Eye-Diagram(眼图-英文)

SR-TN03314October2004–APrimerGuyFosterSyntheSysResearch,Inc.October2004SR-TN033AbstractThispaperdescribeswhataneyediagramis,howitisconstructed,andcommonmethodsoftriggeringusedtogenerateone.Itthendescribesdifferentwaysthatinformationfromaneyediagramcanbeslicedtogainmoreinsight.Italsodiscussessomebasicwaysthattransmitters,channels,andreceiversaretested.Itisdesignedtogiveanengineernewtothisfieldabasicgraspoftheconceptscommonlyused.EyeMeasurementsBasicsEyediagramsareaverysuccessfulwayofquicklyandintuitivelyassessingthequalityofadigitalsignal.Aproperlyconstructedeyeshouldcontaineverypossiblebitsequencefromsimple101’sand010’s,throughtoisolatedonesafterlongrunsofconsecutivezerosandotherproblemsequencesthatoftenshowupweaknessespresentinsystemdesign.Figure1:Overlayingofbitsequencestoformaneyediagram.2Whatdoesitshow?Eyediagramsshowparametricinformationaboutthesignal–effectsderivingfromphysicssuchassystembandwidthhealth,etc.Itwillnotshowprotocolorlogicalproblems–ifalogic1ishealthyontheeye,thisdoesnotrevealthefactthatthesystemmeanttosendazero.However,ifthephysicsofthesystemmeanthatalogiconebecomessodistortedwhilepassingthroughthesystemthatthereceiveratthefarendmistakesitforazero,thisshouldbeshowninagoodeyediagram.Commonwaysofcharacterizinganeyearetomeasuretherisetimes,falltimes,jitteratthemiddleofthecrossingpointoftheeye,theovershootpresentandmanyothernumericaldescriptionsofeyebehaviorinordertocomparedevicesbeingmeasured.Instrumentsusuallyofferautomatedmeasurementsthatsimplifyandspeedupthetakingofsuchmeasurements.TheEffectsofTriggeringonEyeDiagramsManyeyediagramsareconstructedontestequipmentthatusestestpatternsthatrepeat,suchasPRBSpatternsgeneratedbyaBERT(BitErrorRatioTester).Suchequipmentcancommonlygenerateavarietyoftriggersignals:1)Aclocktriggeratthesamerateas,andsynchronouswiththedatasignal2)Adividedclocktriggeratsomedivideratioofthedatarateoftenrelatedtoapowerof2suchas÷4,÷16,etc.3)Apatterntrigger–asignalthatprovidesatriggeronceperpatternrepetition.4)Thedataitselfcanbeusedasatrigger.5)Thelastoptionistoderivethetriggersignalbyusingclockrecoveryonthedatasignal.Eachmethodprovidesdifferentresultswhenusedtoconstructawaveform.ClockTriggerprovidesaclassicaleyediagramcontainingallpossiblebittransitionsinonedisplay.Figure2:AneyediagramformedwithafullrateclocktriggerDividedClockTriggeralsoproducesaneyediagram,andthiscanbeusefulwhentheinstrumentbeingusedtoconstructtheeyehasatriggerinputbandwidthnarrowerthanthedatarateofthesignalbeingviewed.Thismethodcanproduceagoodeyeunlessthepatternlengthdividedbythedivideratioyieldsaninteger–forexamplea128-bitpatternviewedwitha÷4clock.Inthiscase,thetriggersignalwillcoincidewiththesamebitsinthepatterneachtimewhileconsistentlymissingotherpartsofthepattern–leadingtoanincompleteeye.Figure3:AneyediagramformedwithadividedclocktriggerSR-TN03314October2004:AbitsequencecapturedusingapatterntriggerTriggeringonDataistheleastsatisfactorymethodofconstructinganeyeandshouldonlybeusedasaquicklook-see.Longrunsofidenticalcharactersprovidenotransitionstotriggerfrom,andsoacompleteeyeisalmostimpossibletoachieve.Figure5AnincompleteeyediagramformedbytriggeringondataTriggeringfromaRecoveredClock:Whileincreasingcomplexity,thismethodhassomeadvantages:•Insomesituationsaclocksignalisnotavailableandsomustbederived•Inothercases,particularlylongdistancefiberopticcommunications,therelationshipbetweentheclockatthetransmitendandthedataatthereceiveendmaybecorruptedbyrapidlytime-varyingeffectsinthetransmissionpath.•Lastlyincaseswherethereceiverusesclockrecoveryandtheeyeasseenbythereceiverneedstobeexamined–thiscanberequiredbysomestandards,particularlyforjittertesting.Circuitsusedforrecoveringclocktypicallyhavealoopbandwidth,orfilteringfunction,thatremovesfromtheclocksignalsomeofthejitterthatwaspresentonthedatasignal.Dependinguponthemeasurementbeingmade,thiscanbehelpfulorhurtful,butneedstobeunderstood.Narrowloopbandwidthclockrecoverytendstogivearocksolidclocktriggersignalasthereference,andanyjitter,ormovementofedgeswithtime,inthedataeyediagramthatispresentwillbedisplayed.Thisisausefulabsolutemeasurebutmightnotproperlyrepresentthejitterseenbyarealsystemifthereceiverusesclockrecoverytotracksomeofthejitterout.Figure6:Aneyediagramtriggeredfromaclockrecoveredfromthedatasignalusinganarrowloopbandwidthclockrecoveryscheme.SR-TN03314October2004