Pure Signal

8/12/2019 Pure Signal

1/12

PureSignal 20140314 Page 1

IntroductionLinearityofanamplifierimpliesthattheoutputincreaseslinearlywithincreasesoftheinputsignal. I.e.,

ifaninputof5Wproducesanoutputof50W,thenaninputof10Wshouldproduceanoutputof100W.

Unfortunately,the

analog

amplifiers

in

our

power

amplifier

stages

are

not

so

perfect.

A

real

amplifier

mightbehavemorelikethefigurebelow.

TheunfortunatesideeffectofthisNONlinearityisInterModulationDistortion(IMD),someofwhich

resultsin

splatter.

Consequently,

on

the

panadapters

of

our

SDRs

its

easy

to

see

signals

that

extend

outsideanormaltransmitbandwidth. ThisphenomenonismoreevidentonstrongsignalsastheIMD

maybe25dBormoredownfromthepeakofthesignal. ThisIMDpollutesourbandsandraisesthe

interferenceandnoiselevelsforamateursthatmaybeoperatingmanykilohertzawayfromthe

offendingtransmitters. SomeoftheIMDislessvisibleonpanadaptersbecauseitfallsINBAND,i.e.,

withinthebandwidthofthetransmittedsignal. Thisaddsdistortiontothesignal.

Fortunately,severaltechniqueshavebeendevelopedtoimprovethelinearityofanalogamplifiersand

therebyreducetheIMDlevelsthattheygenerate. OnesuchmethodisPredistortionmakingthe

amplifiersoutputmorelinearbypredistortingtheinputsignalinsuchawayastooffsetthedistortion

thattheamplifierwillproduce. Theoutputoftheamplifiercanthereforebecorrectedtobehavevery

muchliketheidealcaseshownabove. (Seefigurebelow.)


2/12


Inadditiontotheamplitudedistortionshownintheabovefigures,thereisalsoaphasedistortion

(amplifierphaseshiftvariesasafunctionofsignalamplitude)thatmustbecorrectedtoachieve

significantreductionsinIMD.

ThecomputingpowerinourSDRscanbeusedtocalculatetherequiredcorrectionsandthentoapply

thosecorrectionstothedigitaltransmitsamples. However,tobeabletocalculatethecorrections,itis

necessarytoknowtheamplitudeandphasecharacteristicsoftheamplifiertobecorrected. Each

amplifierisslightlydifferentanditscharacteristicsalsovarywithfrequency,temperature,load

impedance,andsupplyvoltage. Itfollowsthat,forbestresults,wemusthaveamethodtomeasurethe

characteristicsoftheamplifierwhileitisinuseandtoadaptthecorrectionasconditionschange. This

approach,usedinPureSignal,isoftencalledAdaptivePredistortion.

WecanmeasurethecharacteristicsoftheamplifierbyusingthereceiverofourSDRtoreceivethe

amplifiersoutputandthencomparingthesignalsgoingintoandcomingoutoftheamplifier. Note,

however,thatreceivingtheamplifiersoutputsignalrequiresafeedbackpathtobeestablishedfrom

theamplifieroutputtothereceiverinputduringthetimethatwearetransmitting. Clearlythismustbe

donewithadirectionalcoupler/attenuatororsampler/attenuatorsoastodeliverONLYtherequired

amountofpowerbackintothereceiverinput.

HardwareConfiguration

Feedback

Asmentionedabove,calibrationrequiresthattheamplifieroutputbefedbacktothereceiverduring

transmit. Generally,thiswillinvolveusingoneormorerelayssuchthatthereceiveris(1)connectedto

theantennaduringreceive,and(2)connectedtoANAPPROPRIATELYATTENUATEDsampleofthe

transmitteroutputduringtransmit.


3/12


YOUARERESPONSIBLETOENSURETHATYOURFEEDBACKISALWAYSSUFFICIENTLYATTENUATEDSUCHTHATITDOESNOTDAMAGEYOURRECEIVERHARDWARE.

Forproperoperation,feedbackMUSTNOTcreateanADCOverload. ForPowerSDR_mRX,ADCOverload

will

be

reported

via

aflashing

warning

across

the

bottom

of

the

panadapter.

Foraccuratecalibration,thefeedbacklevelmustbeascloseaspracticaltoADCOverloadwithoutactuallycausinganoverload. Agreenyellowredindicatorisprovidedonthe

Linearityformtoreportthefeedbacklevel. (SeePureSignalControls&Operationbelow.)

