1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G0-%$)!G0G0#(!2!#4%2)34)#3#(!2!#4%2)34)#3G0G0/ further amended)The following Recommendation describes the 1.2/4.4 mm coaxial pair recommended by the CCITT for theinternational service. The use of
2、this pair is defined in Tables 1 and 2 given in the introduction to Subsection 6.2. Whenthe possibility of television or digital transmission has been envisaged, it is expressly mentioned in each provision.1 Characteristics of the pair1.1 Electrical characteristics of the coaxial pair1.1.1 Character
3、istic impedanceThe nominal real part of the characteristic impedance is 75 at 1 MHz.The tolerance is 1.5 for telephony or 1 for pairs that may be used for television transmissions.For information, the impedance values in Table 1/G.622 were obtained at various frequencies on coaxial pairsmanufactured
4、 by different processes.TABLE 1/G.622Means real part of the characteristic impedance measured at various frequenciesFrequency (MHz) 0.06 0.1 0.2 0.5 1 1.3 4.5 12 18Impedance () 79.8 78.9 77.4 75.8 75 74.8 74 73.6 73.51.1.2 Attenuation coefficientThe nominal value of the attenuation coefficient of th
5、e pair, at 12 MHz and at 10C, is 18.0 0.4 dB/km.Table 2/G.622 shows the general trend of the variation of the attenuation coefficient as a function of frequencyfor all pairs which conform to the present Recommendation.TABLE 2/G.622Nominal values of the attenuation coefficient at various frequenciesF
6、requency (MHz) 0.06 0.1 0.3 0.5 1 1.3 4.5 12 18Attenuation coefficient (dB/km) 1.5 1.8 2.9 3.7 5.3 6.0 11 18 22The following equation, in which is expressed in dB/km and f in MHz, gives an approximation of theattenuation coefficient from 2 MHz onwards:= + +0 07 5 15 0 005 .ff.2 Fascicle III.3 - Rec.
7、 G.622Note - By way of information, Annex A shows the values measured or specified in various countries, with thecorresponding deviations or tolerances. In any case, amplifier design must be based on the values measured on the typeof cable which will actually be used.1.1.3 Attenuation distortionThe
8、attenuation distortion required in particular for digital transmission is checked by calculating the ratioff12between attenuation values f 1and f 2measured at two frequencies f1and f2.One of the following three limits should be observed:1)The attenuation distortion is checked in the factory on a sma
9、ll percentage of factory lengths.1.2 Mechanical construction of the coaxial pairThe nominal dimensions are the following:- diameter of solid copper centre conductor: 1.2 mm;- inner diameter of outer conductor: 4.4 mm.The cylindrical outer conductor is obtained using a copper tape with a thickness of
10、 0.15 or 0.18 mm.2 Cable specification2.1 Characteristic impedanceTo check that the value given in 1.1.1 above is met, pulse measurements can be made. The real part ofimpedance at 1 MHz is to be taken as meaning the resistive component of the impedance at 1 MHz of the network withthe best balance ag
11、ainst the coaxial pair measured.2.2 Impedance regularityRoutine control measurements of impedance regularity are carried out by means of pulse echometers from oneor both ends of the factory lengths. The echo curve should be plotted with correction in amplitude and if possible inamplitude and phase.
