Frequency Shift Keying with LMX2571 - TI.com

1 1 SNAA309 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX2571 All trademarksare the propertyof May 2017 FrequencyShiftKeyingwithLMX2571 NoelFungABSTRACTF requencyShiftKeyingor FSKis one of the mostpopularmodulationschemesbeingusedin a low powerPLL with integratedVCO,it can supportFSKmodulationthroughprogrammingor focuson howto applydatato implementanalogFM and pulse-shapedFSKmodulationwith LMX2571in FSKI2S of of Tables1 FSKI2S FSKis one of the mostpopularmodulationschemesbeingusedin is becausethis modulationis very easyto implementin a simpletwo-levelFSK,theserialdatastreamis usedto modulatedthe databit is 0, outputfrequencyis databit is 1.

2 The differencebetweenf0and f1is the drawbackof this FSKis the occupiedbandwidthwill be very ordertoTimeVoltage+1 V-1 VScale toTimeFreq. dev.+2 kHz-2 kHzTime16-bit word0xA012h0x5 FEDhTimeFreq. dev.+2 kHz-2 kHzSamplingLMX2571 Theoryof May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX2571increasespectrumefficiency,the databit is usuallypulse-shapedbeforebeingusedto modulatethe this case,the discrete-leveldigitaldatastreamwill passthrougha digitalfilter,usuallya RaisedCosinefilter or a outputof the filter becomesa continuos-levelanalogsignal,no moreabruptjumpfrom0 to 1 and vice modulatingthe VCO,anotherway to createFSKmodulationisto "modulate"the N-counterof the methodis this case,the frequencydeviationis obtainedby changingthe a low powerPLL with integratedVCO.

3 It can supportdirectdigitalFSKmodulationthrough programmingor FSKSPI and FSKPIN mode,the LMX2571supportsdiscretelevelFSKmodulatio nthat allowsthe N-countervalueto be modulatedbetween2-, 4- or also supportarbitrarylevelFSKor pulse-shapedFSKin FSKSPI FASTand havea machineto manipulatepulse-shapingto incomingdatabits. Pulse-shapingof the databits has to be doneexternallywith a digitalprocessorbeforeapplyingit to OperationThe operationof directdigitalFSKmodulationis similarto the operationof a all, the FSK/FMmodulationsignalgets sampledamplitudesare transformedto a streamof analog-to-digitalconversionis ideallyhandledwith a high frequencycarriergets frequencymodulated,we are indeedinterestedin frequencydeviationvs time insteadof voltagevs beforeA-to-Dconversion,we should scale the voltageofthe modulationsignalto example,assumethe modulationsignalis a sine wavewith amplitudeswingequals1 V, and 1 V is equivalentto 2 kHz everysamplingpoint.

4 The equivalentfrequencydeviationis transformedto a DirectDigitalFSKM odulationWorkflowLMX2571acts as the digital-to-analogconverterin this acceptsthe streamof 16-bitwordandthenreconstructsthe +2 kHz-2 HzOSCin80 MHzPre-div1 MULT1 Post-div1fPD80 MHzVCO5000 ModulationExample3 SNAA309 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX25713 AnalogFM ModulationExampleIn this example,LMX2571is usedto generatea FM modulatedcarrier(similarto an analogFM radiotransmitter).The "modulationsignal"is a puresine 80 MHzRFoutfrequency= 500 MHzFM modulationfrequency= 1 kHzFM frequencydeviation= 2 kHzOver-samplingfactor= 192xOver-samplingrate = 192 kHzFractionaldenominator,DEN= the frequenciesin eachfunctionalblockof a completemodulationsignalperiod,thereare 192 otherwords,on every360 / 192= , the modulationsignalgets frequencydeviationof eachsamplepointis equalto 2 kHz * sin (phase* / 180 ), wherephase= 0.

5 , .Figure3. Over-SamplingFSK_D1 FSK_DVFSK_D0t3t2t1 AnalogFM May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith Equation4 and 6 in the LMX2571datasheet, calculatethe correspondingcodefor FSKI2S codeTIMEFREQUENCYDEVIATIONCORRESPONDINGF SKSTEPSBINARYEQUIVALENTt10 last, feedthe binarycodeto the LMX2571datasheetfor the detailsonFSKI2S FSKI2S AnalogFM SpectrumViewFigure6. AnalogFM Modulation-DomainViewFigure5 showsthe transmitspectrumwhileFigure6 showsthe carrieris movingaroundat a rate of 1 kHzand with a peakfrequencydeviationof 2 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX25714 RaisedCosine4 FSKE xampleThis is a "modulationsignal"is a 80 MHzRFoutfrequency= 500 MHz4 FSKmodulationbaudrate = kSpsRaisedCosinepulse-shapingfilter,BT = 648 Hz and 1944 HzOver-samplingfactor= 20xOver-samplingrate = 96 kHzFractionaldenominator,DEN= designproceduresare sameas the previousexample,we haveto go throughStep1 to Step4.