Witharadioattenuatorsettingof0dB,ADCOverloadoccursatapproximately11dBm. Forbestresults,feedbackshouldbeabove17dBmand,asstatedabove,mustNOTcreateADC

Overload. Withahigherradioattenuatorsetting,youcanrunahigherfeedbacklevelintothe

receiverinput.

Helmut,DC6NY,hasprovidedanexcellentdesignforanRFSamplerthatcanbeusedtocapturethefeedbacksignal.

See: ,RFSamplerforPreDistortionLoop".

AlexFeedback

ForsystemsusingtheAlexfilters,youhavetwooptionsforPureSignalfeedback. OntheSetup>

General>Alex>Antennatab,checkboxes"RX1INonTX"and"RX2INonTX"areprovided. Ifoneof

theseischecked,duringtransmit,othersettingswillbeoverriddenandthechosenRXconnectorwillbe

connectedtothereceiverinputfilters. Therefore,youcaninputyourfeedbacktothechosenRX

connector. Alternatively,ifyouprovideyourownT/Rswitching(suchasusingtherelayonthecoupler

designedbyHelmut,DC6NY,andreferencedabove),youcaninsertfeedbackbetweenthe"OuttoRx"

connectorand

your

receiver

card

input.

ANAN100/DFeedback

FortheANAN100/D,ontheSetup>General>Ant/Filters>Antennatab,twocheckboxes,"Ext2onTx"

and"Ext1onTx"areprovided. Ifoneoftheseischecked,duringtransmit,othersettingswillbeover

riddenandthechosenEXTconnectorwillbeconnectedtothereceiverinput. Therefore,acoupler

providingfeedbackcanbeconnectedtothechosenEXTconnector.

FortheANAN100/D,testershavenotedthatthereisaveryhighlevelofcrosstalkbetweenthe

transmitteroutputandreceiverinputevenwithoutadirectfeedbackconnection. ThatfeedbackMAY

besufficientforaccuratecalibrationofa"barefoot"ANAN100/D. Onedisadvantageofthatapproachis

thatthisfeedbackisduetostraycouplingandthefeedbacklevelislikelytovarysubstantiallyfromband

toband. UsingthisstraycouplingwillNOTworktolinearizeafollowinglinearamplifiersince"clean"

feedbackfromtheoutputoftheamplifiermustbeprovided.

ThefollowingdiagramillustratestheANAN100/Dwithanexternalamplifierandcoupler.


4/12


Amplifiers

ThecurrentPureSignalalgorithmdoesnotattempttodynamicallycompensateformemoryeffects.

MemoryEffectsarephenomenawherebytheamplifiergainandphasearenotonlyafunctionofthe

currentinputsignal,theyarealsoafunctionofpastinputsignals. Theamplifierrememberssignals

thatwentthroughinthepast. Theseeffectscanbethermalorbias/powersupplyrelated. For

example,supposethatastrongsignaloveraperiodofsomemillisecondsheatedtheactivedevicesof

theamplifiertherebychangingtheirgainandothercharacteristics. Thentheamplifierremembers

thosecharacteristicsuntilitcoolsbackdowneventhoughaweakersignalmaypassthroughinthe

meantime. So,forthesameweaksignallevel,theamplifiercharacteristicisdifferentdependingupon

whetheritishotorcooler,dependingupontheprecedingsignals. Asimilarsituationexistswithbias

andsupplyvoltages. Supposeastrongsignalmomentarilycausesaslightdropinpowersupplyvoltage.

Ifthesupplydoesnotrecoverinstantaneously(whichittypicallywillnot)theamplifiersgainwillhave

beenchangedforasubsequentweaksignal.

Amplifierswithweakmemoryeffectswillbemoreeffectivelycorrectedthanthosewithstrongmemory

effects. Amongsolidstateamplifiers,currentexperiencesuggeststhatLDMOSamplifiersusinghigher

voltage(~50V)

supplies

produce

the

best

results.

In

any

particular

amplifier

design,

factors

that

can

minimizememoryeffectsinclude:

Highvoltage/lowcurrentdesignstominimizepowersupplydroop. Stiffpowersourcestominimizevoltagesag. Lowercapacitancedevices(suchasLDMOS). Excellentheatsinking,probablyincludingacopperheatspreader.


5/12


Asyouwouldsuspectfromthislist,13.8VFETamplifierscanbechallengingtocorrect. However,stiff

powersuppliesandgoodcoolingcangoalongway.

