Watkins-Johnson_WJ-8711A_review_NSA_2011.pdf

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1、DOCID:3928940 pproved for release by NSAon12-01-2011 ,TransparencyCase#6385SSIFIED ReceiverDynamics Editors Note: This paper waswrittenbefore the author retired (1995), STATUTORILYEXEMPT InK4weuseanumberofHFreceiversandpurchase theminquantity for our systems. Because we are ofteninapositionofdecidin

2、g what is best forourparticular systemand application,itis good to understand the terminology used byRFengineersandreceiver manufacturersinspecifying the dynamicsofareceiver.Thistechnical note should help you ininterpreting the manufacturers specification whenmakingreceiver comparisons or deciding w

3、hat receiver fits bestinyourapplication. DYNAMIC RANGE Dynamicrangecanbe definedindifferentways,allofwhichhaveapplication. Fundamentally,dynamicrangeis the differenceindB betweentheminimumdiscernible signalandthesignalthatcauses a specifiedamountofharmonicdistortionatthe receiver output.Thissimplist

4、ic definition doesnotdojustice totheway we use receiversina crowdedHFspectrum.Betterdefinitionsareincluded below. Dynamicrangefiguresare importantbecausetheydescribe a very basicparameterandonewithwhich receiverscan be compared.Ofcourse the samecriteriamustbe usedinmakingthecomparison,and independen

5、tstandardtestingis the only way to betrulysurethatyouarecomparing apples to apples. However,manufacturersspecificationsheetsprovide adeparturepoint for comparison.The American RadioRelayLeague(ARRL)performsstandardand independenttestingfortheamateurcommunity. The ARRL recently tested the Watkins- Jo

6、hnsonHF-IOOO,atwinsisterto the WJ-8711 used by someK4divisions,andpublished the results.1AsfarasI know, NSA is not now independentlyevaluatingreceivers.J42 operatesanautomatedreceivertestfacility whichverifiesthatreceiversarewithin specificationafterrepair. Information onactualspecificationscanbe ob

7、tained fromthem for receiverscurrentlyininventory. SENSITIVITY Receiver sensitivity is a measure of a receivers ability to detect weak signals with a prescribed signal-to-noiseratioand IF bandwidth. The valueisusuallyinmicrovolts for a specified2signal-plus-noise-to-noise(S+N)/Nratio. In some casest

8、heterm8INADmaybe used, which indicates signal-plus-noise-and-distortion-to-noise (8+N+D)/N. A variable inmanufacturersensitivityspecifications isthepercentageofAMmodulationorFM 131UNCLASSIFIED DOCID:3928940 UNCLASSIFIEDCRYPTOLOGIC QUARTERLY deviationusedinthetest,whichmayrangefrom30-90percentand3-10

9、kHz, respectively.Because the sensitivitytestusestheaudiooutputofthereceiver,other variablescanaffect the specification, suchasautomaticgain control (AGC) threshold, demodulation mode (SSBorAM),andeventhemethod ofcalibratingthegeneratorinput level. Because of this,ithasbecome common practice to spec

10、ify sensitivityintermsof noise figure (NF), which does not rely onmeasuringthe demodulatedoutputanduses a broadbandstandardnoise source forinputratherthana signal generator. Noise Figure Noise figure is 10 log (noise factor).Thenoise factor (nt) is theinputsignal-to-noise divided bytheoutputsignal-t

11、o-noise. NF=10 log(nO=10 logSilNi)/(SolNoexpressedindB Anoise factorof1, no degradationinsignal to noise, produces a noise figure of 0 dB.HF receiver noise figures willrangefromabout10 to 20 dB. VHFandUHFreceivers will oftenexhibita lower noise figure, 8 to 15 dB, totakeadvantage of the lower atmosp

12、heric noiseenvironmentfound there. The receiver noise figure isprimarilya function ofboth thenoisegeneratedinthefirststageandfirststagegain. Highergainfirst RF amplifiers will normally produce a lower overall receiver noise figure - onethatis close tothenoise figure ofthefirststageitself- however,hi

13、ghgain front endsaresubject to overload by strongsignals. Overloading the frontendofa receiver produces spurious responsesand receiverdesensingorblockingthatrestrictsdynamic range. An applicationrequiringa very low noise figure requirestheuseofanexternallow-noisehigh-dynamicrange amplifierattheanten

14、naanda low-loss RFtransmissionline betweentheantennaand receiver. NoiseFloor Themaximumsensitivityofa receiverisdeterminedby theamountofnoiseinternally generatedinthereceiver (primarilythefirststage)anditsbandwidth.Itisthebasic sensitivityfigure,asanysignal weakerthanthiswill be masked by the noise.