6 The differencein this exampleis the modulationsignalis not a simplesine wavebut a streamof signalis first over-samplingat a rate of 96 ,use Equation4and 6 to convertthe correspondingfrequencydeviationat eachsamplepointto I2S May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith RaisedCosine4 FSKS pectrumViewFigure9. RaisedCosine4 FSKM odulationQualityBecausethe FSKsignalhas beenpulse-shaped,the occupiedbandwidthis very minimal,as showninFigure8. The advantageof usingdirectdigitalmodulationis revealedin Figure9 wherewe can see thatthe modulationqualityis ,this techniqueis immuneto Process,Voltagerail andTemperaturevariationbecauseit is a is "modulationsignal"is a this example,the frequencydeviationis ,DEN,is deliberatelymadesmallerin orderto achievea 80 MHzRFoutfrequency= 470 MHz4 FSKmodulationbaudrate = 125 kSpsGaussianpulse-shapingfilter,BT = 17 kHz and 51 kHzOver-samplingfactor= 10xOver-samplingrate = ,DEN= exceptfor the modulationsignalis filteredwith a May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX2571 Figure10.

7 Gaussian4 FSKS pectrumViewFigure12. Gaussian4 FSKM odulationQualityEvenwith a higherdatarate as well as a higherfrequencydeviation,the modulationspectrumcan meetradiostandard' May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith a generalrule of thumb,the loop bandwidthhas to be greaterthanor at leastcompatiblewith thesamplingrate so that it doesnot degradethe the modulationis implementedbychangingthe N-countervalues, , we are modulatingthe PLL insteadof filterbehavesas a low passfilter to the PLL,out-of-bandmodulationwill get a consequence,loopbandwidthshouldbe greaterthanthe the modulationsignalis a squarewave.

8 As in thecaseof discreteFSKmodulation,loop bandwidthshouldbe at leasttwicethe datasheetof LMX2571 ,it is requiredthat the fractionaldenominatorhas to be equalto 224wheneverFSKis constrainwill thereforelimit the example,in Section4, the maximumachievablefrequencydeviationis just is indeedpossibleto makethe frequencydeviationlargerwith a way that FSKworksin LMX2571is by addingor subtractingthe necessaryFSKstepsfromthefractionalnumera torof the example,if the numeratorof a particularcarrierfrequencyis 1000and the necessaryFSKstep valuefor a 1 kHz deviationis get +1 kHz deviation,theinstantaneousnumeratorvalue is is a validnumeratorvalueso we will be able to get +1 kHzdeviationfromthe ,we are not able to get 1 kHz deviationbecausethe instantaneousnumeratorvalueis an invalidnumberof 1.

9 This situationis alreadytakencarewith whendenominatorisequalto orderto get a largerfrequencydeviation,we can makethe denominatorsmaller,as long as thenumeratorof the carrieris greaterthanthe necessaryFSKstep instance,if we put denominatorequalsto 8000000insteadof 224, the maximumfrequencydeviationwe can get alreadyillustratedin last, thereare a coupleof thingsto remember. Althoughthe fractionaldenominatoris 24-bit,we only use 16-bitto representthe frequencydeviation(or FSKstep). The maximumFSKstep valuefor positivefrequencydeviationis equalto 215- 1. FSKstep valuesare expressedin 2's complementso bothnegativeand positivedeviationcan of the advantagesof directdigitalFSKmodulationis the frequencydeviationis linearoverthe entirefrequencybandof , we can get the samefrequencydeviationat 100 MHzor is not possibleif we wereto modulatethe VCObecausethe VCOgain (akaKVCO)

10 Varieswith ,the adoptionof directdigitalFSKmodulationrequiresa highersignalprocessingpowercapabilityfro mthe is becausewe needover-samplingat a rate muchhigherthanthebaudrate of the N-countergets modulatedat a rate of M kHz,thereare sidebandsappearat multipleof M kHzoffsetfromthe magnitudeof the closestsidebandgets 12 dB downwheneverthe samplingrate is such,around100 kHz is the minimumsamplingrate we thesesidebandscan be reducedby the loop filter,if a highermodulationdistortionor FSKerroris acceptable,we can makethe loop bandwidthsmallerthanthe samplingrate in orderto reducethe amplitudeof +Mfc-Mfc+2 Mfc-2 Mfc+ May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedFrequen cyShiftKeyingwith LMX2571 Figure13.