Limitedtestingofthissoftwarehasbeendonewithvacuumtubeamplifiers. Yourresultsmightbeless

predictablewithvacuumtubeamplifiersbecausethedelaytimesthroughtheamplifiersaremuch

longerdue

to

the

high

Q

tuned

circuits.

THEAMOUNTOFIMDCORRECTIONYOUACHIEVEWILLDEPENDUPONTHEMEMORYEFFECTSOFYOUR

AMPLIFIERANDTHEQUALITYOFYOURFEEDBACKSIGNAL.

FirmwareRequirementsSpecialfeaturesarerequiredinthefirmwareforPureSignaloperation.

FirmwareforHermes/ANAN100mustbeHermes_v2.5orlater. FirmwareforAngelia/ANAN100DmustbeAngelia_v2.2orlater. FirmwareisnotcurrentlyavailabletoenablePureSignalonAtlasbasedsystems.

PowerSDR_mRXSettings

SampleRate: OntheSetup>Audiotab,pleasesetthesamplerateto192K.WhenGigabitEthernetfirmwareisavailable,inthefuture,wewillmoveto384K.

PureSignalControls&OperationFirstofall,youmustbesurethebox"DisablePureSignal"isNOTcheckedonthe

"Setup>General>HardwareConfig"

tab.

Why

is

this

box

even

there?

Having

PureSignal

enabled

MAY

requiremorenetworkbandwidth. Therefore,forusersthatareoperatingwirelessorhavelimited

bandwidth,itwouldbebesttochecktheboxunlesstheywanttousePureSignal. Thisisverysimilarto

thesituationwiththe"LimitStitchedReceivers"checkboxjustabove. Atthispoint,wehavemade

StitchedReceiversandPureSignalmutuallyexclusive.

ClickingLinearityonthemenustripalongthetopofthePowerSDR_mRXconsolewillopenthe

PureSignalform.


6/12


Operationofthebuttonsandindicatorsisasfollows.

Information: Clickinginformationopensapdfdocumentgivinganoverviewoflinearity,predistortion,andhowtousePureSignal.

AutoCalibrate: Activates/deactivatesautomaticcalibration. ThebuttonbackgroundcolorwillbegreenwhenautomaticcalibrationisturnedON. NotethatmerelyturningONAutoCalibrate

doesnotensurethatyouareactuallycalibratingandcorrectingyoursignal. Moreonthislater.

Whenautomaticcalibrationisfunctioningcorrectly,thesoftwarewillberepeatedlymeasuring

theamplifierresponseandadaptingthecorrectiontoadjusttocurrentoperatingconditions.

SingleCalibrate: Theremaybeinstanceswhereyoudonotwishtorepeatedlycalibrate;but,youinsteadwishtojusttakeonesetofamplifiermeasurements,calibrateoncebasedupon

thosemeasurements,andcontinuetocorrectwiththatcalibrationdata. Clickingthisbutton

initiatessuchasequence.

OFF:

OFF

turns

off

any

current

correction

and

collection

of

amplifier

data.

It

leaves

PureSignal

inaresetstate. YouwillneedtoclickAutoCalibrate,SingleCalibrate,orRestoreacalibration

datasettoreturnPureSignaltoanoperatingmode.

Correcting: TheLEDlabeledCorrectingwillbegreenwhencorrectionisenabledandavalidsetofcalibrationdatahasbeencomputedorloaded. Whengreen,yourtransmitsignalwillbe

correctedusingthecalibrationdata.


7/12


MOXWait: AfteryouactivatePTTortripVOX,asmallamountoftimewillberequiredbeforesignalisactuallyflowing,relayshaveengaged,amplifierbiashasbeenenabled,etc.MOXWait

istheamountoftimefromyourinitiationoftransmituntilthesoftwarewillbegincollecting

dataforacalibration. Thedefaultof0.1secondsshouldbemorethansufficientformostcases.

CalibrationWait: Theamountoftime(inseconds)betweenthecompletionofcomputationofacalibration

and

the

beginning

of

data

collection

for

the

next

calibration.

If

set

to

zero,

calibrationswillproceedinrapidsuccessionaslongasvalidsamplesetscanbecollected.

HighernumberswillincreasethetimebetweencalibrationsandsaveafewCPUcycles.

AmplifierDelay: Allanalogamplifiersintroducesomeamountofsignaldelay. ForsolidstatePAs,thelargestcomponentofdelayisgenerallyintroducedbythelowpassoutputfilters. The

algorithmusedtocomputecalibrationisreasonablytolerantofdelayandtoanextent

automaticallyadjustsforit. However,youmayfindthatyougetimprovedresultsbyadjusting

thiscontroltomanuallyoffsetthedelaythroughtheamplifier. Thisoffsetcanbeadjustedin

20nsincrements.