15、Another termyou may encounter isminimumdiscerniblesignal(MDS)sensitivity.MDSis commonly definedintwo ways. The 3dBMDSisthevalue oftheinputsignal,measuredin dBm or microvolts (uV)thatisjustperceivableattheoutput,thatis,aninputcausing a signal torise3dBoutofthedevice noise.MDSis also definedasthevalue

16、 ofthenoise floor andisusedinterchangeablywiththatterm. WhereMDSis usedinthistechnical note,it isequal tothenoise floor. Noise Floor where:NF BW -174 UNCLASSIFIED =NF+10logBW-174 =noise figure (ratio ofinputtooutputSINratio expressedasdB) =bandwidth in Hz =noise floor(-174dBm2900K) 132 DOCID:3928940

17、 RECEIVERDYNAMICSUNCLASSIFIED Amanufacturersspecification ofsensitivityinuV or dBmatacertainsignal-to-noise ratiois a different valuethanthenoise floor. For instance,theCW sensitivityoftheHF- 1000 is given by Watkins-Johnsonas-116dBm(.35uV) for 16 dBS+N/N,.3kHz bandwidthwith preamplifier offover 500

18、 kHz to 30MHz.3TheARRLmeasuredtheMDS (noise floor)as-133dBmat14 MHzusingthesame filter; a 17 dB difference calculating thenoise floor usingtheaboveequationandWJs 14 dB noise figure specification, which yields-135.3dBm. The difference couldbeinthe noise figure specification, which was notmeasuredby A

19、RRLandis providedasamaximumvalue by Watkins-Johnson. BlockingDynamicRange Blocking dynamicrange(BDR) indicates how wellthereceiver handlesstrongnearby signalsbefore desensitization occurs.Thisisanimportantparameterwhenattemptingto hearweakstationsinthe presenceofstronglocal signals.Blocking dynamicr

20、angeis referenced totheMDSandisthevalue ofaninputsignalthatcauses thegainto drop 1 dB. Therefore,ifa - 25 dBminputsignalcauses 1 dB ofgaincompression for a receiverwitha MDS of-135dBm, the blocking dynamicrangeis110 dB.Thereceiver filterbandwidth andthedistanceinkHzbetweenthetwosignalsmustbe specifi

21、ed tomakethis measurementmeaningful.Manufacturersoftenuse 100kHzspacingbutarenot consistent. Two-tone DynamicRange Two-tone dynamic range, also knownasintermodulation distortion (IMD) dynamic range,indicates the rangeofsignalsthatcan betoleratedbythereceiverbeforean undesirable spurious response is

22、developed.A spurious response is a distortion product thatresultsfrom receiver nonlinearities. Normally, receiver filtersrestricttheworst case tothethirdorder difference products.Fortwo frequenciesfland2,these productsare 2fl -2and22-fl.Thesumproducts (2fl +f2and22+1)arealso produced,butare outsidet

23、hereceiver filter bandwidthsandthusarenot considered. For instance, a signal at8030 kHzand8050 kHz will produce the following products: (Spur1)=(2x 8030) - 8050=8010 kHz (Spur2)=(2x 8050) - 8030=8070 kHz Inotherwords, twostrongsignalsat8030 kHzand8050 kHzarelikely tocreatea signal at8010.kHzand8070

24、kHzthatisreceivertunableifthethird-ordertwo-tone intermodulation distortion dynamicrange(lMD3) is exceeded.Ifa signal ofinterestexists on 8010 kHz, the intermod could easily coveritup. The IMD3 dynamicrangeis defined astheinputfrom twogeneratorsusing a specified frequencyseparation(20 kHz)that cause

25、s athirdorder spurious response toappearabovethenoise (MDS).Thatis: Two-tone dynamicrange(IMD)=MDS -1Mlevel 133UNCLASSIFIED DOCID:3928940 UNCLASSIFIEDCRYPTOLOGIC QUARTERLY For instance,ifa combinedsignalof -50 dBm causes a spurious signal toappearon 8010 kHz for a receiver with a-135dB MDS,theIMD is

