
2/12

LastUpdatedJune8,2011


Applic

ationN

ote

Whyusefiber?

TransmissionofRFandMicrowaveSignalsviawaveguidesorcoaxialcablesuffershighinsertion

loss and susceptibility to interference (EMI). SingleMode FiberOpticAnalogRF (akaRFoF)

Transceivers provide an excellent alternative for this type of application. Fiber Optic

transmissionofferssignificantadvantagesforthereliabletransportofRFsignalsintheirnative

format over many types of optical networks and across a broad range of frequencies. For

analog type signals, especially at high frequencies, in which premium performance at high

SpuriousFreeDynamicRange(SFDR)isdesirable,therearebutafewmethodstoachievesuch

agoal.

Lesssignaldegradationpermeter

Highersignalcarryingcapacity/bandwidth

Lesscostlypermeter Lighterandthinnerthancopperwire

Freefromelectromagneticinterference(Poorweatherdoesnotaffectsignal)

Lowertransmitterlaunchingpower

Flexible(usedinmedicalandmechanical imagingsystems)

Functionality&HowitWorks?

The incoming RF signal is input to the Transmitter Module, which contains RF signal

conditioning,provides complex impedancematchingbetween50Ohm input impedance and

theLaser,LaserBiasControl,APC,MonitoringandAlarmelectronics.ThetransmittermoduleutilizesanIntensityModulationschemetoconvertRFtolight,whichistransportedthroughan

opticalfiberintotheOpticalReceiver.TheReceiverModuleconvertsthemodulatedlightback

intoanRFsignal.TherecoveredRFsignalisagaincomplex impedancematchedandamplified

before itbecomesavailableattheoutputofthereceiver.Generallythephotodiode isahigh

impedance current source with an impedance around 2 kohms, followed with several

amplifiers.Thebroadbandmatchingisachievedwithvarietyofschemesthatwillbedictatedby

the overall user modulation bandwidth and Noise density and Intermodulation distortion

requirements.TherearemanytypesofRFOFDirectmodulationtransmitters includingCWDM

grade,butformostpart. Thefollowingtwotypescovermajorityofapplications.

ThetwotypesofLinksare,withoutanyLowNoiseAmplifiers(LNA)andotherwithLNAbuiltin.

TypicalRFoF Linkwithout LNAhasaGainoffering from1dB+/ 1,buthigherGain is

availablebasedoncustomeroverallbandwidth,NFandIIP3requirements.

Typical RFoF Linkwith LNA has a Gain offering from 20dB +/ 1, but higher Gain is

availablebasedoncustomeroverallbandwidth,NFandIIP3requirements.


3/12



Applic

ationN

ote

RFAttenuationvs.OpticalLoss

1dBoofopticallosscorrespondsto2dBeofRFloss.

Thephotodiodegeneratedcurrentcanbecalculatedfromthefollowingexpression:

Ipd=rpdxPopt Where,

Ipd =photodiodecurrent(A),

rpd=smallsignalphotodioderesponsitivity(A/W),

Popt=opticalpowerdetectedbyphotodiode(W).

TheoutputRFpowercanalsobecalculatedusingthephotoDiodegeneratedcurrent

Prf=Ipd^2xRloadWhere

Prf=powerdeliveredtoloadresistanceconnectedtophotodiode(W),

Rload=loadresistanceconnectedtophotodiode().

WithsomeSimplesubstituting:

Prf=rpd^2xPopt^2xRload

Thus the convertedRFpower is related to the squareof theopticalpower,anddue to this

relationship, a 1dB loss of optical powerwillbecome a 2dB loss of RF power. To eliminate

confusionbetweenopticallossesandelectricallossestheunitdBoanddBehasbeenadopted

respectively.

SimpleRFoverFiberLinkBlockDiagram

LNA

DC Power

CurrentSource

RF

Input

DC Power

Laser

Complex

impedanceMatching

Bias-T

Complex

impedance

Matching

PA

High Gain

Power Amplifier

Low Noise

Ampl if ier

RF

RF+DC

DC

RFOutput

FO TX: Fiber Optic

Transmitter

FO RX: Fiber Optic

Receiver

Photo

Diode

Fiber

OpticalIsolator


4/12



Applic

ationN

ote

LinkGain

RFoFTotalLINKGAIN

RFoFTotalLinkGain(dB)=GT

LNAGain(dB)=GLNA

OpticalTXRFEOefficiency(mW/mA)= TX (includes50ohmstoLaserMatching losses

andLaserslopeefficiency)

OpticalRXRFOEefficiency(mA/mW)= RX(includes matching losses of PD high

impedanceto50ohmsandPDresponsitivitylosses)

Fibercablelosses(dB)=Lopt(includesfiberlosses,andcouplingorsplicesetc)

GT(dB)=GLNA(dB)+20log(TX. RX) 2Lopt

OutputOpticalPower

(mW)

Laser Current (mA)

ThresholdCurrent(mA)

Typical Laser Diode Transferfunction Curve.

