofQualcommWTR1605Lis20mV.Because

theIRdropforthesepowertracesrelatedtoVCO,PLL,LO,andsynthesizermust

belessthan20mV,weshouldatleastmaketheirtracewidthmeettherule:

1A=40mil.

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Asshownbelow,ifthewaveformissinglesinusoidalsource,thereisnospursin

spectrum[48].

Therealmeasurementinspectrum[49]:

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Nevertheless,singlesinusoidalwaveformcombinedwithasquarewavewill

generatealotofspurs,asshownbelow[48]:

Asaresult,weshouldkeepthesetraceswithhighfrequencynoiseawayfrom

VCOpowersupply.

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Let’stakeQualcommRTR6285Aforexample,theADCintegratedinPMIC

convertsthe19.2MHzsinglesinusoidalwaveformintothedigitalreference

frequency.That’stosay,theXO_OUTtrace,whichisrichinharmonics,shouldbe

keptawayfromVCOpowersupplyandwellisolatedtopreventfromaggravating

VCOphasenoise.AndtheXO_OUTtraceshouldalsobekeptawayfromGPSsignal

duetoits82orderharmonics(19.2MHzx82=1574.4MHz)[51,56].

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Bytheway,insomeplatforms,suchasQualcommWTR2965,theADCis

integratedintransceiver.AseriesDCblockcapacitorisrequired[59].

Besides,sincepowersupplyripplecanbeamajorcontributortoVCOspurs[43],

decouplingcapacitorsareusedontheVCOsupplytoimprovetheripple.As

shownbelow,withlargerdecouplingcapacitor,comeslowerphasenoise[98].

48

Author : Criterion

Consequently,takeQualcommWTR1605Lforexample,wemustadddecoupling

capacitorsonpowertracesrelatedtoVCO,PLL,LO,andsynthesizertoreduceXO

orLOspurs[97].

Nowthatwealreadyknowthatthepoorpowersupplyresultsinlargephase

noiseandspurs.Asmentionedabove,thephasenoiseandspursofVCO

deterioratessensitivity,ifwesuspectthatthepoorsensitivityisbecauseofpoor

VCOpowersupply,wecanuseasetofbatteriestoprovidetheexternalclean

powersupplytominimizethepossiblenoiseintroducedbytheoriginalpower

supplysystem[49].

49

Author : Criterion

Asshownbelow,withtheexternalcleanpowersupply,thephasenoiseandspurs

improvemuch[99].Itindicatesthattherearesomeproblemsattheoriginal

powersupplysysteminput.

Ingeneral,theVCOphasenoiseforGPSapplicationshouldbelessthan

-92dBc/Hz@100-KHz[44].

50

Author : Criterion

Asmentionedearlier,theVCOoutputfrequencyisdependentoncontrolvoltage.

Asshownabove,Kvcoistheslope.WeshouldmakeKvcolowtoreduceVCO

modulationsensitivitytokeeptheoscillatingfrequencyasstableaspossible.In

general,theKvcoforGPSapplicationshouldbelessthan50MHz/V[44].

Besidesphasenoise,theVCOshouldalsohaslowfrequencydrift.Asshown

below,theXOisthesourceofVCO.

Asaresult,youshouldselecttheXOwithlowfrequencydrift.Ingeneral,inGPS

application,thefrequencydriftshouldbewithin±5ppm(±7.877kHzforGPS

and ±8.028kHzforGLONASS)[23]

51

Author : Criterion

LayoutandplacementConsideration

Asshownabove,withthethesameimpedance,themoredistancebetweensignal

traceandreferenceGNDis,thewidersignaltracewidthandthelessILwillbe.

52

Author : Criterion

Furthermore,ifthesignaltraceistooclosetoadjacentlayer,theILmayincrease

asaresultofparasiticcapacitance.

Let’ssupposetheparasiticcapacitanceisa0201size,750ffcapacitorshunting

toGND.

Asshownabove,thereis0.16dBadditionalILatGPSfrequency.Althoughthe

assumptionisnotcorrectcertainly,itisdoubtlessthatparasiticcapacitance

contributestoadditionalIL.

53

Author : Criterion

Asaresult,ifdielectricthicknessisnotsufficient,thenextlayermayneedtobe

clearedtomitigateparasiticcapacitancetoreduceIL[17].

