Updating the Bob Smith Termination Technique.docx
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UpdatingtheBobSmithTerminationTechnique
UpdatingtheBobSmithTerminationTechnique
WhiletheBobSmithTerminationtechniquehaswonacclaimworldwide,ithasitsflaws.Fortunately,anewterminationtechniqueisproposedherethatcansolvemanyoftheproblemsdesignerswillencounterwhenusingtheBobSmithapproach.
ByJimSatterwhite,TeltestElectronicLaboratories
FordesignersofEthernetsystems,theBobSmithTerminationtechniqueisregardedasanimportantapproachformigratingpotentialRFemissionandsusceptibilityproblemsforEthernetinfrastructure.But,whatifthetechniquehasflaws?
Unfortunately,therearesomeflawswiththeterminationtechniquepatentedbyBobSmithINthe90s.ThisarticlepointsoutpotentialflawsandthenshowshowaproposedterminationtechniquecansolvesomeoftheproblemsencounteredusingaBobSmithterminationapproach.Thearticlealsoprovidesaphysicalverificationmethodthatdesignerscanusetoanalyzetheimpactoftheproposedterminationtechnique.
Theproblematicresultsofimproperterminationaredifficultdifficulttomeasureandverify.Theyshowupasproblemswithelectromagneticcompatibility(EMCcertificationandworseasintermittentanddifficulttoidentifyperformanceproblemsinservice.Thus,thisarticlewillalsoprovideasoundtheoreticalapproachforevaluatingtheproposedterminationscheme.
ProblemswithMr.Smith
InaU.S.patent,RobertW.(Bob)Smithdescribedamethodtoreducethelongitudinalorcommonmodecurrentonmulti-pairconductorsystemswherethepairsareinterrelatedinauniformmanner.HealludestothefactthatthepairtopairrelationshipsofaCAT5cableformtransmissionlinesinthemselves.
Smithassertsincorrectlythatsuchtransmissionlinesexhibitacharacteristicimpedanceofapproximately145ohms.Hisdescriptionisconfusingandmisleadingbecausehedoesnotdefinetheconfigurationinwhichthisissupposedlyexhibited.Hecorrectlygoesontosaythatthecablecansupportandradiatecommon-modesignals.Moreover,hestatesthatthereisamethodtoreducethecommonmoderadiationbymatchingthecommonmodeimpedanceofthecable,andthenpresentsamethodofterminatingthecommonmodetransmissionline,thatdoesnotinactualityaccomplishthatgoalwell.Histerminationtechniquedoesreducethecommonmoderadiation,butnotoptimallyandnotfromthecorrectunderstanding.
ForaCAT5cable,therearetwocommonmodesofinterest.Thefirstisthepair-wisecommonmodeandthesecondisthecable-wisecommonmode.Inthearticletofollow,we'llnoteswilladdressthepair-wisecommonmode.
Thepair-wisecommonmodehasacommonmodecharacteristicimpedancethatcanbereadilycharacterized,whilethecable-wisecharacteristicimpedanceisgrosslydependentonimplementation.Actually,thecharacteristicimpedanceofthecable-wisecommonmodeisnotameaningfulconceptexceptinsomewell-definedcircumstances.
Withthatbackgroundinmind,let'slookatthepair-wisecommonmodecharacteristicimpedancerelationshipswithinaCAT5cable.AsFigure1pointsout,thepair-wisemodecallsforfourpairswithanintrinsiccharacteristicimpedancebetweeneachpair.Duetotheapproximatesymmetryintheconstructionofthecable,eachpairhasthesamerelationwithanyotherpair.Itisprudenttodefinetherelationshipstobedescribed(Smithdoesn'tdothat).Sincetheinherentcharacteristicimpedancescannotbemeasureddirectly,toverifythetheoryitisnecessarytomeasurethemincombinationsandresolvetheindividualelements.
Figure1:
Pair-wisecharacteristicimpedancesonaCAT5cable.
Thefirstrelationshipisbetweenanytwopairs(Z1-1).Thisparameterissomewhatmeaninglessinapplicationbecauseallfourpairsareinterrelated,butitisusefulandcanbecharacterizednone-the-less.Thesecondandmoremeaningfulrelationshipisthecharacteristicimpedanceofonepairrelativetotheotherthreepairs(Z1-3).Thethirdistherelationshipoftwopairstotheothertwopairs(Z2-2).Forthecaseofonepairversusanyotherpairwiththetworemainingpairsfloating,themeasuredcharacteristicimpedanceisapproximately100ohms.
Forthecaseofonepairversustheotherthreepairstiedtogetherthemeasuredcharacteristicimpedanceisapproximately70ohms.Forthecaseoftwopairstiedtogetherversustheothertwoparstiedtogetherthemeasuredcharacteristicimpedanceisapproximately50ohms.NowheredoesthereexistarelationshipthatthecharacteristicimpedanceisanywherenearSmith'sstated145ohms.
Figure2showsthedefinitionsofsomeofthepossiblemeasurablecharacteristicimpedances.
Figure2:
Definitionsofpossiblemeasurablecharacteristicimpedances.
Table1showssomemeasureddatasamples.TheseweretakenusingtheAgilent4396BNetworkAnalyzer.
Table1:
MeasuredDataSamples.