FeedbackLevel: ThisLEDindicatesthestrengthofthefeedbacksignal(theamplifieroutputsignalthatisbeingreceivedbackthroughyourreceiver). TheindicatorwillbeupdatedONLY

whenacompletesetofdatahasbeencollectedforcalibrationandwillthengraduallyfadeto

thebackgroundcolorafteritsupdate. BlueindicatesthatyouareclosetoADCOverloadand

shouldprobablyDECREASEyourfeedbacklevel;greenindicatesanexcellentfeedbacklevel;

yellowindicatesthatfeedbackislowerthanrequiredforoptimumresults;redindicatesthatthe

feedbacklevelisverylowanderraticorerroneousresultsarelikelytobeproduced. Notethatif

acompletesetofdataisnotabletobecollected,thisindicatorwillneverupdate;itwillremain

atthebackgroundcolor.

TwotoneGen: Forconvenience,atwotonegeneratorbuttonisprovidedherefortesting. ThisbuttonactivatestheTwotonegeneratorontheSetup>Teststab. ForusewithPureSignal,set

theLevelto0.0dBsothatPureSignalcancollectdataacrossthefulloutputrange.

CorrectionSave/Restore: Anytimeavalidsetofcalibrationdataisinuse,thatdatacanbesavedtoafileforlaterretrieval. ClickingSaveopensthefilesavedialogforthatprocess.

ClickingRestoreopensthefiledialogtoretrieveapreviouslystoredsetofcalibrationdata.

Restoringasetofdataautomaticallyactivatescorrection.

RelaxTolerance: CheckingthisboxisNOTrecommendedfortypicaloperation. Itrelaxesarequirementonconsistencyofsamplescollectedforacalibration. Whenmightthisbeuseful?

IfyouhaveanamplifierwithseverememoryeffectsforwhichPureSignalrefusestocalibrate,

OR,inasituationsuchasatubetypeamplifierwithACrippleontheplatesupplythatis

modulatingthe

output.

The

risk

in

checking

the

box

is

that

it

increases

the

possibility

of

an

invalidcalibrationthatthealgorithmbelievesisvalid.

AmpView: ClickingtheAmpViewbuttonwillopentheAmpViewform. AmpViewprovidesautilitytoeasily,inrealtime,visualizethegainandphasecharacteristicsofyouramplifierANDto

simultaneouslyvisualizethecorrectionthatisbeingappliedbyPureSignal.


8/12


Thebluedotsshowtheoutputmagnitudeversusinputmagnitudeoftheuncorrectedamplifier.

Similarly,the

yellow

dots

show

the

uncorrected

phase

characteristic

versus

input

magnitude

for

the

amplifier. Notethatthesearediscretedotsatthepointsofthesamplescollectedtoperformthe

PureSignalcalibration. Theredandgreenlinesthenshowthecalculatedmagnitudeandphase

correctionsthatarebeingappliedtooutgoingtransmitsamples.

ThisformcurrentlyrequiresquiteafewCPUcycles. Checkingthe"LowRes"boxwillreducethenumber

ofplottedsamplesbyafactoroffour,therebyreducingtheCPUrequirements. (Ifsomeonewantsto

writesomeefficientgraphicscodeforthis,theCPUrequirementcouldundoubtedlybedecreased

substantially!)

Checkingthe"ShowGain"boxchangestheleftMagnitudeAxistoaGainAxis. Therefore,youhavethe

optionoflookingatGainversusInputMagnituderatherthanOutputMagnitudeversusInput

Magnitude.

Notethatviewingtheamplifiercharacteristicswillallowyoutospotproblemssuchasnoisyfeedbackor

significantmemoryeffects. Itcanalsobeusefulforthingslikebiasadjustments.