Asmentionedearlier,thecascadedNFisprimarilydependentonthefirstLNA’s

NFandgaininthechainaswellasanylossesincurredpriortotheLNA(e.g.,

causedbypre-SAW)forlossesincurredposteriortoLNAwillbeattenuatedby

thereciprocaloftheLNA’sgain.IfweareabletoreducetheILofRFtracepriorto

theLNA,thesensitivitywillimprovefurther[57].

54

Author : Criterion

Asshownabove,alargetransientcurrentflowsfromtheexternalsupplytothe

PA,creatingIRdropsacrosstraceresistancesRT1andRT4.Thegoodcasekeeps

theRT4componentoutofthePMIC’ssupplyinput,whereasthebadcaseplaces

theRT4traceresistanceinthePMIC’spath.SincethePMICprovidesreceiver

supplyvoltages,anytransientsonitsinputsupplyvoltagemayleakontothe

suppliesofsensitivecircuitssuchasVCO,PLL,LO,andsynthesizer.Thiswill

aggravatetheXOorLOspursandphasenoise,therebydegradingthesensitivity.

Asshownbelow:

55

Author : Criterion

TrytopositionthedevicesandroutetheirsuppliessothatthePMICandPA

routinggoesinoppositedirectionsfromtheprimarypowernode,andkeep

sensitivecircuitsfarfromthePA’sreturncurrentpath,includingthedecoupling

capacitorsonPA’spowersupply.RoutethePAsupplytracesandreturnpathsfar

fromsensitivecircuitsthatmightpickuptransientenergyfromtheswitching

current[17].

56

Author : Criterion

Careneedstobetakeninthelayouttoisolatethenoise-generatingpinsfromthe

noise-sensitivepins.TakeQualcommRTR860xforexample,thesepinsare

summarizedasbelow[17]:

Particularly,allPLLsuppliesandTx/Rxoscillatorsarenoisyplusnoise-

sensitive[17].

57

Author : Criterion

InsteadofsharingacommongroundfloodwithallRFtransceivergroundpins,

theoppositegroundingmethodistokeepmultiplesubgroupingsseparatefrom

eachotheruntiltheyconvergeonthemainPCBgroundplane[17].

Themethodcanhelpisolatethesenoise-generatinggroundpinsfromthose

noise-sensitivegroundpins.

58

Author : Criterion

SinceIandQbasebandoutputsaresensitiveanalogsignals,routethemcarefully

topreventfromdesense.TechnologiesusingnarrowerRxchannelbandwidths

willbeinfluencedmore.Theworstcasesare

GSM/LTE1.4MHz/CDMA/WCDMA/GPS[50,51].

AvoidroutingRX_I/QnearordirectlyunderPMICSMPS(SwitchingModePower

Supply)PCBareas,forPMICSMPSswitchingnoisewillcouplemagneticallyand

electricallytoRX_I/Qtraces[50].Besides,avoidoverlappingreceiverandPMIC

SMPSareasondouble-sidedboarddesignsbecauseevenusingagroundplaneto

separatethePMICSMPSareasfromRX_I/Qtraceswillnotblockmagnetic

coupling[50].

59

Author : Criterion

Asshownbelow,thecasewheretheI/Qareroutedonadifferentlayervs.the

PMICswitchingnodewithgroundplaneseparation;magneticcouplingcould

stillcausedesense[50].

60

Author : Criterion

PlacingthePMICbetweenthereceiverandbasebandchipswillputRX_I/Qlines

indirectpathofnoisySMPScurrents.Ifsuchaplacementisinevitable,atleast,

I/QtracesshouldberoutedawayfromPMICandSMPS;nooverlapping

occurs[50].

Nevertheless,longRX_I/Qtracesaremoresusceptibletonoisecoupling.If

possible,placereceiverclosetothebasebandchiptoshortenwhoselength[50].

Atleast,thelongRXI/Qtracesshouldberoutedininnerlayerforstripline

provideshigherisolationasaresultofthefactitissurroundedbyground

planes[51].

61

Author : Criterion

Becauseoflimitedareasondouble-sidedboard,thebasebandchip,PMIC,and

receiverarelocatedondifferentlayers.Therecommendedplacementisas

shownbelow:

BasebandchipandreceiverareawayfromPMICandtherearenooverlapped

areas.BasebandchipisclosetoreceivertopossessshortRX_I/Qlines.But,in

thatway,thepowertracesbetweenPMICandreceiverwillbelong.Consequently,

itshouldberoutedininnerlayertoavoidradiatingEMInoiseoutside.Moreover,

careneedstobetakenforIRdropissue.

