AN100 - Crystal Selection Guide (Rev. C) - TI.com

1 ApplicationReportSWRA372C November2013AN100- CrystalSelectionGuideFredrikKervelABSTRA CTRF systemsare highlydependenton accurateclocksfor deviationin clockfrequencyis directlyreflectedas a deviationin degradeRF performance,violatelegalrequirementsor in worstcaselead to a scopeof this applicationreportis to give the designengineera quickguideon how to design,tuneand verifythe clockcircuitfor TI s list of suggestedcrystalsthat can be usedwithCC253xand CC254xis designermustpick a crystalthat satisfiestheirrequirementsonsize,toleran ceand othercrystalsare to be used,fundamentalmode,AT cut crystalsis highlyrecommendeddue to accuracyand easeof of AddedSeriesResistance[FA-128on CC2540EM].. trademarksare the propertyof November2013AN100- CrystalSelectionGuideSubmitDocumentation FeedbackCopyright 2013,TexasInstrumentsIncorporated'Cx' '1 2' '1 2 CCCLCC=+ circuitusedas high frequency,high accuracyclocksourcefor TI s low powerRF productsis calleda PierceOscillator and is shownin Figure1.

2 The oscillatorcircuitconsistsof an invertingamplifier(normallya regularinverter),a feedbackresistor,two capacitorsand a first two componentsare internalin the IC whilethe capacitorsand the crystalare externaland mustbe selectedfor ,the crystaland the capacitorsforma pi filter providing180 phaseshift to theinternalamplifier,thus keepingthe oscillatorlockedat the total toleranceof a Crystalis dependenton threefactors:productiontolerance,tempera turetoleranceand of thesevaluesare givenin [ppm](partsper million)and can be foundin the manufacturer' get the total maximumtoleranceof a certaincrystalall thesefactorsmustbe directlytransferredto RF deviation,thus a deviationin the crystaloscillatorfrequencyof X ppmleadsa deviationin carrierfrequencyof X ,it is importantto selectacrystalwith performancewithinthe limitsof the RF course,the end productsexpectedage and operatingconditions(temperature)mustbe takeninto (RF4CE),the max deviationin carrierfrequencyis limitedto 40 ppm.

3 [1]For BluetoothLow Energy,the limit is 40 ppm.[2] the crystaloscillatoris dependenton the valuesof the two externalcapacitors,C1andC2in Figure1. Thesecapacitorstogetherwith any parasiticcapacitancein the PCBand the crystalterminalscomposethe total load capacitanceseenby the optimumload capacitancefor thecrystal,CL, is givenin the crystaldatasheetand C1and C2shouldbe matchedto this valueaccordingtoEquation1:(1)Where,is the sum of the capacitancein Cx, the parasiticcapacitancein the PCBtraceand thecapacitancein the terminalof the sum of the two latterpartswill typicallybe in the rangeof2 8 CrystalSelectionGuideSWRA372C November2013 SubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporated0500100 015000123456x 10-4 Series resistance (exc. RSof Crystal ) [ohm]Startup time [sec]Startup @ 2 VStartup @ V5 RnegRs< easiestway to find the optimumload capacitorvaluesfor a givencircuitand layoutis radiocan be set to put out a constantcarrierat a givenfrequencywith measuringthe outputfrequencywith a SpectrumAnalyzer,the offsetcan easilybe (ESR)and NegativeResistanceRSor equivalentseriesresistance(ESR)is a parameterof the Crystal ,whichcan be foundin the a parameterof the completeoscillatorcircuit,includingcapac itorvalues,crystalparametersand ensurecorrectfunctionof the crystaloscillator,negativeresistanceshou ldbe at least5 timesgreaterthanRS.