Analysis
Analysisofthemeasuredvaluestodeterminetheinherentcharacteristicimpedancevaluesiscomplex.Sometechniquesinclude:
Z1-3(1,234)=Z14//Z12//Z13
Z2-2({12,34)=Z14//Z23//Z13//Z24
Z1-3(1,234)=Z12//(ComplicatedcombinationofZ14,Z34,Z23,Z13,Z24)
Thisdoesnotappeartobereadilymanageable.Therefore,asimulationwasdevelopedselectingvaluesforZnntogivevaluessomethingnearthemeasureddata.Fromthis,theintrinsiccharacteristicimpedanceZnnappearstobeinthevicinityof200ohms.Fromthissimulation,itcanbesuggestedthatthevalueofZnnvariesalongthelengthofthecableaswellasfrompairtopair.Thus,toproperlyterminatethecommonmodetransmissionlines,thetechniqueinFigure3mustbeused.
Figure3:
Diagramshowingtheproposedterminationtechniqueapplied.
OnewaytovisualizeaproposedterminationmethodistoterminatetheZ1-3characteristicimpedance.First,thattransmissionlinemustbeterminatedwithavalueequaltoitscharacteristicimpedance.Asingleresistorterminationwillaccomplishthis,howevertheotherthreepairsarenottiedtogetherinnormaluse.
Amoregeneralapproach,intheory,istoformaresistornetworkcomprisedoffourresistorsofequalvalue,oneinserieswiththeotherthreeinparallel.Oncethenetworkissetup,designerscanthencomputetheresistancevaluesuchthatthenetworkmatchestheZ1-3characteristicimpedanceofthecable(Figure4).
Figure4:
Matchingequations.
Smithclaimsthatallfourresistorsshouldbe75ohms(Figure5).Thismeansthata75-ohmresistorinallfourterminationswouldmatchacommonmodeimpedanceof100ohmsforZ1-3.
Figure5:
DiagramshowingtheSmithterminationtechnique.
ButSmithclaimsthecharacteristicimpedanceof"some"common-modeconfigurationthatheisattemptingtomatchis145ohms.Sinceit'sdifficulttounderstandinthepatenttheconfigurationSmithisdescribingandconfigurationstypicallyarenotanywherenear145ohms,itcanonlybeconcludedthatSmith'snumbersandapproachtoderivingthemarewrong.Additionally,hismatchingtechnique,givenacommonmodecharacteristicimpedance,doesnotproduceamatch.
Thus,Smithhasn'tpresentedareallyworkablesolutiontotheproblemhehasrecognized.Forexample,forCAT5andCAT5Ecableanimprovementinreturnlossachievedbyreplacingthe75ohmresistorsby52.3ohmresistors,wouldbefrom15dBtomorethan28dB,givenavariationincommonmodeimpedanceof+/-5ohms.Thevariationofcommonmodecharacteristicimpedanceistakenintoaccountinthecalculationofreturnloss(Table2).
PhysicalVerification
Fortheaboveconclusionstobeaccurate,it'simportanttohaveaphysicalverificationmethod.Fortunately,thereisarelativelysimplemethodfordoingverifyingtheaboveclaims.
Undertheproposedverificationmethod,designersconfigurealengthofcablewiththeterminationresistors,theninsertacommonmodepulsewhilemonitoringthatinsertionwithanoscilloscope(apoorman'sTDR).Apulsegeneratorwithanoutputimpedanceof50ohmsandarisetimeofapproximately1nswasused(Figure6).
Figure6:
Diagramshowingtheverificationsetup.
Intheverificationmethod,ifthecableisproperlymatched,noreflectionshouldbeseen.Ifnot,areflectionwillbeobserved.
Intheexampledescribedinthisarticle,terminationsofthreevalueswereappliedateachendofacablesevenfeetlong.Theresistorvaluesemployedweresetsof52.3,75and100ohms.Thecableemployedwas7feetlong.
TheseriesofscopepresentationsinFigure7showimplementationsofthevariousterminationsforcomparison.Thefactthataterminationinthevicinityof52.3ohmsisproperforthematchcanbeclearlyseen.
Figure7:
Diagramshowingreflectionmatchviews.
Forcalibrationpurposes,severalconditionsareshownareshowninFigure7.Thefirstscopepresentationshowstheoutputofthepulsegeneratorwithnoload.ThesecondshowsthecableconnectedtothepulsegeneratorasinFigure6andthefarendopencircuited.Thethirdisthesameconditionswiththefarendshort-circuited.Theremainingthreescopepresentationsshowtheresultsforthethreevaluesoftheterminationresistance.
Itisclearthatforthe100ohmand75ohmresistancesthereisareflection,whereas,with52.3ohmsthereisnot.Theconclusionisthatinthevicinityof52.3ohmsiscorrectandthatBobSmith's75-ohmvalueisnotcorrect.
Asanote,CAT6cablehasahighercommon-modeimpedance.Thisisexpectedduetothesignificantdifferenceinitsconstruction.Itisanticipatedthatalsoduetoitsmethodofconstructionthatthecharacteristicimpedanceswillbemuchmoreuniformbothversuseachotherandalongthelengthofthecable.
TheoreticalAnalysis
Figure8showsthemodelforcomputationofthetermination