62

Author : Criterion

AsforPMIC,keyaggressorsarelistedasbelow:

l SMPSVSWnode:VSW_Sx

l SMPSswitchinginductors

l SMPSground:GND_Sx

l SMPSinputcapacitors

l VREG_S

l Othernoisydigitalsignals,orswitchingpowersupplyrails

63

Author : Criterion

AsforSMPSinputcapacitors,whichshouldbeasclosetoPMICaspossible,or

thetransientcurrentfromPMICmaycoupletoothertraces,andthetransient

currentfromotherchipmayleakintoPMICaswell,therebyaggravatingGPS

performance.

Furthermore,SMPSinputcapacitorsshouldbegroundedthroughisolatedarea,

orthetransientcurrentfromPMICmayleakintootherchipsthroughcommon

GND,andthetransientcurrentfromotherchipsmayleakintoPMICthrough

commonGNDaswell,therebydeterioratingGPSperformance.

Besides,addamassofgroundviasandconnectthisisolatedgroundareatothe

PCBmaingroundplanedirectly[103].

64

Author : Criterion

AsforSMPSswitchinginductors,

Withlargerinductorvalue,comeslessrippleandEMInoise.But,largerinductor

valueresultsinmoreturns,therebyincreasingDCR(DCresistance)andIRdrop

issue.

Bytheway,withthesameinductorvalue,thelargersizeis,thelessDCRwillbe.

So,theinductorsizeshouldnotbetoosmall.Andweshouldselectthepower

inductorwithlessDCR.

65

Author : Criterion

WithlessinductorvalueandlessEPC(EquivalentParallelCapacitance),comes

higherSRFandwiderinduciblerange.

BecausetheSRFshouldbeatleast

(DC-DCSwitchingFrequency)*10

Forexample,switchingfrequencyis1.2MHz,theSRFshouldbeatleast12MHz.

That’sthereasonwhyweneedwideinduciblerange.

Thus,theinductorvalueisneitherthelargerthebetternorthelessthebetter.It

isthemoreprecisethebetter.Certainly,intermsofinduciblerange,theEPC

shouldbeassmallaspossible.

66

Author : Criterion

Isat(saturationcurrent)isthecurrentlevelcausingpowerinductorvaluetodrop

30%.IfthecurrentgoesabovetheratedIsat,therippleaggravates.Asshown

below:

Thatis,theIsatofpowerinductoristhelargerthebetter.

67

Author : Criterion

Ingeneral,withthesameinductorvalue,multi-layertypehashighertransfer

efficiencythanwire-woundtype(duetolowDCR).Butmulti-layerhaslower

saturationcurrentthanwire-woundtype.

DCR(Ohm) Isat(mA)

Wirewound 0.26 0.68Multilayer 0.14 0.28

Thepowerinductorselectionguideislistedasbelow:

l LowEPC

l LowDCR

l HighIsat

l Accuratevalue

68

Author : Criterion

Moreover,thepowerinductoranddecouplingcapacitorshouldboth

beasclosetoPMICaspossibletoshrinktheswitchingnoiselooparea.

Otherwise,theswitchingnoiseloopareaenlarges.

69

Author : Criterion

Inthatway,thewaveformdistortsandEMInoiseaggravates.

Also,theGPSantennamaypickupradiatedEMInoise,thendesenseissueoccurs.

70

Author : Criterion

Furthermore,theswitchingnoisemaycoupletoPApowersupplyinput,thereby

mixingwithRFsignaltoproduceIMDproductsnearRFsignal.

Inthatway,ACLRdeteriorates,therebyincreasingtheGPSnoisefloorand

degradingsensitivity.

71

Author : Criterion

Besides,asmentionedearlier,SMPSinputcapacitorsshouldbegrounded

throughisolatedarea.But,bydoingthis,itseemstobedifficulttoaddnumerous

GNDviasinthegroundislandduetoitslimitedarea.Thus,extendtheground

islandtotheareaunderneaththepowerinductortoenlargethegroundisland

areatoaddGNDviasasmanyaspossible.Also,themethodiscapableof

generatingtheminimumswitchingnoiselooparea.

