1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G0-%$)!G0G0#(!2!#4%2)34)#3#(!2!#4%2)34)#3G0G0/ further amended)1 Pair characteristicsIt is necessary to have throughout the international network types of coaxial pairs having the same electricalc
2、haracteristics, in order to enable transmission systems to operate on any cable meeting the specifications of thisRecommendation. The use of these pairs is defined by Tables 1/G.623 and 2/G.623 given in the introduction to 6.2.1.1 Electrical characteristics of the coaxial pair1.1.1 Characteristic im
3、pedanceThe characteristic impedance of the coaxial pair follows a well-defined law depending on frequency given by:where f is the frequency measured in MHz1). There is therefore no point in specifying values at all frequencies.The figure of 74.4 (impedance at infinite frequency) is subject to a tole
4、rance of 1 .1.1.2 Attenuation coefficientThe nominal attenuation coefficient of the coaxial pair at a frequency of 60 MHz and a temperature of 10 Cshould be within the limits of 18.00 0.3 dB/km2).The rate of the variation of the attenuation with frequency, for a nominal value of 18.00 dB/km at 60 MH
5、z, isindicated in Table 1/G.623.TABLE 1/G.623Nominal attenuation coefficient at various frequenciesFrequency (MHz) 0.06 0.3 1 4 12 20 40 60 150 300Attenuation (dB/km) 0.59 1.27 2.32 4.62 8.01 10.35 14.67 18.00 28.6 40.7_1)This formula is equivalent to Zfj=+ 74 4 0 92 1.(./ )( ). If this latter formu
6、la is used, a correcting factorshould be applied to the tolerance indicated in the text.2)For internal reasons, some Administrations considered it advantageous to use pairs of larger dimensions, withsmaller attenuation, making it possible to use longer repeater sections (2 km). Cables manufactured b
7、y assembly of thesepairs may be regarded as meeting the requirements of this Recommendation for 60-MHz systems provided the electricalcharacteristics of the repeater sections built up with these cables comply with this Recommendation and provided theline equipments are exactly the same as those used
8、 with the cables referred to in this Recommendation. The FrenchAdministrations 3.7/13.5-mm pairs described in 1 fall within this category.2 Fascicle III.3 - Rec. G.623The following equation, in which is expressed in dB/km and f in MHz, gives an approximation of theattenuation coefficient from 1 MHz
9、onwards:= + +0 01 2 3 0 003 .ffNote - In designing amplifiers, the values measured on the cable to be used must be taken as reference.1.1.3 Attenuation coefficient tolerances - Attenuation distortionTo guarantee proper adaptation between the coaxial pair and the transmission equipment, in addition t
10、o thetolerances at frequency 60 MHz, set at 0.3 dB/km, it is also necessary to establish the limits of attenuation distortionaccording to frequency.Table 2/G.623 gives the nominal values and tolerances of the quantity f(in mB km-1 MHz-1/2)at various frequencies (f in MHz).TABLE 2/G.623Nominal values
11、 and tolerances of the quantity fcharacterizing attenuation distortion at various frequenciesFrequency (MHz) 4 12 20 40 60Nominal value 1.1 1 0.8 0.4 0Tolerances 1.5 1.1 0.8 0.4 0To check the attenuation distortion beyond 60 MHz, which is necessary in particular for digital transmission, itis necess
12、ary to calculate the ratio between the attenuation values measured at the frequencies of 240 MHz and 60 MHz(after eliminating any peaks). The limit to be observed is:The attenuation distortion is checked in the factory on a small percentage of factory lengths.1.2 Mechanical construction of coaxial p
13、airsa) The inner conductor is a solid copper wire 2.6 mm in diameter.b) The insulation is such that the permittivity of the combination of gas and low-loss solid dielectric materialis low enough to meet the requirements of this specification.c) The outer conductor consists of a copper tape 0.25-mm t
14、hick formed into a cylinder of internal diameter9.5 mm around the insulation.d) For reasons of crosstalk, the outer conductor should be surrounded by soft steel tapes.Another form of construction having the same electrical characteristics but with an inner copper conductor of2.8-mm diameter and an a
15、luminium outer conductor of 10.2-mm internal diameter is used by some Administrations. Thistype of construction is described in detail in Annex A.Fascicle III.3 - Rec. G.623 32 Cable specification2.1 Characteristic impedanceTo check that the value given in 1.1.1 above is met, either sine-wave signal
16、 measurements or pulsemeasurements can be made.For sine-wave signal measurements, the check is often made in terms of the smooth impedance/frequencycurve.For pulse measurements, a sine-squared pulse having a half-amplitude duration of less than 100 ns should beused. One may either balance the impeda
