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].

2 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. 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.

3 Productiontolerance,temperaturetolerance and 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.[1]For BluetoothLow Energy,the limit is 40 ppm.[2] the crystaloscillatoris dependenton the valuesof the two externalcapacitors,C1andC2in Figure1.

4 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.

5 (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.

6 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.

7 Thesecrystalsare testedwith theCC2540on the two EVMboards CC2540 EMCrystalEval and CC2540 EMHC49smdEval . 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).

8 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. 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.

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