26、-135- (-50)or85 dB. The receiver filter should be specifiedaswellasthesignal spacing. Intercept Point UP) Someyearsago the conceptofintercept point was introducedandhasbecome a useful andpopular specification forcomparingthequalityofvariousnonlinearelectronic components (amplifiers, mixers, couplers

27、, receivers). Theinterceptpoint isthetheoretical levelatwhich two-tonedistortionproductsintersectthesingle tonetransfercurveona plot ofoutputvsinputlevels.Normallythethirdorder productsareplotted; however,the intercept pointcanalso be found for the secondorotherorders.Tounderstandthis concept,rememb

28、erthattheoutputof alineardevice,sayanamplifier, will followthe inputaccording to the formula: Output=A (Input) .WhereA=thedevice gain. Thirdorderproducts(2fl-2)and(22-fl)will followtheformula: Output(3rd)=3A (Input) Thethirdorderoutputwill have a slopethatisthreetimesthatofthe desiredfundamental sig

29、nal. By plottingtheabove two equations withinputandoutputon the xandy axis, respectively, the two curvescanbe projected tointersectatsome point ontheplot;that point is knownasthethirdorderintercept point (lP3). The intercept pointcanbe givenin termsofinputoroutputlevel. TheoutputIPis theinputIPtimes

30、thegainofthe device. Deviceswithhigherinterceptpointsarebetterthanones with lowerinterceptpoints. The IP3 isrelatedto MDSandIMD3 by IP3=MDS+1.5(1MD3). Most commercial manufacturersspecify IP3 fortheirdevices.For instance,thebrochure on the Ten-Tec 330A receiver specifies a IP3of+30 dBm (input IP) wi

31、thpreamplifieroff.4Inmost cases thespecification is fortheinputintercept point.Itmay be worthaskingifyou seean unusuallyhigh IP specificationthatisnot designatedaseitherinputoroutput. Theinterceptplot (see Fig.1)shows both theinputandoutputIP#.Notethatthisis nottheactualtransfercurve,buta knee,thepo

32、int wherethereceiver goes intosaturation stylized plot which doesnotshow theactualtransfercurve. Sp.u-Free Dynamic Range Spur-free dynamicrange(SFDR) is the difference betweeninputnoise levelandinput signal level wheretheIMD3curveis equal to the noise level. Usingthisdefinition, wecan seethatSFDRis

33、the distancealongthex-axis betweenthetwo curves onanIP3 plotthat isextended to the noise floor.TheSFDR is shown ontheIP3 plot above. SFDR is defined as SFDR (dB)=2/3 (IP3-NF-101ogBW+174) UNCLASSIFIED134 DOCID:3928940 OutputIP3 o u t P u t d B m MDS InputdBm RECEIVER DYNAMICS Fundamental +-SFDR-t UNC

34、LASSIFIED InterceptPoint derProduct I;oise Floor InputIP3 Fig.1.Thirdorderinteceptpointplot Withalittlethought,you will recognizethisasjustarestatementoftheIP3equation above,andSFDR is equal to IMD3.Thesetwotermsareusedinterchangeably. Radio (BW=3kHZ) Racal21745 WJ-8711A Ten-Tec 330A NF dB 13 14 20

35、IP3 dBm +20 +30 +30 SFDR dB 88 104 100 Fig. 2.SFDRderivedfromreceiverspecificationsheets Sincemostmanufacturersprovide noisefigureandIP3 specifications,theSFDRnumber canbederivedandused forcomparisonpurposesacross anumberofsimilarradios. LOPhaseNoise Thereceiverlocal oscillatorexhibitsshort-terminst

36、abilityintheform ofphaseand frequency modulation.The effect oftheFM is to widentheLOto include frequencies aboveandbelowthecarrierintheformofFM noise.Inthemixingprocess,thisnoise is transferredtothesignalsinthepassbandofthereceiver. AsthedistancefromtheLO frequencyincreases,thisnoise is reduced. A t

37、ypical specification isindB belowthecarrier (-dBc)ina onehertzbandwidthatsome offset fromthecarrier.Thespecification forthe 135UNCLASSIFIED DOCID:3928940 UNCLASSifiEDCRYPTOLOGICQUARTERLY WJ-8711 is -110 dBc/Hzat1 kHz offset, typica1.6Thelowerthisnumberthebetter,and lowernumbersmeasuredcloser totheca

38、rrierarebetterthanthosemeasuredfarther away.Thetransferoflocaloscillatorphasenoise tosignalsinthemixerduringthe frequency conversion processiscalledreciprocal mixing.Phasenoise willtendtomask weaksignals inthepresenceofstrongsignalsasthephasenoise onthestrongsignalmay belargerthantheweaksignal. IFPa

39、ssband dBm MDS Strongsignal Phasednoise Desired signal Frequency Fig.3.Weaksignalmaskedbystrongsignalphasenoise Receiversusingphaselocked loop techniquestendtoexhibitmorephasenoisethan thoseusingcrystaloscillatorsordirectdigitalsynthesis(DDS)techniques.Notall manufacturersspecify reciprocal mixing.