4 (2)If this is not true the oscillatormightnot operateas expectedor it mightnot startup at negativeresistanceof the circuitcan be foundby introducinga resistorin serieswith the is recommendedto use a 0201resistorfor this thresholdof resistancewherethe oscillatoris no longerable to startup is approximatelythe sameas the exampleof resistancevs. startuptime and negativeresistancethresholdis higherload capacitorvaluesdecreasesthe negativeresistanceand risesthe StartupTimevs. AddedSeriesResistance[FA-128on CC2540EM] CC253xand CC254xradioshavethe possibilityto add extraload capacitanceon-chipfor capacitancevalueis adjustedby the 4 last bits (3:0)of the FREQTUNE defaultvalueof the registeris 0x0 Fwhichcorrespondsto no eachdecrementin the registervalue,extracapacitanceis addedto the oscillatorcircuit,reducingthe levelof tuningpossible,also knownas pullability,will differfromcrystalto crystalas showninFigure3.

5 Somemanufacturerspresentthis valuein the datasheetgivenin parametersas Rnegand startuptime are dependenton the load capacitance,so changingFREQTUNE valuealso showshow startuptime will vary with addedload bothFigure3 and Figure4 the load capacitanceis tunedsuchthat FREQTUNE= 0x0 Fcorrespondstoapproximately0 November2013AN100- CrystalSelectionGuideSubmitDocumentation FeedbackCopyright 2013,TexasInstrumentsIncorporated0123456 789 ABCDEF200220240260280300320340360380 FREQTUNE settingStartup time [us]TZ1983 ANX3225GA0123456789 ABCDEF-160-140-120-100-80-60-40-20020 FREQTUNE settingFrequency offset [ppm] FREQTUNEP ullabilityExampleFigure4. StartupTimevs. FREQTUNES etting2 SelectingCrystalsSomesuggestedcrystalsar e presentedin Table1 and Table2. Thesecrystalsare testedwith theCC2540on the two EVMboards CC2540 EMCrystalEval and CC2540 EMHC49smdEval.

6 The firstboardhas footprintsfor crystalsvaryingin size from2016through4025whilethe latterboardhasfootprintsfor testshavebeenperformedwith CC2540,the crystalswill havesimilarperformanceon anyCC253xor CC254xdevicesas crystalsare listedwith theirkey datasheetvaluesin Table1. Table2 containsdatameasuredon measureddatawill differfromboardto boardas a resultof layoutand validfor the TI crystalevaluationboards;othervaluesmight be a specificapplicationwill oftenbe dependenton threefactors:size (footprintarea,height),performance(accur acyovertemperature,lifetime)and cost (for example,higherperformanceandsmallerpacka ge= higherprice).4AN100- CrystalSelectionGuideSWRA372C November2013 SubmitDocumentationFeedbackCopyright 2013, DataSheetValuesTempAgainTempESRCL freqToleranceTolerance(ppm/RangeManufact urerMPNP ackage( )(pF)(MHz)(ppm)(ppm)year)[ C]EpsonFA-12820166010321010585-40 ToyocomEpsonFA-20H25206010321010385-40 ToyocomNDKNX3225GA3225501232203085-40 NDKNX3225JA32256010322020700 Tai-SawTZ1983 AHC49smd50103210201700 TechnologyTable2.

7 Measurementson TI EM @25 CStartuptimeNegativeFREQTUNEMPNC231C2212 V3 VResistancePullabilityFA-12812 pF12 pF290 s251 s1k -54 ppmFA-20H12 pF12 pF408 s352 s-35 ppmNX3225GA12 pF15 pF236 s216 vs820 > Rneg> 680-78 ppmTZ1983A10 pF12 PF228 s208 s-150ppm3 References1. (IEEE download)2. IEEEGet (IEEE download)5 SWRA372C November2013AN100- CrystalSelectionGuideSubmitDocumentation FeedbackCopyright 2013,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,enhancements,improvementsandotherchang estoitssemiconductorproductsandservicesp erJESD46,latestissue,andtodiscontinueany productorserviceperJESD48, (alsoreferredtohereinas components ) aresoldsubjecttoTI s ,in accordancewiththewarrantyin TI s ,testingofallparametersofeachcomponentis productsandapplications, ,eitherexpressorimplied,is grantedunderanypatentright,copyright,mas kworkright,orotherintellectualpropertyri ghtrelatingtoanycombination,machine,orpr ocessin licensetousesuchproductsorservicesora licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, is solelyresponsibleforcompliancewithallleg al,regulatoryandsafety-relatedrequiremen tsconcerningitsproducts,andanyuseofTIcom ponentsin itsapplications.

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