Slope Efficiency =?P/? I (mW/mA)

Laser Bias

Current

InputOptical

Power

(mW)

Photodiode outputCurrent

Typical Photo Diode Transferfunction Curve.

Photodiode

ResponsitivityAmp/Watt

Fiber Losses


5/12



Applic

ationN

ote

EffectsofLaserSlopeEfficiency

InallLaserstheslopeefficiencyparameterisverytemperaturesensitive.AstheLaser

temperaturechanges,sodoestheslopeefficiencyoftheLaser,andconsequently,alloftheothercriticalLaserparameterssuchasGain,OMI,NF,IP3,etc..Thetemperaturecharacteristics

oftheLaserdiodearesuch,thatasthethresholdcurrentincreases,theslopeefficiencyofthe

Laserdevicedecreases,andanincreasingLasertemperatureresults. Thisphenomenonmakes

theLaserlessefficient,thusreducingtheRFsignalGainandincreasingthelinkNoisefigure,as

wellascausingadditionaldegradationintheLaserlinearity.

Onemethodtoovercomethisphenomenon,whichyieldshighperformance,istousean

integralThermoelectricCooler(TEC)withDFBLasers. Thistechnologyassuresahighlevelof

LaserstabilityandassuresexcellentRFperformance,AtypicalcooledDFBLaserhasahigh

slopeefficiency,whichmeansthattheLaserishighlysensitiveandrequiresalower

modulationcurrentinordertoachievetheusualhighmodulationindex.

LaserSlopeEfficiencyoverTemperature

InCaseofCooledDFBLaserTransmitter,theslopeEfficiencyofthelaserisnotchangedeven

thoughtheTransmittermodulewillbeexposedtowidetemperaturevariationsofupto 40Cto

+85C


6/12



Applic

ationN

ote

TotalLinkNoiseFigureAnalysis

EquivalentLinkInputNoiseDensity(dBmHz),EINistheamountofNoiseattheinputof

theRFoFlinkthatproducesoutputNoiseDensity(EON)atthereceiverendifthetotallinkitselfwereNoiseless)

Noisefigure(NF)isameasureofdegradationoftheSignaltoNoiseRatio(SNR),caused

bycomponentsinaRFsignalChain.Thenoisefigureisthustheratioofactualoutput

noisetothatwhichwouldremainifthedeviceitselfdidnotintroducenoise.

EIN=EON GT Where, GT(dB)=GLNA(dB)+20log(TX. RX) 2Lopt

NF=EIN+174dBmHz Where,KBTisidealdevicethatisterminatedbyapassiveloadat

temperature290Kelvin(T0).KB=1.38x10^23J/KBoltzmannconstant

TheEndtoendlinknoiseisduetovarietyofsources,suchastheLaserRelativeIntensityNoise

(RIN),PhotodiodeShotNoiseandtheThermalNoiseduetothereceiverpostamplifiers

followingthePD.Asyoucanseeabovetheopticallossesduetothefiberinherentlossesand

anycouplinglossesisalsodegradestheLinkequivalentinputnoisedensityEIN.

EIN=EINlaserRIN+EINPDshotNoise+EINreceiverThermalNoise(W/Hz)

TotalLinkNoiseandAllItsContributorsvs.OpticalLoss

RFoF Input Noise Density

-150.00

-140.00

-130.00

-120.00

-110.00

-100.00

-90.00

0 5 10 15 20 25 30

Optical Losses dB

EIN(dBm-HZ)

EIN Total

EIN Thermal

EIN Shot

EIN RIN


7/12



Applic

ationN

ote

HowtoReduceNoiseFigure(NF)?

OnewhattoreducetheNFofaRFoFLink(EndtoEnd)istoincreaseTransmitterRFto

ConversionefficiencyotherwisetheTXGain.UsingaLowNoiseamplifierBeforetheLaserishighlyeffectivewayachievingjustthat.ChoosingtheLNAproperlyisextremelyimportantto

maximizetheeffectivenessofsuchapproach,andparameterssuchasGain,NFandIIP3of

amplifierplayakeyroleinallthat.

TheWayitworksisasfollowing:

NFTotalRFoFLINK=NFLNA+(NFRFoFLink1)/GainLNA

ForExample:

LNAGain20dB

LNANF3dB

RFoFLinkNFwithoutLNA=40dBNFTotalRFoFLINK=2+(100001)/100=102

NFTotalRFoFLINK=10xLOG(102)=20dB

20dBimprovementinNF!

