XITRON XT2640 - Page 116

XT2640OperatingManual 13July2016 Page116of187
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shortestandlongestsuchmeasurementperiod.Forexample,ifoneVPAismeasuringmainsatafrequencyof50Hz,and
anotherismeasuringtheoutputofanelectroniclightingballastatafrequencyof15.23456kHzandbotharesetfora
measurementperiodof100ms,thenthefirstmeasurementperiodwillbethatof5cycles(i.e.100ms)whilethesecondwillbe
thatof1523cycles(i.e.99.97ms)sothisincursanadditional30µsofdifference(i.e.anerrorof0.03%ofchangebetween
measurementperiods).Itshouldbenotedthatthiserroristypicallyfarlessthanthatcausedbythecommonindustry
practiceoferroneouslyforcingallmeasurementperiodstobeexactlythesameasthatcausesallbutoneofthesignalstobe
incorrectlymeasuredwhetherthesignallevelsarechangingornot.TheXT2640doesnotforceallmeasurementperiodstobe
identical.
Rangechangestypicallyhavethemostdominanteffectastheyverysignificantlyelongateameasurementperiod(possiblyby
10:1ormore)onlyintheaffectedchannel(s).Thereisacommonindustrypracticeofrequiringarangechangeforchangesin
voltageand/orcurrentlevelsof3:1orlesswhichisverydominantinthesesituationsassuchachangeforceslossof
synchronizationandtheinaccuracycausedbyforcingthehighestsuitablerangeisgenerallymuchlargerthantheinter‐
measurementdelayerrors(e.g.ifinaccuracyisspecifiedas0.05%rangethenthatisanerrorofupto0.45%causedby
selectingafixedrangewhichisonlyonerangehigherthanneededforthatparticularlevel).IntheXT2640thisisoffarless
consequenceasarangechangecanonlyoccurinDtypecurrentoptionchannels,onlytakes1ms,isonlyasinglerangechange,
onlyoccurswhenaverylargechangeincurrentleveloccurs(1000:1),andhasanegligibleeffectonaccuracy.
Recommendation:
Asdiscussedabovethereisconsiderablemulti‐castLANtrafficwhenmeasurementperiodsarebeingsynchronizedbetween
multipleXT2640’sinthismanner.Thiscanhaveaslighteffectonothernetworktrafficifitisallowedtotraversetheentire
network.Multi‐casttrafficisnotpassedthroughacorrectlyconfiguredrouterandalsoIGMP“snooping”switchescanbeused
whichlimitmulti‐casttraffictoonlythosenodeswhichrequestit(i.e.onlyXT2640’s).WhenmultipleXT2640’sare
synchronizedinthismanneritisrecommendedtolimitanyeffectsontheremainderofthenetworkbyusingsuchdevicesas
needed.
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