40、LOSp.us Thelocal oscillatormayproducespurioussignals (spurs)resultingfromtheprocess usedinsynthesisoftheoscillator signal.Spursthatrise abovethenoisecanbe detected by terminatingthereceiverinputandtuningthroughthereceiverthroughitsrangewhile listeningfor signals.Spursshould be fewandweak,barelyrisin

41、gabovethenoise. Spuriousresponsesarespecified bythemaximumvalueoftheiramplitudeandnumber. Cross Modulation Cross modulation (CM) istheabilityofa receiver to reject modulationofa signal inits passbandby astrongsignaloutsidethepassband.Theundesiredsignalmustbeof sufficientamplitudeto drive oneoftherec

42、eiverstagesintononlinearoperation.Itis usuallygivenintermsofapercentageofeM-inotherwords,theamountofmodulation theundesiredsignalimposes onthedesiredsignal.Thedesiredandundesiredsignal amplitudes,modulation percentage,andfrequencyseparationarealsoparametersofthe specification. Because ofthevariables

43、andlack ofstandards,eM,althoughimportant,is notparticularlyuseful for receiver comparison purposes. UNCLASSIFIED136 DOCID:3928940 Image Rejection RECEIVER DYNAMICSUNCLASSIFIED In asuperhertrodynereceiverthereexistsanimage frequency which when mixed with theLOwill produce a signalintheIFbandpassalong

44、withthesignalof interest. For a receiver withanLOhigherthanthe tuned frequency,theimageis2IF+f,whereIFisthe IFfrequencyandf is thetunedfrequency.Forinstance,for a receiver tuned to500kHz withanIFof1100kHz,anLOof1600kHz,theimageis2700kHz.7Thismeansthata signalat2700kHz could bepresentintheIFalong wit

45、hthedesired500kHzsignalsif imagerejection is not high. Imagescanbe identifiedintheIFpassbandbecausethey tune backwards. Image rejection should be high,75dB,meaningthattheimage is morethan 75dB down from the signalofinterestintheIFpassband. Normally image rejection is given forthefirstIF. Image rejec

46、tion can be improved by converting thetunedsignalup infrequency,aswas doneinthecase above. Most modernHFreceiver systemsconvertthe RFinputsignal to ahighfirstIFinthelowVHFrange, which causestheimage to be well outof thepassbandofthereceiverandthusimprovestheimagerejection specification. This also im

47、provestherejectionofsignalsattheIF(IF rejection) byputtingthemwellout ofthereceivertuningrange.IFrejection specifications should begreaterthan80dBina modern communications receiver. dB Signal FsLO Image Fi LO-Fs=IF Fi-LO=IF Fig.4.Imagesignalrelationshipto LOandtunedsignal,Fs Manufacturersoften use m

48、icrovolts (uV)anddecibels referenced to onemilliwatt(dBm) interchangeablythroughouttheirspecificationsheets.ToconvertuV to dBm,the impedance(Z)mustbe known. In most casesthisis50or75ohms. Using the factthat power, p=e2/r, converted by dBm=10log (uV2/Z)(103) Need more information? K4hasseveralexperts

49、inthe receiver dynamicsareawho would be willing tointerpretspecificationsheetsandanswerdetailed questions.Your systemengineercan lead you to these people. 137UNCLASSIFIED DOCID:3928940 CRYPTOLOGIC QUARTERLY Notes 1.David Newkirk, Product Review - WatkinsJohnsonHF-lOOOGeneralCoverage Receiver, QST December 1994, ARRL Newington, CT, 76-79. 2. The specifiedS+NINdepends onthedemodulation selection, with 10 dB, 16 dB,and17dB being used by WatkinsJohnsonforAM/SSB,CWandFM, respectively. This isnotconsistentacross manufacturers. 3. Watkins-Johnson HF-1000 specificationsheet,October 1993. 4.Te