17、nce against a variable reference impedance or measure the reflection coefficientagainst a fixed reference standard.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 sho
18、uld be plotted with correction in amplitude and if possible inamplitude and phase. If the equivalent error is measured, it must be corrected. However, for routine measurements,correction may be dispensed with if the test length is so short that the correction is small.Table 3/G.623 shows the various
19、 values to be obtained, according to the purpose for which the cable isintended.Note 1 - For 0.06-6 MHz analogue systems, the provisions are the same as for 0.3-20 MHz analogue systems.Note 2 - To detect systematic irregularities, return wave attenuation measurements should be carried out on asmall
20、proportion of fabricator lengths. The limits to be observed are given in Table 4/G.623.Note 3 - The percentage figures given in the tables relate to all the pairs of a batch of cables submitted forcontrol or delivered at the same time.TABLE 3/G.623Echometric measurement of factory lengthsType of sys
21、tem Analogue DigitalFrequency range or bit rate 0.3-20 MHz 4-70 MHzHigh bit rate(140 Mbit/s)Very high bitrate(565 Mbit/s)Maximum pulse duration 50 ns 10 ns 10 ns 10 ns a)General Maximum100% 50 dB 48 dB 48 dBprovisions peak95% 56 dB 54 dB b)54 dB b)AdditionalA Mean of 3 maximum peaks 53 dB 51 dB 51 d
22、Boptionalprovisionsc)B EquivalentresistanceerrorL 500m0.6 0.8 0.81 1.2 1.6 1 1.2 1.6 4 Fascicle III.3 - Rec. G.623TABLE 4/G.623Measurement of factory lengths using sine-wave signalsType of system Analogue DigitalFrequency range or bite rate 0.3-20 MHz 4-70 MHz High d)Very highReturn wave attenuation
23、 on irregularitiesPercentage of lengths concerned none about5%about5%about5%Frequency band explored 4-62 MHz 20-100 MHz 62-500 MHzMinimum measured value100% 35 dB 30 dB 20 dB95% 38 dBMean return power in a 10-MHz band(Transmission of television signals in the 60-MHz system)Frequency band concerned N
24、one 52-62 MHzMean power return coefficientL 250 m 41 dB 35 dB 28 dBL 500m 40 dBNotes to Tables 3/G.623 and 4/G.623a)If investigations or definition studies show that measurements with shorter pulse durations are required, the duration of 2 nswill be adopted.b)Provided that no more than one value bet
25、ween 48-54 dB is encountered on one and the same coaxial pair of an elementarycable section.c)It is enough to check that one of the two conditions A or B is fulfilled.d)The provisions for 4-70 MHz analogue systems are certainly adequate. However, much lower values have also beenproposed. Agreement s
26、hould be reached on the values to be specified and the frequency band to be explored (4-100 MHz or62-500 MHz).2.3 Attenuation coefficientThe attenuation of pairs should be such as to allow of compliance with the provisions of 3.3 below3).If reference is made to the length measured along a generation
27、 of the cable sheath, the linear attenuation coefficientshould be multiplied by the take-up factor, the values of which are given as an indication in Table 5/G.623._3)At this stage of manufacture, attenuation and crosstalk measurements are merely prototype measurements.Fascicle III.3 - Rec. G.623 5T
28、ABLE 5/G.623Take-up factor valuesNumber of pairsin cableTake-up factor,last layerWeighted take-up factor,entire cable4 or 681218 or 201.0091.0121.0031.0051.0071.0102.4 CrosstalkThe crosstalk between pairs should be such as to allow of compliance with provisions of 3.4 below 3).2.5 Dielectric strengt
29、hThe pair should withstand for one minute an a.c. voltage of 2000 V r.m.s. at 50 Hz (or 3000 V d.c.) appliedbetween the centre conductor and the outer conductor connected to the sheath. This dielectric strength test should bemade on each factory length.2.6 Insulation resistanceThe insulation resista
30、nce 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 electrification for one minute at atemperature not lower than 15 C. The measurement of the insulation resistance should be made a
31、fter the dielectricstrength test. This measurement should be made on each factory length.3 Elementary cable section specificationThe Administration and the supplier must agree on whether tests are to be carried out on all sections or whethersome percentage or even a type-approval test alone will be
32、sufficient, especially in the case of measurements which aredifficult to carry out under field conditions.3.1 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 elementary cable se
33、ction. Referenceshould be made to one of the columns in Table 6/G.623, according to the purpose for which the cable is intended.Note 1 - Notes 1 and 3 to 2.2 in connection with Table 3/G.623 still hold good. However, for 0.06-6 MHzanalogue systems, the provisions of column 0.3-20 MHz apply, but the
