AN-1811 Bluetooth Antenna Design (Rev. B) - TI.com

1 ApplicationReportSNOA519B March2008 intendedfordesignersusingtheLMX5251orLMX 5252 Bluetooth ,matchingcircuitdescription, Non-50 (PIFA).. March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, , , , , (PhycompAN-2700orMurataANCM12G) (MitsubishiMaterialsAHD1403) March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, >50%canbeconsidereda ,a countlessvarietyofantennascanbeused, : WireMonopole Thisconsistsofa simplewiresolderedatoneendfromwhichit is is thatit is notlowprofilebecauseit projectsabovethePCB. PIFA ThePrintedInvertedF Antennais likea monopoleprintedona PCB,butit hasa groundpointandfeedpointalongthemainreson antstructure. Helix Similartothewiremonopole,exceptthatit is coiledarounda centralcore(usuallyair) providesexcellentperformance,butprojects abovethePCB. Ceramic Surfacemountdielectricantennasarethesmal lesttypesofantennasavailable,becausethey areprintedona high-dkceramicslab,whichmakestheelectric fieldconcentratedallowingtheantennatobem adesmallwhilekeepinga ,low-profile,smallest, >100 MHzwithVSWR< >60%.

2 Theantennawilldetuneif anyobjectis placedclosetoit (initsnearfield).Thishasaneffectofpullin gthefrequency, criticalin staticornon-acceleratingchargewillresult in a ,considera printed g/4elementonmicrostrip,asshownin March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, a networkanalyzeris connectedtothefeedpoint,it wouldindicatea ,thegroundplanemustbemovedawayfromthemic rostripelementwhichmakesthefringingfield covermoredistance,asshownin Figure2. Butit shouldbenotedthatif thegroundplaneis movedtoofar,thenthefringingfieldstopsalt ogether,andthereis ,thepositionandsizeofthegroundplaneis vitalin thedesignofa March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, ,transformingtheimpedanceofa microstripline(50 ) tothatoffreespace(377 ), whichallowsthepowertobetransferredfroma whichthephysicalsizeis muchsmallerthenwavelength(L<< ) is almostsymmetricalin alldirections,asshownin Figure3.

3 ThepatterncanbecontrolledonlywhenL is similarorgreaterthan .5 SNOA519B March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, networkanalyzerlookslikethatshownin Figure4, withthefullbandcoveredwithVSWR< ,howeverin realconditionswhentheantennais detunedduetohandlingorplacementofcompone ntsclosetoit, a VSWRof3 to4 March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, 20to25mm, requiredaroundtheradiatingelementwhichis fedfroma pointalongit, asshownin Figure5. it wereanysmaller,it three-elementmatchingnetworkgoingintothe feed, , (PIFA)ThePIFAis placedontheedgeofthemotherboardPCB,assho wnin Figure6. Theareaaroundthecorneris keptcopper-free, March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, ceramicdielectricantennais smallerthana PIFA oranyotherPCBantennabecausetheactiveelem entis woundarounda high-dkceramicslab, PIFA,a copper-clearedareaanda groundplanearerequired,asshownin Figure7.

4 A smallergroundplanecanbeused, March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, shownin shownin March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, PIFA,in thatit canbedetuned,hasa symmetricalradiationpattern,andhasaneffi ciencyofapproximately70%. March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, ,approximately14 10mmin verysimilartothatoftheLMX5251/LMX5252, ,themetalshieldprotectsthecomponentsin themodulefromtheelectricfieldoftheantenn a,soit is ,theshieldingalsoactsasa groundplane,solessunpopulatedgroundareai s ceramicdielectricantennashownin Figure12, themoduleis theshieldis well-grounded, March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, smallerthanthe9820/A,approximately6 9mm,howeverit is cannotbeplacedasclosetotheantennaactivee lement,elsetheE-fieldmaygiverisetounwant edcouplingeffects, ,suchassurface-mountceramicdielectricant ennas,willbematchedto50 inputimpedancewithreturnloss<-7dBover100 MHzbandwidth, ,thisis onlyasmeasuredonthemanufacturerstestboar d,in ontheapplicationPCB.

5 In whichtheground-planelayoutmaybedifferent ortheremaybedetuningcomponentssuchasfilt ersplacednearby, three-elementPInetwork, capacitorpairandaninductor,oraninductorp airanda capacitor, ,one-portonlymeasurementsusingtheopen,sh ort, March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, ,NoConnectionBeforesolderingthesemi-rigi dcabletothePCB,it is connectedtotheendofthenetworkanalyzercab le,andtheelectricaldelayis shortcableattachment(lessthan5 cm), adjusteduntilit is measuringa perfectshortontheSmithChartasshownin , ata ,it is importanttohavethesetupona woodenornon-detuningsurfaceandtokeepyour handsawayfromthesetup, typicalreturnlossmeasurementis shownin March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, ,AntennaConnectedInthisexample,theresona ntfrequencyoftheantennais ,thereturnlossis , ,it impedancehastobetransformedto50 , asshownin March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, (point1)

6 ,adda shuntinductorwilltransformtheimpedanceat point2 tothecenterofthechart,whichis thenormalized50 shownin onlya ,theinductorshaveparasiticresistanceandc apacitance, ,theexactvaluesshownabovemaynotbeavailab lein a populartypeofmatchingnetworkis thePI-network,consistingoftwoshuntcompon entswithoneseriescomponentin ,it ,considerthedatapointontheSmithChart:( + ) March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, tomovearounda constantresistancecirclefromposition1 to2 byaddinga seriescapacitanceandthenfrom2 to3 aroundaconstantconductancecirclebyadding a shuntcapacitance,asshownin March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, tomovearoundtheconductancecircleandthent heresistancecirclebyaddingashuntandserie scapacitancerespectively,asshownin ,a PI-padmustbeusedwiththeredundantgapbridg edusinga zero- ,sofarwehaveonlymatcheda single-pointfrequencyto50.