9/12


VERYIMPORTANTNotesAboutCalibration&Feedback

Collectionofasetofamplifierdataforcalibrationrequiresmeasuringtheentireamplifiercharacteristicfrom0%to100%oftheinputlevelforaparticularDrivesetting. (This0%to

100%referstoyourdigitalsignallevelasmeasured,forexample,bytheALCmeter;itdoesNOT

relatetoyourDrivesetting.) YoursignalMUSTtraversethatentirerange(orverynearlyso)

forcalibrationtoproceed. ThehighersignallevelsarewheremostoftheIMDisgenerated;so,

gettingtovirtually100%isespeciallyimportantandcannotbeoverlooked. Therefore,innormal

operationyourALCmeterSHOULDbeperiodically(evenfairlyfrequently)hitting0dB. Thereare

alsootherreasonsthisisgoodpractice,forexample,inordertousetheDACsfulldynamic

range. YouwilllikelywantafewdBofLevelergainand/orafewdBofcompressiontofacilitate

thisprocess. Unlikeanalogandmanyotherdigitalradios,hittingthemaximum(0dB)doesnot

generateany"flattopping"ormaterialdistortion.

AutomaticCalibrationisrecommendedbecauseitadaptsdynamicallytochangingoperatingconditions. ShouldyouchoosetousetheSave/Restorecapability,beawarethatcorrection

thatisRestoredmustbeusedunderexactlythesameconditionsasitwasSavedtoachieve

goodresults. Itsfairlyobviousthatsignificantchangesinfrequency,supplyvoltage,andload

impedanceareconditionsthatwouldinvalidateaSavedcorrection. However,alsobeawarethe

anychangeintheDrivesettinginvalidatesthecorrection.

Duetomemoryeffects,calibrationwithatwotonesignalwillgenerallyNOTproduceanoptimumcalibrationforvoiceandviceversa. Again,automaticcalibrationisrecommendedto

allowforadaptationtochangingconditions.

PureSignalwillattempttocorrectwhateverdistortionit"sees." Sinceit"sees"thePAoutputbywayoftheSDRReceiverpath,itisCRITICALNOTTOINTRODUCEANYDISTORTIONBETWEEN

THEPAOUTPUTANDTHERECEIVERADC. PureSignalwillattempttocorrectwhateveris

presentedto

the

ADC

input.

Therefore,

ifyou

have

some

element

in

the

receive

path

that

distortsthePAoutputbeforeitgetstotheADCinput,PureSignalwilltrytolinearizethat

distortionaswellasthePAoutput. Indoingso,itmaymaketheactualtransmittedIMDworse

ratherthanbetter! So,hereappliesthesaying"garbagein,garbageout." Tomakematters

worse,thepanadapter"sees"thesamesignalasPureSignal"sees". Therefore,PureSignalmay

havemadethatsignallookquitefine;whereas,inreality,thePAoutputmaybedistorted. The

importantmessagehereis,makesureyourfeedbackpathisfreeofanyelementsthatdistort

yourfeedbacksignal!

SimilarlytothepointaboutdistortioninthepathtotheADC,DONOTOVERLOADTHEADC.Thatwillproduceasimilarlydistortedresult.

SomehavereportedthatontheANAN100/D,the6MeterLNAisapparentlyoverloadedbyinternalcrosstalkduringtransmit. Thiscausesthetypeofdistortiondiscussedabove.

PowerSDRnowoffersanoptionto"bypass"the6MeterLNA,removingitfromthefeedback

path,duringtransmit.


10/12


CorrectionBandwidthPureSignalcanonlycomputecorrectionforIMDproductsthatitcan"see." I.e.,itcanonlycomputecorrectionforIMDproductswithinthebandwidthofthereceiverusedtoreceivetheoutputofthe

amplifier. So,forexample,at192Ksamplerate,andallowingforalittlefilterrolloff,wehavea

bandwidthof~186K,or+/ 93Kfromthetransmitfrequency.

However,PureSignalcanonlycorrectwithinthebandwidthofthechannelbetweenthecorrectionalgorithmandtheamplifiertobecorrected. Weusea48KsampleratetotransfertransmitI/Qsamples

fromthecomputertotheradiohardware. So,allowingforalittlefilterrolloff,thecorrection

bandwidthis~40Khz,or+/ 20Khzfromthetransmitcenterfrequency.

Unlessyouhaveaverydirtyamplifieroraresignificantlyoverdrivingyouramplifier,nearlyALLofyour

IMDenergyshouldfallwithinthis+/ 20Khzbandwidth.


11/12


ExampleResults

Example1: "barefoot"Hermescard on6M(resultsfromWarrenPratt,NR0V)

Uncorrected

Corrected


12/12


Example2:ANAN100D,StrayCouplingFeedback(resultsfromBillDiaz,KC9XG)

AdditionalPureSignalInformation

SeedocumentspostedbyBillDiaz,KC9XG,ontheApacheLabsYahooGroup.

Pure Signal

Documents

Transcript of Pure Signal