72

Author : Criterion

IftheGNDviasarenotmanyenough,theimpedanceofthegroundislandmaybe

notlowenoughaswellastheswitchingnoisemaycoupletoothergroundarea,

therebyleakingintootherchipsthroughground.Inthatway,thegroundislandis

notabletoisolatetheswitchingnoisethoroughly.

Nevertheless,donotextendthegroundislandtoinputcapacitorsarea.Although

thismethodcanalsoenlargethegroundislandareatoaddplentifulGNDvias,

theswitchingnoiseloopareaenlargeseventhoughSMPSpowerinductorand

inputcapacitorsareextremelyclosetoPMIC.

73

Author : Criterion

Inlayer2,theareabelowgroundislandontoplayershouldbegroundislandas

well.Also,thereshouldbenotracesonthesegroundislands.

Besidestheinductorvalue,placement,andlayout,itsorientationhasalarge

impactonsensitivitytoo[50].

Asshownabove,thepowerinductorisawire-woundtypewithexposedwire

endsononeside.ExposedwiresfacingPMICresultinradiatednoisepickedup

byPMIC,therebyaggravatingGPSperformance[50].

74

Author : Criterion

Furthermore,thepowertracefromSMPSpowerinductorandinputcapacitor

maybeprovidedforotherchips.Donotusedaisy-chainconfigurationwith

sharedpowertracesfromcapacitortomultiplechipsthatareBGApackage

type[51].Sincethetransientcurrentornoisefromupriverpinsmayleakinto

downriverpins,asshownbelow:

Also,thepowertracelengthfordownriverpinswillbeverylong,thereby

deterioratingEMInoiseandIRdrop.

75

Author : Criterion

Instead,usestarconfigurationwithdedicatedtracesfromcapacitortoeachchip

pin.

Inthatway,eventhoughthereistransientcurrentornoisefromupriverpins,

whichwillbefilteredbycapacitorratherthanleakingintodownriverpins.Thus,

thepowertracelengthfordownriverpinswillnotbetoolong,therebymitigating

EMInoiseandIRdrop.

76

Author : Criterion

Nevertheless,branchatcapacitoronlyinsteadofotherplacesawayfrom

capacitor[51].

Otherwise,thetransientcurrentornoisefromupriverpinsmaystillleakinto

downriverpins[51].

77

Author : Criterion

InadditiontoRFtrace,I/Qsignal,andpower,careneedstobetakenforhigh

speeddigitalsignalaswell.SuchasSSBI(Single-lineSerialBusInterface),

whichisclockedat19.2MHz(referenceclockfrequency)andshouldbewell

isolated,sogoodlayouttechniquesareextremelyimportant[51].

DDR(Double-Data-Rate)clock,suchas50MHz,100MHz,200MHz,400MHz,and

533MHz,theharmonicsmaygeneratewidebandjammerradiatingintoGPS

antennathroughlikelyradiationpathsuchaspower/GND[10].Forexample,the

fourthorderharmonicsof400MHzDDRclockcauses15dBdesenseto

GLONASS(1600MHz)[10].Changingtheclockfrequencyisapossiblemethodto

mitigatetheissue.

78

Author : Criterion

Goodplacementisshownasbelow[104]:

AlmostallthecomponentswithhighspeeddigitalnoisearefarawayfromGPS

antenna,andtheirdistancearelistedasbelow[104]:

Badplacementisshownasbelow[104]:

ThesubcameraistooclosetoGPSantenna,andthedesenseisroughly10dB.

79

Author : Criterion

AsformicroSDcard,thebadsocketsareshownasbelow[104]:

Theirshieldingeffectisverybad,anddesenseissuemayoccur.Besides,the

relatedtracesofmicroSDcardshouldbeshortandroutedininnerlayerfor

striplinesurroundedbygroundplanesprovideshigherisolationthanmicrostrip

line.Furthermore,weareabletoaddabsobersonEMInoisychipstomitigatethe

desenseissue[106].

80

Author : Criterion

AsforFPC(FlexiblePrintedCircuit),takeLCDforexample,

SincethereisalinkbetweenLCDandPCBthroughFPC.Namely,regardlessof

noisesource,whichisfromLCDorPCB,unnecessaryradiatingnoisesare

producedfromtheFPCactingasanantenna.GPSantennamaypickupthe

radiatingnoisesanddesenseissueoccurs[105].