12、If the equivalent resistance error is measured, it must be corrected. However, for routinemeasurements, correction may be dispensed with if the test length is so short that the correction is small.Table 3/G.622 shows the various values to be obtained according to the purpose for which the cable is i
13、ntended._1)These three conditions are equivalent. Accordingly, only one of them is to be used for checking attenuationdistortion.Fascicle III.3 - Rec. G.622 3TABLE 3/G.622Echometric measurement of factory lengthsType of system Analogue DigitalFrequency range or bit rate 0.06-6 MHz 0.3-20 MHzMedium b
14、itrate(6-34 Mbit/s)High bit rate(140 Mbit/s)Maximum pulse duration 100 ns 50 ns 50 ns 10 nsGeneral Maximum100% 45 dB 48 dB 48 dB 48 dBprovisions peak95% 50 dB 50dB 50dB 49dBAdditionalAMean of3 maximum peaks48dB 51dB 51dB 47 dBoptionalprovisionsa)B Equivalentresistance error1.2 1.6 1.6 2.5 a)It is en
15、ough to check that one of the two conditions A or B is fulfilled.Note 1 - For 0.06-1.3 MHz analogue systems, the provisions are the same as for 0.06-6 MHz analogue systems.Note 2 - To detect systematic irregularities, return wave attenuation measurements should be carried out on a small proportion o
16、ffactory lengths. The limits to be observed are set out in Table 4/G.622.Note 3 - The percentage figures given in the table relate to all the pairs of a batch of cables submitted for control or delivered atthe same time.4 Fascicle III.3 - Rec. G.622TABLE 4/G.622Return wave attenuation on irregularit
17、iesType of system DigitalFrequency range or bit rateMedium bit rate(6-34 Mbit/s)High bit rate(140 Mbit/s)Percentage of lengths concerned about 5% about 5%Frequency band explored 1-40 MHz 20-100 MHzMinimum measured value100% 20 dB 20 dB95% 23 dB 23 dB2.3 Attenuation coefficientThe attenuation of pair
18、s should be such as to allow compliance with the provision of 3.3 below2).If reference is made to the length measured along a generatrix of the cable sheath, the attenuation coefficientshould be multiplied by the take-up factor, the values of which for different numbers of pairs contained in the cab
19、le aregiven as an indication in Table 5/G.622.TABLE 5/G.622Take-up factor valuesNumber of pairsin cableTake-up factorlast layerWeighted take-up factor,entire cable4 or 6812-1824481.0041.0051.0081.0021.0031.0031.0041.0062.4 CrosstalkThe crosstalk between pairs should be such as to allow compliance wi
20、th the provisions of 3.4 below2)._2)At this stage of manufacture, attenuation and crosstalk measurements are merely prototype measurements.Fascicle III.3 - Rec. G.622 52.5 Dielectric strengthThe pair should withstand an a.c. voltage of 1000 V r.m.s. at 50 Hz (or a d.c. voltage of 1500 V) applied for
21、 atleast one minute between the centre and outer conductors.If, in normal use, the outer conductors of the coaxial pair are not earthed, a dielectric strength test is madebetween the outer conductors and the earthed metallic sheath. The conductors of the auxiliary quads or pairs areconnected to the
22、outer conductors of the coaxial pairs or to the sheath, according to the kind of system used for thesequads or pairs. Under these conditions, an a.c. voltage of 2000 V r.m.s. or more at 50 Hz will be applied for at least oneminute (or a d.c. voltage of 3000 V or more).Note - The test voltages recomm
23、ended take account of the normal safety margins applied in the variouscountries. Polythene insulation, however, might reasonably withstand considerably higher test voltages. In any case,some other dielectric might conceivably be used in the future.2.6 Insulation resistanceThe insulation resistance b
24、etween the centre and outer conductors of the coaxial pair, measured with a perfectlysteady voltage of between 100 and 500 V, should not be less than 5000 M-km after electrification for one minute, at atemperature not lower than 15 C. The measurement of the insulation resistance should be made after
25、 the dielectricstrength test. This measurement should be made on each factory length.3 Elementary cable section specification3.1 End impedanceThe conditions described in 1.1.1 and 2.1 above are applicable.3.2 Impedance regularityImpedance regularity measurements are carried out from each end of the
26、elementary cable section. Referenceshould be made to one of the columns in Table 6/G.622, according to the purpose for which the cable is intended.3.3 Attenuation coefficientAt 1 MHz, the real attenuation coefficient must not differ from the nominal figure by more than 0.2 dB.Attenuation measured on
27、 a cable at an average temperature of tC is referred to 10 C by the formula:The coefficient k, of the variation in attenuation with temperature is about 2 x 10-3per C at frequencies of500 kHz or more. It increases slightly at lower frequencies (about 2.8 x 10-3per C at 60 kHz).3.4 CrosstalkThe far-e
28、nd crosstalk ratio between two coaxial pairs in a cable transmitting in the same direction at anyfrequency in the band actually transmitted must be not less than the values given in Table 7/G.622.6 Fascicle III.3 - Rec. G.622TABLE 6/G.622Echometric measurement of elementary cable sectionsa)Type of s
29、ystem Analogue DigitalFrequency range or bit rate 0.06-6 MHz 0.3-20 MHzMedium bitrate(6-34 Mbit/s)High bit rate(140 Mbit/s)Maximum pulse duration 200 ns 100 ns 100 ns 50 nsGeneral Maximum100% 42 dB 42 dB 42 dB 40 dBprovisions peak95% 46 dB 46 dB 46 dB 44 dBAdditionalAMean of3 maximum peaks.Uncorrete
30、dmaximum45 dB48 dB45 dB48 dB45 dB48 dB43 dB46 dBoptionalprovisionsa)EquivalentBEnergycorrected( km -1/2)2 2.5 2.5 3.5resistanceerrorC Uncorrected () 1.8 2.0 2.0 2.5a)It is enough to check that one of the three conditions A, B or C is fulfilled.Note 1 - Notes 1 and 2 to Table 3/G.622 still hold good.
