单片机外文文献翻译基于MSP430的FSK调制解调.docx
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单片机外文文献翻译基于MSP430的FSK调制解调.docx
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单片机外文文献翻译基于MSP430的FSK调制解调
中文翻译材料
英文题目FSKModulationandDemodulation
WiththeMSP430Microcotroller
中文题目基于MSP430的FSK调制解调
学院:
计算机科学与技术学院
专业:
通信工程
学生姓名:
指导教师:
二O一三年六月
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Copyright♥1998,TexasInstrumentsIncorporated
Contents
1Introduction...................................................................................………………………1
2DemodulationTheory..........................................................................
2.1ChoosingtheSamplingRate...............................................................
2.2FrontEndProcessing.....................................................................
2.3FSKDemodulation........................................................................
2.4BitSynchronization.......................................................................
3ModulationTheory.............................................................................
3.1ChoosingtheSamplingRate...............................................................
3.2ConstructingtheLookUpTable............................................................
3.3FSKModulation..........................................................................
2
2
2
2
3
4
4
4
4
4DataConversion..............................................................................………………………..5
4.1A/DConversion.........................................................................………………………….5
4.2D/AConversion.........................................................................………………………….5
5PowerConsumption...........................................................................………………………6
6ExercisingtheSoftware........................................................................…………………………7
6.1FSKReceiver...........................................................................…………………………..7
6.2FSKTransmitter.........................................................................…………………………..7
7ExampleCircuits..............................................................................………………………...8
7.1UsingtheMSP430C325asMainProcessor.................................................……………………………….8
7.2ExampleTelephoneInterface.............................................................……………………………..8
8Summary.....................................................................................……………………….10
9References..................................................................................………………………….11
FSKModulationandDemodulationWiththeMSP430Microcontroller
i
FSKModulationandDemodulationWiththeMSP430
Microcontroller
ABSTRACT
ThisapplicationreportdescribesasoftwareprogramforperformingV.23FSKmodem
transceiverfunctionsusinganMSP430microcontroller.Itmakesuseofnovelfilter
architecturetoperformDSPfunctionsonaprocessorwithonlyshiftandaddcapabilities.
1Introduction
Manymeasurementapplications(forexample,electricandgasmeters)require
awaytocommunicateelectronicallywithacentralofficesothatmeasureddata
canbereportedbacktothecentralofficeandnewtariffscanbesetintheremote
site.Telephonyprovidesaconvenientmeansofdatacommunication.
Frequencyshiftkeying(FSK)anddualtonemultifrequency(DTMF)aretwo
popularmethodsofrepresentingbinarydataovertelephonecircuits.This
applicationreportdescribesaV.23-compliantFSKtransceiversoftwaremodule.
Integratingthemeasurementandcommunicationfunctionsontothesamechip
yieldscostaswellaspower-savingbenefits.UsingtheMSP430,ahighMIPsultra
lowpowermicroprocessor,allowspowertobedrawnfromthetelephonelinein
somecases.
ThisreportdescribesthemathematicalformulasforFSKsignaltransmissionand
detection.Alistofthesoftwaremodulesisincludedwithareferenceschematic
fortelephoneinterfaceandlowcostA/Dconverter.Theschematicisonlya
reference,sincethepreciseimplementationcanvaryfromcountrytocountry.
1
DemodulationTheory
2DemodulationTheory
AquadraturedemodulatorprovidestheFSKdemodulation.Inthistypeof
demodulation,thesignalanditsdelayedversionaremultipliedtogetherandthen
low-passfiltered.Ifthedelay,T,issetsuchthatWcarrier⋅T=ð/2,thenthe
low-passfilterresultisproportionaltothefrequencydeviationfromthecarrierand
thereforerepresentsthebitvaluesent.
IfwWcarrier
wherew=2π⋅f:
+
"WdeltaandT
+
Wcarrier
+p2
³
2.1
cos[wt].cos[w(t–T)]coswT
coswTsin["Wdelta]
+
)cos(2wt–wT)³LowPassFilter
+"sin[Wdelta]
_
ChoosingtheSamplingRate
ThesamplingischosentobeFcarrier4forthepurposeofobtainingthe
delayedsamplewithoutcomputationaloverhead.ForV.23,theFcarrier
frequencyis1700Hzandthereforethesamplingratebecomes6800Hz.Using
a32768-Hzcrystalyields6793.3Hz,whichis0.1%out.Thesamplingfrequency
issetbythe8-bitintervaltimer.Becausethistimerislimitedto256counts,the
interruptratedissettotwicethesamplingrateandtheprocessingisdividedinto
twohalveswithsignalsamplingperformedeveryotherinterrupt.
2.2
FrontEndProcessing
MostA/Dconverters,includingthesuccessiveapproximationA/Dconverterin
theMSP430C325,needadcbias;thisyieldsanunsignedintegersamplewith
anoffset.Beforethissamplecanbeprocessedfurther,itneedstogothroughan
unbiasfiltertotakeoutthedcbiasandturnthesampleintoasignedintegervalue.
Thisunbiasfilteringalsogives30dBorsoofrejectionformainfrequencies.
2.3
FSKDemodulation
Thesignedintegersampleanditsdelayedversionaremultipliedtogether;inthis
application,an8×8signedmultiplicationloopisused.
Theproduct,madeupoftwofrequencyelements,islow-passfilteredtoremove
thedoublefrequencyelement.Theremainderisasignedintegervalue
representingtheoriginalbitvaluetransmitted.
Thelow-passfilterusesthedigitalwavefilteringtechnique.Thistechniquegives
stablecharacteristicswithverygoodcoefficienttolerance.Allmultiplicationis
donethroughshiftsandaddswithth
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