81

Author : Criterion

Thus,ametalfoilshouldbeattachedtotheFPCforshieldingunnecessary

radiatingnoises[105],asshownbelow:

InadditiontocoveringtheFPCwiththeshieldinglayer,theshieldinglayer

shouldelectricallybestuckonthemetalframeratherthanbeingfloating.Since

themetalframehasalargemetalsurfaceareaworkingasastablegroundand

theradiatingnoisesareeffectivelyreduced.Otherwise,theunnecessaryradiating

noisesarenoteffectivelyreduced[105].

82

Author : Criterion

Certainly,ifpossible,keepFPCawayfromGPSantenna[106].

83

Author : Criterion

Let’sintensifytheimportanceofgroundingfurther.Ifwefixontheclockrateof

LVDS(Low-voltagedifferentialsignaling)ofLCM(LCDModule),60MHz.As

shownbelow,therewillbespursappearingatregularintervals(60MHz)dueto

harmonicsinspectrum.The26thorderharmonicsisnearGPSsignal

(60MHz*26=1560MHz),andthenapproximately10dBdesenseoccurs[104].

84

Author : Criterion

Consequently,weneedtoaddgasketonPCBtopsidetointensifythegrounding

betweenPCBandLCMmetalframe(markedasA),andsodoesPCBbottomside

tointensifythegroundingbetweenPCBandmetalbackcover(markedasB),as

shownabove[104].

Certainly,youcanmodifytheclockrateaswelltopreventGPSsignalfrombeing

interferedbyharmonics[104].

85

Author : Criterion

Besidesgasket,duringtheinitialdesignstages,addgroundviasonallthethe

shieldingframepadsasmanyaspossibletointensifythegrounding.

Otherwise,theEMInoisefromnoisychipmayradiatetoGPSantennathrough

shieldingcanactingasradiator(duetocavityresonatormechanism).

86

Author : Criterion

Also,thismethodisabletoprovidetheRFblockwithmoreeffectiveshielding

effectandpreventRFblockfrombeinginterferedbyoutsidenoise.

Asforshieldingcan,youcannotstrengthenitsgroundingtoomuch.Inwireless

test,sometimes,poorgroundingisworsethannogroundingforshieldingcan

willactasradiatorasaresultofcavityresonatormechanism.

87

Author : Criterion

ThereisoftenbacklightdriverchipforLCMapplication.TheSMPStrace

generatesstrongEMInoise,asshownbelow[106]:

88

Author : Criterion

Sincepowerinductorisusedtostabilizetheswitchingtransientcurrentand

reducetheripple.Inotherwords,thecurrentbetweenbacklightdriverchipand

powerinductorisextremelyunstable,whichisaterriblystrongaggressor,as

shownbelow[106]:

Consequently,thetracebetweenbacklightdriverchipandpowerinductormust

beshort,andthetraceposteriortopowerinductorshouldberoutedininner

layerasaresultofbetterisolation[106].

89

Author : Criterion

Besides,theshieldingcanoughttobealsowelldesigned,especiallyfornoisy

chips.TakeMMD(MemoryModuledesign)forexample[104]:

Asshownabove,theMMDshieldingcanwithgapsisveryclosetoantennafeed

point(1cm),andtheEMInoisemayleakintoGPSantennathroughthesegaps.

Initially,thedesensewasapproximately10dB.Aftersealingthesegapswith

copperfoil,

Thedesenseimprovedabout5dB.Thus,inadditiontokeepingthenoisychips

area(includingshieldingcanandconnector)awayfromGPSantenna,sealing

theseshieldingcangapsisimportantaswell.

90

Author : Criterion

Moreover,perhapstherearenumeroustestpointsonthesehighspeeddigital

signal,especiallyinterfacesuchasI2S,PCIE,etc..Yououghttothinkofthemas

noise-generatingaggressorsfortheyareaportionofthesehighspeeddigital

signalaswell.Therefore,allofthemmustbetakenawayfromGPSantenna,or

desenseissuewilloccuraswell[106].

Evengroundpad,youshouldregarditasaportionofthesehighspeeddigital

signalaswell,andtrytokeepitawayfromGPSantenna[106].

91

Author : Criterion

MIPI(MobileIndustryProcessorInterface)isoftenusedasLCMinterface,which

isrichinharmonics[106].

