ITU-T G.621 (11 1988) (英) Characteristics of 0.7 2

INTERNATIONALTELECOMMUNICATIONUNION)454'TELECOMMUNICATIONSTANDARDIZATIONSECTOROFITU42!.3-)33)/.-%$)!#(!2!#4%2)34)#3#(!2!#4%2)34)#3/&MM#/!8)!,#!,%0!)23)454Recommendation'(ExtractfromtheLUEOOK)NOTES1ITU-TRecommendationG.621waspublishedinFascicleIII.3oftheBlueBook.ThisfileisanextractfromtheBlueBook.WhilethepresentationandlayoutofthetextmightbeslightlydifferentfromtheBlueBookversion,thecontentsofthefileareidenticaltotheBlueBookversionandcopyrightconditionsremainunchanged(seebelow).2InthisRecommendation,theexpression“Administration”isusedforconcisenesstoindicatebothatelecommunicationadministrationandarecognizedoperatingagency.ITU1988,1993Allrightsreserved.Nopartofthispublicationmaybereproducedorutilizedinanyformorbyanymeans,electronicormechanical,includingphotocopyingandmicrofilm,withoutpermissioninwritingfromtheITU.FascicleIII.3-Rec.G.6211RecommendationG.621FascicleIII.3-Rec.G.621CHARACTERISTICSOF0.7/2.9mmCOAXIALCABLEPAIRS(Geneva,1976;amendedatGeneva,1980)Administrationswhichdecidetousefordigitaltransmissions,andpossiblyalsoforparticulartypesofanaloguetransmission,coaxialpairssmallerthanthe1.2/4.4-mmcoaxialpairshouldasfaraspossiblechoosepairscomplyingwiththespecificationsgiveninthisRecommendation.TheuseofthesepairsisdefinedinTables1and2givenintheintroductiontoSubsection6.2.1Paircharacteristics1.1Electricalcharacteristicsofthecoaxialpair1.1.1CharacteristicimpedanceThenominalvalueoftherealpartofthecharacteristicimpedanceat1MHzshouldbe75Ω.Themeanrealpartoftheimpedanceofacoaxialpairat1MHzshouldnotdifferfromthenominalfigurebymorethan±2.5Ω.Table1/G.621showsthegeneraltrendofthevariationoftheimpedanceasafunctionoffrequency.TABLE1/G.621MeanrealpartoftheimpedancemeasuredatvariousfrequenciesFrequency(MHz)0.20.51251020∞Impedance(Ω)77.775.97574.273.47372.872.21.1.2AttenuationcoefficientThenominalvalueoftheattenuationcoefficient,at10°Candat1MHz,isequalto8.9dB/km.Table2/G.621showsthegeneraltrendofthevariationinattenuationcoefficientasafunctionoffrequencyatthetemperature10°C.TABLE2/G.621MeanvaluesoftheattenuationcoefficientatvariousfrequenciesFrequency(MHz)0.20.51251020Attenuationcoefficient(dB/km)4.56.58.912.619.828.039.61.2MechanicalconstructionofthecoaxialpairThepairhasthefollowingconstitution:a)nominaldiameterofsolid-copperwireinnerconductor:0.7mm;2FascicleIII.3-Rec.G.621b)nominalinternaldiameterofouterconductor:2.9mm;c)outerconductorconsistingofacoppertapewithathicknessoftheorderof0.1mm,laidlengthwisewithoverlap1);d)screenconsistingofasteeltapewithathicknessoftheorderof0.1mm,laidlengthwisewithoverlap1).2Cablespecification(factorylengthsofabout500m)2.1CharacteristicimpedanceTocheckthatthevaluegivenin§1.1.1ismet,pulsemeasurementscanbemade.Themeanrealpartoftheimpedanceat1MHzistobetakenasmeaningtheresistivecomponentoftheimpedanceat1MHzofthenetworkwiththebestbalanceagainstthecoaxialpairmeasured.2.2ImpedanceregularityRoutinecontrolmeasurementsofimpedanceregularityarecarriedoutbymeansofpulseechometersfromoneorbothendsofthefactorylengths.Theechocurveshouldbeplottedwithcorrectioninamplitudeandifpossibleinamplitudeandphase.Table3/G.621showsthevariousvaluestobeobtainedaccordingtothepurposeforwhichthecableisintended.TABLE3/G.621Echometricmeasurementoffactorylengthsa)TypeofsystemDigitalBitrateMediumbitrate(6to34Mbit/s)Maximumpulseduration100nsGeneralprovisionsMaximumpeak100%36dB95%39dBAdditionaloptionalprovisionsa)AMeanof3maximumpeaks39dBBEquivalentresistanceerrora)ItisenoughtocheekthatoneofthetwoconditionsAorBisfulfilled.Note1-Thepercentagefiguresgiveninthetablerelatetoallthepairsofabatchofcablessubmittedforcontrolordeliveredatthesametime.Note2-Withtheconstructiontechniquesusedsofar,systematicfaultsdonotgiverise,insteady-statemeasurementsofregularityreturnloss,topeaksatfrequenciesbelow60MHz.Forthisreason,andtaking_______________1)Asinglebimetalliccopper-steel-coppertapemayalsobeusedtoserveasouterconductorandscreen.FascicleIII.3-Rec.G.6213intoaccountthebitrateenvisaged,steady-statemeasurementsofregularityreturnlossdonotseemnecessary.Forothertypesofconstructionwhichmightbeusedinfuture,supervisionoftheregularityreturnlossmightbewise;insuchcases,thevalueshouldbe20dBfrom4to60MHz.2.3AttenuationcoefficientTheattenuationofpairsshouldbesuchastoallowcompliancewiththeprovisionsof§3.3below2).2.4Near-endcrosstalkattenuationThenear-endcrosstalkattenuationbetweencoaxialpairsusedfordifferenttransmissiondirections,measuredinthefrequencyband0.5-20MHzonfactorylengths,mustbeabove135dBfor100%ofmeasurements.2.5DielectricstrengthThepairshouldwithstandana.c.voltageof1000r.m.s.at50Hz(orad.c.voltageof1500volts)appliedforatleast1minutebetweenthecentreandtheouterconductor.Ifinnormalservicetheouterconductorsofthecoaxialpairsarenottobeearthed,adielectricstrengthtestmustbecarriedoutbetweentheouterconductorsandtheearthedmetalsheath.Forthistest,ana.c.voltageofatleast2000voltsr.m.s.at50Hzorad.c.voltageofnotlessthan3000Vwillbeapplied.2.6InsulationresistanceTheinsulationresistancebetweenthecentreandouterconductorsofthecoaxialpair,mea