31、 However, for 0.06 to 1.3 MHz analogue systems, the provisions of column0.06 to 6 MHz apply, but the pulse duration may attain 400 ns for elementary cable sections longer than 4 km.Note 2 - Measurements using sine-wave signals on elementary cable sections are unnecessary unless there are serious gro
32、unds forbelieving that systematic irregularities may have been introduced during the laying or installation of the cable. In such cases, themeasurement results should not be less than 20 dB.TABLE 7/G.622Minimum far-end crosstalk ratio between two 1.2/4.4 mm coaxial pairsLength of the sectionFar-end
33、crosstalk ratio (dB)(km)Without phase inversionWith phase inversionat repeaters864328789939599-80-83-Fascicle III.3 - Rec. G.622 7There is no need to specify a near-end crosstalk ratio when the former limits are chosen for the far-end crosstalkratio.When phase inversion is used, the near-end crossta
34、lk ratio for pairs transmitting in opposite directions must beat least 84 dB for a section about 6 km long, and 87 dB for a section about 3 km long.Note - These limits enable a far-end crosstalk ratio of 65 dB to be obtained on the worst homogenous 280-kmsection, assuming that for the frequencies in
35、 question only far-end crosstalk due to the cable is to be considered3). It isassumed that the variation in the minimum far-end crosstalk ratio as a function of the distance approximately follows a20 dB/decade law for distances below a limit distance L1and a 10 dB/decade law for distances above L1.
36、The valuesdepend on a number of factors, mainly the system used, the type of cable and the considered frequency. A value of 30km appears suitable in most cases, although values of L1ranging from a few kilometers to 30 kilometers have beenobserved in practice, ensuring the consistency of the limits i
37、n Table 7/G.622 with the 65 dB limit on a 280 km section.3.5 Dielectric strengthThe pair must withstand a d.c. voltage of at least 1000 V applied during at least one minute between the innerand the outer conductors.In addition, a test of dielectric strength between the coaxial pair and earth shall b
38、e made as described in 2.5,using a d.c. voltage of at least 2000 V applied for one minute.Note - The recommended test voltages take account of the normal safety margins applied in the variouscountries. Polythene insulation, however, might reasonably withstand considerably higher test voltages. In an
39、y case,some other dielectric might conceivably be used in the future.3.6 Insulation resistanceThe insulation resistance between the centre and outer conductors of the coaxial pair, measured with a perfectlysteady voltage of between 100 and 500 V, should not be less than 5000 M-km after electrificati
40、on for one minute. Themeasurement of the insulation resistance should be made after the dielectric strength test. This measurement should bemade on every elementary cable section._3)In practice it is possible to forget the influence of line equipment on intelligible crosstalk, but this is only truef
41、or low frequencies of the band (less than 300 kHz).8 Fascicle III.3 - Rec. G.622ANNEX A(to Recommendation G.622)Examples of attenuation coefficient measured or specified in some countries(Values given as an indication)TABLE A-1/G.622Values measured on a type of pair whose outer conductor is 0.15 mm
42、thickFrequency (MHz) 0.060 0.1 0.3 0.5 1 4 12 18 52Attenuation (dB/km) 1.54 1.85 2.89 3.67 5.21 10.4 18.0 22.0 37.5Tolerance (dB/km) 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.5Temperature coefficient 0.0028 0.0026 0.0024 0.00225 0.0020 0.0020 0.0020 0.0020 0.0020TABLE A-2/G.622Values specified in certain countries for a type of pair whose outer conductor is 0.18 mm thickFrequency (MHz) 60 100 200 300 500 700 1000 1300 4500Specific attenuation (dB/km) 1.49 1.80 2.42 2.91 3.73 4.43 5.30 6.05 11.2Tolerance (dB/km) 0.1 0.1a) a) a) a) 0.2 0.2 0.2a)Not specified.
