1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G0-%$)!G0G0#(!2!#4%2)34)#3#(!2!#4%2)34)#3G0G0/ amended at Geneva, 1980)Administrations which decide to use for digital transmissions, and possibly also for particular types of analoguetransmission
2、, coaxial pairs smaller than the 1.2/4.4-mm coaxial pair should as far as possible choose pairs complyingwith the specifications given in this Recommendation. The use of these pairs is defined in Tables 1 and 2 given in theintroduction to Subsection 6.2.1 Pair characteristics1.1 Electrical character
3、istics of the coaxial pair1.1.1 Characteristic impedanceThe nominal value of the real part of the characteristic impedance at 1 MHz should be 75 .The mean real part of the impedance of a coaxial pair at 1 MHz should not differ from the nominal figure bymore than 2.5 .Table 1/G.621 shows the general
4、trend of the variation of the impedance as a function of frequency.TABLE 1/G.621Mean real part of the impedance measured at various frequenciesFrequency (MHz) 0.2 0.5 1 2 5 10 20 Impedance () 77.7 75.9 75 74.2 73.4 73 72.8 72.21.1.2 Attenuation coefficientThe nominal value of the attenuation coeffic
5、ient, at 10 C and at 1 MHz, is equal to 8.9 dB/km.Table 2/G.621 shows the general trend of the variation in attenuation coefficient as a function of frequency atthe temperature 10 C.TABLE 2/G.621Mean values of the attenuation coefficient at various frequenciesFrequency (MHz) 0.2 0.5 1 2 5 10 20Atten
6、uation coefficient (dB/km) 4.5 6.5 8.9 12.6 19.8 28.0 39.61.2 Mechanical construction of the coaxial pairThe pair has the following constitution:a) nominal diameter of solid-copper wire inner conductor: 0.7 mm;2 Fascicle III.3 - Rec. G.621b) nominal internal diameter of outer conductor: 2.9 mm;c) ou
7、ter conductor consisting of a copper tape with a thickness of the order of 0.1 mm, laid lengthwise withoverlap1);d) screen consisting of a steel tape with a thickness of the order of 0.1 mm, laid lengthwise with overlap1).2 Cable specification (factory lengths of about 500 m)2.1 Characteristic imped
8、anceTo check that the value given in 1.1.1 is met, pulse measurements can be made. The mean real part of theimpedance at 1 MHz is to be taken as meaning the resistive component of the impedance at 1 MHz of the network withthe best balance against the coaxial pair measured.2.2 Impedance regularityRou
9、tine 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.Table 3/G.621 shows the various values to be obtained accord
10、ing to the purpose for which the cable is intended.TABLE 3/G.621Echometric measurement of factory lengths a)Type of system DigitalBit rateMedium bit rate(6 to 34 Mbit/s)Maximum pulse duration 100 nsGeneral provisions Maximum peak100 % 36 dB95 % 39 dBAdditional optional provisions a)A Mean of 3 maxim
11、um peaks 39 dBB Equivalent resistance errora)It is enough to cheek that one of the two conditions A or B is fulfilled.Note 1 - The percentage figures given in the table relate to all the pairs of a batch of cables submitted forcontrol or delivered at the same time.Note 2 - With the construction tech
12、niques used so far, systematic faults do not give rise, in steady-statemeasurements of regularity return loss, to peaks at frequencies below 60 MHz. For this reason, and taking_1)A single bimetallic copper-steel-copper tape may also be used to serve as outer conductor and screen.Fascicle III.3 - Rec
13、. G.621 3into account the bit rate envisaged, steady-state measurements of regularity return loss do not seemnecessary. For other types of construction which might be used in future, supervision of the regularity returnloss might be wise; in such cases, the value should be 20 dB from 4 to 60 MHz.2.3
14、 Attenuation coefficientThe attenuation of pairs should be such as to allow compliance with the provisions of 3.3 below2).2.4 Near-end crosstalk attenuationThe near-end crosstalk attenuation between coaxial pairs used for different transmission directions, measured inthe frequency band 0.5-20 MHz on
15、 factory lengths, must be above 135 dB for 100% of measurements.2.5 Dielectric strengthThe pair should withstand an a.c. voltage of 1000 r.m.s. at 50 Hz (or a d.c. voltage of 1500 volts) applied for atleast 1 minute between the centre and the outer conductor.If in normal service the outer conductors
16、 of the coaxial pairs are not to be earthed, a dielectric strength test mustbe carried out between the outer conductors and the earthed metal sheath. For this test, an a.c. voltage of at least2000 volts r.m.s. at 50 Hz or a d.c. voltage of not less than 3000 V will be applied.2.6 Insulation resistan
17、ceThe 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 10 000 M-km after electrification for one minute at atemperature not lower than 15 C. The measurement of the insulation r
18、esistance should be made after the dielectricstrength test. This measurement should be made on every factory length.3 Elementary cable section specificationIt will be a matter for agreement between the Administration and the supplier whether tests are to be carried outon all sections or whether some
19、 percentage or even a type-approval test alone will be sufficient, especially in the case ofmeasurements which are different to carry out under field conditions.3.1 Mean impedanceThe mean real part of the impedance of a coaxial pair at 1 MHz must not differ from the nominal value (asdefined in 1.1.1
20、) by more than 3 . Measurements should be affected as described in 2.1.3.2 Impedance regularityMeasurements are effected as described in 2.2 above. Table 4/G.621 indicates the various values to beobtained according to the purpose for which the cable is intended. Note 1 of 2.2 remains valid._2)At thi
21、s stage of manufacture, attenuation measurements are merely prototype measurements.4 Fascicle III.3 - Rec. G.621TABLE 4/G.621Echometric measurement of elementary cable sectionsType of system DigitalBit rateMedium bit rate(6 to 34 Mbit/s)Maximum pulse duration 100 nsGeneral provisions Maximum peak100
22、 % 30 dB95 % 33 dBAdditional optional provisions a)A Mean of 3 maximum peaks 33 dBB Equivalent resistance errora)It is enough to check that one of the two conditions A or B is fulfilled.3.3 Attenuation coefficientAt 1 MHz, the real attenuation coefficient must not differ from the nominal figure, as
23、defined in 1.1.1, bymore than 0.4 dB.Attenuation measured on a cable at an average temperature of tC is referred to 10 C by the formula:The coefficient of the variation in attenuation as a function of temperature kis about 1.8 10-3per C forfrequencies above 2 MHz and about 1.9 10-3per C for 1 MHz.3.
24、4 CrosstalkThe near-end crosstalk attenuation between coaxial pairs used for different transmission directions, measured inthe frequency band 0.5-20 MHz on 2- and 4-km sections, should be above 130 dB.3.5 Dielectric strengthThe pair must withstand a d.c. voltage of at least 1000 V applied during at
25、least 1 minute between the internaland external conductors.In addition, a test of dielectric strength between the coaxial pair and earth shall be made as described in 2.5 using a d.c. voltage of at least 2000 V applied for 1 minute.3.6 Insulation resistanceThe insulation resistance between the centr
26、e 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 1 minute. Themeasurement of the insulation resistance should be made after the dielectric strength test. This measurement should bemade on every elementary cable section.