*ThedrawbackinsuchapproachisalsoreductioninInput3rdOrderInterceptPointofthetotal

linkwhichmustbeconsideredinoverallLinksystemDesign.

HowtoCalculateCarriertoNoiseRatio(CNR)?

NoiseEquivalentBandwidthisdefinedsuchasWhenwhitenoise(flatspectrumoffrequencies)

ispassedthroughafilterhavingafrequencyresponseH(f),someofthenoisepowerisrejected

bythefilterandsomeispassedthroughtotheoutput.ThisisalsoknownasSignalChannel

Bandwidth(BW).

ThustoCalculateCarriertoNoiseRatioofaRFoF

linknotonlyoneneedtoknowtheinputsignal

levelandlinkEINbutalsotheNoiseEquivalent

Bandwidth.

Sinput=RFinputSignalLeveldBmPNoise=TotalNoiseattheinputdBm

PNoise=EIN+10Log(BW)

CNR=SinputPNoise (dB)


8/12



Applic

ationN

ote

3rdOrderInputInterceptPoint?

AInputthirdorderinterceptpoint(IIP3)isameasureofnonlinearityoftheRFoFlink.The

interceptpointisapurelymathematicalconcept,anddoesnotcorrespondtoapracticallyoccurringphysicalpowerlevel.Inmanycases,itliesbeyondthedamagethresholdofthe

device.Butinmultichannelsystemknowingthisparameteriscriticalparametertocalculate

theoverallIntermodulationdistortionproducedbytheRFoFlink.

CarriertoIMDratio=2x(IIP3Signallevel) dBc,(IntermodulationDistortionlevelsbelow

Carrier)

AnotherparameterishasbeenusedtomeasurenonlinearityoftheRFoFlinkisOIP3,which

istheOutputthirdorderinterceptpointwhichismeasuredattheoutputofaopticalreceiver.

IIP3=OIP3LinkGain

OIP3=SignalLevel(dBm)+[(SignalLevel(dBm)3rdorderdistortion(dBm)/2]

ToCalculatetheOIP3ofaRFoFlink,istoinputTwoTonesintotheRFoFTransmitterand

measurethe3rdorderdistortionproductsasisshownbelow.

3rdOrderInputInterceptPointMeasurement


9/12



Applic

ationN

ote

3rdOrderInputInterceptPointPlots


10/12



Applic

ationN

ote

CascadedIIP3Calculations

IncasetherearePreamplifiers(LNA),orotherdevicesbeforeand/orafterRFoFlinktheoverall

CascadedLinkIP3canbecalculatedasfollowing:

1/(OIP3EndEnd)=1/(OIP3LNA*GRFoF*GPA)+1/(OIP3RFoF*GPA)+1/(OIP3PA)

1/(OIP3EndEnd)=1/(1000*1*10)+1/(1000*10)+1/10000=1E4+1E4+1E4

1/(OIP3EndEnd)=3.33E3OR+35.22dBm

IIP3EndEnd=35.2230=+5.22dBm

SpuriousFreeDynamicRange(SFDR)

SpuriousFreeDynamicRange(SFDR),isdefinedasthepowerlevelrangeofapairoftwoinput

signalsinwhichthetwosignalsareabovethenoisefloorandthe3rdorderIntermodulation

distortionproductsarebelowthenoisefloor.

SFDR=2/3x(IIP3EIN10LOG(BW))

ThelargerSFDRbecomesthehigherdynamicrange,theRFoFlinkposses.TheSFDRcan

beincreasedseveralways,amongthembyreducingthebandwidthorreducingtheTotalinput

NoisedensityorincreaseIIP3ofthelink.


11/12



Applic

ationN

ote

Conclusion

Thispresentationisanoverviewoftheoverallprocessofdesigning,andspecifyingafiberoptic

systemforanalogRFtransport anEngineeringGuide.RFoverFiberdesignersneedtoknowhowtofactorinthekeyparametersofaRFoFlink(suchaslinkGain,NFandIP3,)fortheir

systemanalysisanddesign.Equallyimportantarethequestionsyoumustanswerpriortonew

developmentproject,suchas:

1. Whatarethesystemperformanceparametersthattheapplicationyouconsider

requires?ThosearetheENDtoENDRFperformanceofthelink.

2. Shouldyouusefiberopticsinyourcommunicationsproducts?

3. Whatareitsadvantages?

4. HowdoyouspecifyaRFoFlinktoallowuserstochoosetheproperproductfortheir

application?

Wehaveansweredallquestionsexceptitem4,sotocompletethispresentationswehave

accumulatedlistofquestionsbelowfortheusertocompleteenablingthemtooptimizethe

overalllinkperformance.


12/12


f

Applic

ationN

ote

RFoF Link Design Guideline

Documents

Transcript of RFoF Link Design Guideline