gps rf front end considerations
TRANSCRIPT
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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Author : Criterion
Spreadingcodeshaveverylowcross-correlationandareuniqueforeveryuser
(lowinterferencewithothersignals).Transmissionbandwidthismuchwider
thanwhichofinformation.Asshownbelow[94]:
TheL1signalisaBPSKsignalmodulatedinphasebytheC/A-codeandthe
informationofthenavigationmessage.TheL1frequency=1575.42MHz
=154x10.23MHz.AndthechippingrateofC/Acodeis1.023Mcps[49].
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Author : Criterion
Mostcommercialfront-endshaveahighgainexceeding80dB.Thehighgainis
achievedatacostofeitherahighnoiselevelorhighpowerconsumption[49].For
example,theRTR6285AofQualcommhasapproximately82dBgain.Because
GPShas43dBprocessinggain,theGPSbasebandsignalhas123dB(80+43=
123)gain[49].
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Author : Criterion
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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Author : Criterion
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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Author : Criterion
AnotherreasonforrequiringalowerNFliesintheincreasinguseofGPS
receiversinurbanenvironments.Thereceivedsignalpowerisreducedinplaces
withhighbuildingsandnarrowstreets.Inthiscase,thereceiverisunableto
detectthesatellitesignalorprovidesanimpreciseposition.Moreover,another
factortoconsiderinurbanenvironmentsisthepresenceofinterferencesignals
thatthereceivercancapture,increasingCNRdegradation,andallthe
performance,suchasTTFF,positioningaccuracy,andsensitivity,willaggravate
aswell[49].
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Author : Criterion
Pre-SAW
Asshownbelow[31],
TheseoutbandblockersmayleakintotheGPSreceiver’spathandhaveagigantic
impactonthereceiver’ssensitivitybyoverloadingthereceiver’sLNAor
backend[31].Thestrongeroutbandblockeris,themoredegradationofSNRwill
be[41].
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Author : Criterion
Asshownbelow,iftheseblockersareclosetoLNA’sP1dB,thegainwilldecline
duetosaturation:
Asshownbelow,whentheblockerislargerthan-10dBm,thegainwilldrop
dramatically,whereastheNFwillincreasesignificantly[39].
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Author : Criterion
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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Author : Criterion
Asshownbelow,ALM-1912haspre-SAW,butALM_1612doesn’t.Ifwefixonthe
blockerstrength(-10dBm),itcanbeseentheNFofALM_1612increasesmuch
morethanwhichofALM-1912.That’swhyaddingpre-SAWpriortoLNA
forrejectionofthejammerisnecessarytoachieveacceptablesensitivitywith
strongoutbandblocker[55].
13
Author : Criterion
Furthermore,weneedtoputDC-blockatLNAinputtoavoidmakingDCOffset
feedintoLNA.
Nevertheless,theDCblockatinputisoptionalasitisusuallyprovidedbythe
pre-filterbeforetheLNAinmanyGPSapplications[75].
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Author : Criterion
AccordingtoFriisformula:
Theimportantvariablescontributingtosystemsensitivityarethepre-SAW’s
IL(insertionloss)andLNA’sNF,whileLNAsubsequentblockshaveminimal
impact[28]:
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Author : Criterion
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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Author : Criterion
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.
18
Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
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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Author : Criterion
Let’stakeAVAGOALM-1912forexample,theschematicisshownabove,L2isa
RFchokeisolatingtheGPSsignalfromtheDCsupply[55].Moreover,theL2
biasinginductortogetherwiththeC2bypasscapacitorsetstheoutputmatching.
Asaconsequence,L2hasinfluenceonLNA’sperformanceaswell.
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Author : Criterion
Inputandoutputreturnloss:
Gain:
NF:
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Author : Criterion
Linearity:
Assummarizedbelow:
Asaresult,inadditiontomatching,youcanalsoadjusttheL2valuetoachieve
thebestperformance.
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Author : Criterion
Ingereral,withhigherVdd,comesbetterlinearity.Asshownbelow[55]:
IfwefixontheVdd(2.2V),itcanbeseenthatlinearityvariesfordifferentIdd.
Thus,weareabletoadjustR2valuefordesiredIddcurrenttoachievebest
linearity[55].
Inadditiontolinearity,otherperformancecanalsobeobtainedbyincreasingthe
supplyvoltage[74].
Consequently,weshouldpreventsupplyvoltagefromIRdroptopreventfrom
deterioratingperformance.
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Author : Criterion
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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Author : Criterion
Asmentionedearlier,poorlinearityresultsinpoorsensitivity,therebyincreasing
theTTFF.Asaresult,weoughttoselectaLNAwithrelativelygoodlinearity.With
thesamelevelblocker,thebetterLNA’slinearityis,theshorterTTFFwillbe[25].
Ingeneral,theLNAP1dBshouldbeatleast-5dBm,andIIP3shouldbeatleast5
dBm.
.
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Author : Criterion
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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Author : Criterion
Takingpreviouspre-SAWconsiderationsintoaccount,theinputimpedanceof
themixerhasbeensettooptimizethepowerconsumption,gain,NF,and
linearityoftheentireRFmixer[49].
Nevertheless,asshownabove,themodernGPSreceiveralmosthasnoexternal
matchingcomponentsbetweenLNAandmixer.
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Author : Criterion
But,somereceiversmayaddexternalSAWfilterbetweenLNAandmixer.It
indicatesthattherewillbeexternalmatchingcomponentsatmixerinput.As
mentionedearlier,theinputimpedanceoftheexternalSAWfiltermustbe50Ωto
avoiddegradingSAWfilterperformanceduetomismatchedimpedance.Similarly,
theinputimpedanceofmixermustbealso50Ωtopreventfromdeteriorating
mixerperformance(powerconsumption,gain,NF,andlinearity)dueto
mismatchedimpedance.
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Author : Criterion
Inordertogetlowconversionlossfrommixers,typicallyahighLOpoweris
needed[49,69].
Asshownabove,thehigherLOpoweris,thehighergain(thelowerconversion
loss)ofmixerwillbe.Especiallyforapassivemixer,whichactslikeswitches
controlledbythesignalfromLO[38].ThehigherLOpowerisessentialtoachieve
thelowerconversionloss,therebybeingabletodrivetheADC[40].
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Author : Criterion
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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Author : Criterion
VCO
Voltagecontrolledoscillator,asthenameimplies,theoutputfrequencyis
dependentoncontrolvoltage[49]:
Asaconsequence,VCOsareverysensitivetonoiseonthesupplysources[45].To
achieveaccuratefrequency,theprecisetuningvoltageisessential[23].
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Author : Criterion
Asshownabove,theidealspectrumofoscillatorshouldhasno“skirt”.
Nevertheless,inrealworld,it’simpossible.Ifthewaveformofoscillatorsignalin
timedomainhastimingerror,i.e.,phaseerrororjitter,therewillbephasenoise
resultinginlarger“skirt”,therebyaggravatingsensitivityinspectrum.Besides
phaseerror,phasenoiseisgeneratedbypowersupplyinputaswell[45].
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Author : Criterion
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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Author : Criterion
Asshownbelow,ifthewaveformissinglesinusoidalsource,thereisnospursin
spectrum[48].
Therealmeasurementinspectrum[49]:
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Author : Criterion
Nevertheless,singlesinusoidalwaveformcombinedwithasquarewavewill
generatealotofspurs,asshownbelow[48]:
Asaresult,weshouldkeepthesetraceswithhighfrequencynoiseawayfrom
VCOpowersupply.
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Author : Criterion
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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Author : Criterion
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