34、pulse duration may attain 200 ns for elementarycable sections longer than 5 km.Note 2 - Measurements using sine-wave signals on elementary cable sections are unnecessary unless there areserious grounds for believing that systematic irregularities may have been introduced during the laying or install
35、ation ofthe cable. In such cases, the measurement results should not be less than 33 dB for the 4-62 MHz band.6 Fascicle III.3 - Rec. G.6233.3 Attenuation coefficientFor a cable of any given manufacture with a nominal attenuation coefficient defined by the limits given in 1.1.2 above, the difference
36、 between the maximum and minimum attenuation coefficient values measured at 60 MHz onthe coaxial pairs of all elementary sections of 1.5 km must be below 0.4 dB/km (referred to 10 C).Attenuation measured on a cable at an average temperature of tC is referred to 10 C by the formula:TABLE 6/G.623Echom
37、etric measurement of elementary cable sectionsType of system Analogue DigitalType of system 0.3-20 MHz 4-70 MHzHighbit rate (140Mbit/s)Very highbit rate (565Mbit/s)Maximum pulse duration 50 ns 10 ns 10 nsc)10 nsa)General Maximum 100% 50 dB 46 dB 46 dB 46 dBprovisions peak95% 50 dB 50 dB 50 dBAdditio
38、nalAMean of3 maximum peaks.Uncorrectedmaximum51 dB54 dB49 dB52 dB49 dB52 dB49 dB52 dBoptionalprovisionsb)EquivalentresistanceBEnergycorrected( km-1/2)0.8 2 2 2errorC Uncorrected () 1 1.5 1.5 1.5a) If investigations or definition studies show that measurements with shorter pulse durations are require
39、d, theduration of 2 ns will be adopted.b) It is enough to check that one of the three conditions A, B or C is fulfilled.c) As long as there does not exist an echometer with impulses of 10 ns capable to explore half a repeater section, themeasurement will be done with 50 ns impulses.3.4 CrosstalkThe
40、far-end crosstalk ratio between two coaxial pairs of a cable at any frequency in the band transmitted shouldbe at least equal to the values listed in Table 7/G.623.Fascicle III.3 - Rec. G.623 7TABLE 7/G.623Lengths(km)Frequency band(MHz)Far-end crosstalk radio(dB)94.51.50.06-4.30.3-12.54-628594a)130a
41、)If the cable operates both in the 0.3-12 MHz frequency band and the lower frequencyband with longer repeater sections, the value of the far-end crosstalk should be increasedby a few decibels to frequencies higher than 300 kHz to allow for the differences inlevels across some points of the cable. A
42、limit of 100 dB suffices.With cables operating at 60 MHz, the near-end crosstalk attenuation at 60 MHz between pairs transmitted inopposite directions should be at least 140 dB. No limit is fixed for other systems, previous studies having shown that thenear-end crosstalk ratio under service conditio
43、ns was greater than the far-end crosstalk ratio. These values include thecontribution of accessories which are associated to elementary cable section, such as flexible cords and coaxialconnector.Note 1 - The values given for cables operating at 60 MHz are derived from general considerations on cross
44、talkbetween sound-programme circuits given in Recommendation J.18 2. These values are easy to obtain, although in thepresent state of the art it is difficult to test them with ordinary measuring equipments.Note 2 - The values given for cables operating at 12 MHz or less suffice for telephone transmi
45、ssion. For sound-programme circuit transmission, this value must be increased to 105 dB, a value which is easily obtained with all typesof cable at frequencies above 300 kHz.Note 3 - These limits enable at far-end crosstalk ratio of 65 dB to be obtained on the worst homogeneous280-km section, assumi
46、ng that for the frequencies in question only far-end crosstalk due to the cable is to beconsidered4). When there is no phase inversion, it is assumed that the variation in the minimum far-end crosstalk ratio asa function of. the distance approximately follows a 20 dB/decade law for distances below a
47、 limit distance Lland a 10dB/decade law for distances above Ll. The value of Lldepends on a number of factors, mainly the system used, the typeof cable and the considered frequency. A value of 30 km appears suitable in most cases, although values of Llrangingfrom a few kilometers to 30 kilometres ha
48、ve been observed in practice, ensuring the consistency of the limits in Table7/G.623 with a 65 dB limit on a 280 km section.3.5 Dielectric strengthThe pair should withstand for one minute a d.c. voltage of 2000 V applied between the centre conductor andthe outer conductor connected to the sheath. Th
49、is dielectric strength test should be made on each elementary cablesection on completion of laying.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 electrification for one minute; themeasurement of the insulation resistance should be made after the dielectric strength test. This measurement should bemade on ev
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