Someusersmayusedynamicwallpaperfortheircellphones,

92

Author : Criterion

ThedynamicwallpapermaycauseseriousdesenseissueduetoMIPIof

LCM[106].ThereisusuallyEMIfilterontheMIPIpathofLCD.

Forthosedifferentialhighspeeddigitalsignals,weoftenfocusonthecommon

modenoiserejection.Asaresult,selecttheEMIfilterwithbettercommonmode

noiserejection.

MaintainasolidgroundwithnobreaksintheplanereferenceforMIPIclocksto

providealow-impedancepathforreturncurrents[103].IftheMIPItraceislong,

routeitininnerlayerforstriplinesurroundedbygroundplanesprovideshigher

isolationthanmicrostripline[106].

93

Author : Criterion

Moreover,SSC(spreadspectrumclock)isalsoatechniquetomitigatedesense

issue.

Asshownabove,thetechniqueisabletospreadtheMIPIclockenergywithwider

bandwidth,andthenavoidalltheenergyisconcentratedinGPSchannel.Thereal

spectrumisasshownbelow[106]:

Surely,tomodifytheMIPIclockfrequencyisalsoatechniquetomitigatedesense

issue.

94

Author : Criterion

BesidesFPC,connectoristheconnectionbetweenPCBandLCMaswell.Inother

words,thenoise,whichisfromLCMorPCB,mustgothroughconnector.Thus,we

havetokeepinmindthatconnectorisanaggressorrichinnoise.Let’sanalyze

thefollowingcase[106]:

ThenoisecouplestocameradatabusfromLCMconnector,andthenoiseflows

intocameramodulethroughconnectorandFPC.SincetheFPCisnearGPS

antennaandactsasanantenna,whichradiatesnoisetoGPSantennaandcauses

desenseissue.Consequently,avoidmakingLCMconnectorbeingclosetotraces

ofthecomponentsnearGPSantenna.

95

Author : Criterion

Let’sanalyzethefollowingsimilarcase[106]:

AsaresultofthefaultthatmemorytracesoverlapmicroSDcardtraces,andthe

microSDcardconnectorisnearGPSantenna.Thenoisefrommemorytraces

couplestomicroSDcardtraces,flowingintomicroSDcardconnector

,andthentheradiatingnoisefrommicroSDcardconnectoractingasaradiator

ispickedupbyGPSantenna.Therefore,desenseissueoccurs.

Asmentionedearlier,ifpossible,keepanyFPCandconnectorawayfromGPS

antenna.

96

Author : Criterion

AsforSIMcard,

Asshownabove,theSIMcardtracesontoplayerweretoolong,whichcaused

approximately5dBdesense[106].Aftercoveringthetoplayertraceswithcopper

foilandintensifyingthegroundingofSIMcardconnector,thesensitivity

improvedroughly3dB[106].Consequently,asmentionedearlier,thenoisytraces

shouldbeshortandroutedininnerlayerasaresultofbetterisolation.Moreover,

asshieldingcan,youcannotstrengthenthegroundingofconnectortoomuchfor

poorgroundingisworsethannogroundinginwirelesstestduetothefactthat

connectorwillactasradiatorbecauseofcavityresonatormechanism.

97

Author : Criterion

AsforLNA,weoughttopayattentiontonotonlyRFtrace,butalsotopowerand

enabletrace[104].

Asshownabove,theLNAenabletraceissurroundedwithLCMRGB

(Red,GreemBlue)traces,andthenoisemayleakintoLNAthroughenabletrace,

therebycausing10dBdesenseissueinconductedtest.Asshownbelow[104]:

98

Author : Criterion

Furthermore,careneedstobetakeninthethermalplacement[103]:

Duetothermalnoise,withhighertemperature,comesworsesensitivity.Some

considerationsarelistedasbelow:

l KeepthePAawayfromtheotherheatsources.

l Keepveryhotcomponentsawayfromthebattery.

l KeepthePMICawayfromthebasebandchipset.

l KeeptheXOsawayfromtheheatsources/gradients.

99

Author : Criterion

Asforheatflowunderthethermalcomponents,firstly,itisimportanttofillthe

heatsourcemountsidewithcopper.Ahighercopperdensityoralargeamount

ofcopperprovidesbetterthermalreliefandheattransport[103].

Secondly,addampleGNDviasunderornearthehotspots,andconnectthem

directlytomaingroundplane.ViasonthePAgroundpadareveryimportant,and

shouldbeasmanyaspossible.

100