7 Inthecaseofa realpassivedevicesuchasanantenna,theenti reBluetoothbandhastobematchedascloselyas possibleto50 . Atleastthreefrequencypointshavetobematch ed,asshownin March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, broadbandmatchtoonefrequencypointis thattheothertwowillgoevenfurtherout!Fore xample, byaddinga shuntcapacitor, (advanceddesignsystems).If thisis notpossible,thena lotofmanualtweakingis ,theinputimpedanceofthedetunedantennais measuredusinga networkanalyzerandsavedasanS-parametersb lock, ,asshownin ,it is March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, ,themodelis enteredintoADS,andanoptimizationprocedur eis setuptoreduceS11asmuchaspossiblein it is lower,thenthecomponentsaretweakedagainin thesamedirectionuntilthebestoptimizedsol utionis , Non-50 ActiveSource/LoadImpedanceIf thesourceandloadimpedancesarebothnon-50 , thentheycanbematchedin muchthesamewayasa 50 impedanceasshownin March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, ImpedanceInthisexampletheloadis at20+ j10andthesourceat84+ seriescapacitanceandshuntinductanceis , theycanlieanywhereontheSmithChartandmust bemeasuredusingavectornetworkanalyzer(VN A)

8 Receiverorpassiveantennais ,whenmeasuringtheoutputimpedanceofa poweramplifierortransmitter, , a ,it caninfluenceitsoutputpowerwhichis perfectconjugatematchis appliedtotheoutputofthePA,thepowermeasur edonthepowermeterwillbeatitsmaximum, March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, achieved, detachedfromthedirectionalcoupleranda theconjugateoftheoutputimpedanceofthePA, orif theimpedancemeasuredontheVNAis R + jXthentheoutputimpedanceofthePAwillbeR - :thetermconjugatematchmeansthatif in onedirectionfroma junctiontheimpedanceis R +jX,thenin theoppositedirectiontheimpedancewillbeR load,in whichtheimpedanceseenlookingtowardtheloa data pointin a transmissionlineis , twooptionsareavailablefortheantennadesig ner,eitherassumea 50 pointfora simplerdesign,in whichcasea smallmiss-matchwillresultcausinga smalldegradationin matchingnetworkasdescribedabovebetweentw onon-50 50 input/outputimpedanceis assumedthepowerlossthatwillresultsis asfollows.

9 WorsecaseRxinputimpedance= 32 VSWR atthispoint= 10 LOG(S11)= 7dBThroughtransfercoefficientS21= SQRT(1-[S11]^2)= [S21]^2= non-50 March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, frontoftheantennais filterwouldlookdisplayedona hasthreemainfeatures:withinthepass-bandi s anunwantedInsertionLoss(IL)whichattenuat esthetransmitandreceivesignals,outsideth epass-bandis a wantedrejectionwhichattenuatesinterferen ce,andattheedgesofthepass-bandis thefilterroll-offwhichshouldbeassteepasp ossibletoforma a capacitorandinductorin ,considertheresponseofa singlecapacitorandinductorin seriestoa Figure30, atDCit hasveryhighIL, ;atDCitsILis ,andtheratebywhichit changeswillbedependentonits Q (steeproll-off)ata , is importanttonotethatthehigherthefrequency ofthefilter, 1-pFcapacitorin serieswitha March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, ,evena ,it willprovide10dBofrejectionbelow1 , betterbutmoreexpensivesolutionis Muratafilteris (fo)2450 MHzBandwidth(BW)fo 50 MHzInsertionLossin @25 CInsertionLossin @-40to+85 @1710to1910 MHzAttenuation(AbsoluteValue)III6 @4800to5000 MHzRipplein BW2 (Nom.)

10 50 +85 CTheILmaybeslightlyworsethananLCfilter,b uttheout-of-bandrejectionis significantlybetter(20to30dB).Thefilteri s approximately2 size,andit is ratedoverthefullautomotivetemperatureran ge(-40to+85 C).A noteworthybutunwantedfeatureis , , March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, wantedsignalis setto2460 MHzat3dBabovereferencesensitivity,andani nterferingsignalis appliedatthelevelsshownin steppedin intervalsof1 ,andateachofthesepointsthebiterrorrate(B ER) totalof24exceptionsareallowed,becauseit is ,eitherduetodirectsaturationofthefronten dif thelow-noiseamplifier(LNA)is , , ,theBluetoothQualificationTaskForce(BQTF )usestheTS8960testsettolinktothedeviceun dertest(DUT)andplaceit ona signalgeneratorandcombineris controlledwithautomatedtestequipment(ATE ), ,howeverattimestheBERoncertainblockingfr equenciesgoessohighthatthelinkis dropped,anda , is theresponsibilityoftheDUTmanufacturertot estthesefailingfrequenciesmanuallyanddet erminewhetheradditionalfilteringis ,theATEsystemsometimeslogsmorefailuresth anareactuallypresent, Figure31orFigure32is fora ceramicfilterandblockingcapacitorwithgoo dout-of-bandrejection, Figure32maybeusedtoforma March2008 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2008 2013, PIPad6 AntennaVendorsTable3 March2008 RevisedMay2013AN-1811 BluetoothAntennaDesignSubmitDocumentatio nFeedbackCopyright 2008 2013, (continued) ,Shibuya-kuTokyo150-0002 JapanForexample,M1seriesPhone:+813 54696123 Fax:+813 approximately15monopoleefficiency,doesno trequireofthePCBmmlong, approximatelyceramicchipon ,12mmlong, approximatelyorotherprintedon , Manytypesofantennasareavailable.