ITU-T G 343-1993 4 MHz SYSTEMS ON STANDARDIZED 1 2 4 4 mm COAXIAL CABLE PAIRS《1 2 4 4mm标准同轴电缆线对上的4MHz系统》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3).$)6)$5!,G0G0#(!2!#4%2)34)#3G0G0/ further amended)Preliminary noteThe present Recommendation describes a system designed to carry a maximum of 960 carrier telephon

2、echannels on a 1.2/4.4 mm coaxial pair (see Recommendation G.622).A system of this kind is produced by halving the length of the repeater section of a 1.3 MHz system (asdescribed in Recommendation G.341) if this length is 8 km, corresponding to a nominal repeater spacing of 4 km for a 4MHz system.1

3、Line frequenciesThe CCITT recommends the two plans in Figure 1/G.343. Plan 1 shows the supergroup allocations and Plan 2the mastergroup allocations.It may be desirable to make provision for the through-connection of entire mastergroups or a supermastergroupto this system. This can be effected in acc

4、ordance with the frequency arrangement of Plan 2 in Figure 1/G.343.Plan 2 uses the three lowest mastergroups in the 12 MHz system on a 2.6/9.5 mm coaxial pair. It permits, inparticular, direct interconnection with a 12 MHz coaxial system using the Plan 1A frequency allocation shown inFigure 1/G.332

5、and with a radio-relay link of 900 or 1800 channels operated according to Recommendation G.423(Figures 4/G.423 and 8/G.423).2 Pilots and additional measuring frequencies2.1 Line-regulating pilotsThe frequencies recommended for the various cases indicated in 1 above and shown in Figure 1/G.343 are as

6、follows:Plan 1 - The CCITT recommends the use of the following frequencies:i) 60 kHz or 308 kHz for the lower line-regulating pilot;ii) 4092 kHz or 4287 kHz for the upper line-regulating pilot.However, each Administration, when so requested by another Administration, should permanently send a line-r

7、egulating pilot at 4287 kHz.Plan 2 - The line-regulating pilots recommended in Recommendation G.332 for the 12 MHz system in thesame frequency band.2 Fascicle III.2 - Rec. G.343In every instance, the recommended stability is 1 x 10-5, the power level recommended is -10 dBm0, whilethe tolerances at t

8、his level are the same as in Recommendation G.332, 2.1. The harmonics of the 60 and 308 kHz pilotshould each have a level not higher than -70 dBm0.2.2 Frequency comparison pilotsPlan 1 - For a national routine frequency cheek as described in Recommendation G.225, a frequency of either60 kHz or 308 k

9、Hz may be used for the frequency-comparison pilot.The power level of a frequency-comparison pilot should be adjusted at the output of the transmit amplifier, to anominal value of 10 dBm0. The harmonics of the frequency-comparison pilots should each have a level not higherthan -70 dBm0.The frequency

10、1800 kHz is provisionally reserved for international frequency comparisons, as required.However, if the Administrations concerned so desire, this frequency 1800 kHz may be used for the frequency-comparison pilot.Administrations concerned with an international carrier system on coaxial cable may agre

11、e to use (if theyconsider it desirable) one of the lower line-regulating pilots (either 60 or 308 kHz) for level control as well as forfrequency checking.In any case, it is desirable that one of the following two solutions should always be applied, so as to allow theline-regulating pilots to be used

12、 at the same time for frequency checking:- provide, in each regulated-line section, a master oscillator which is regularly compared, directly orindirectly, with a national frequency standard;- if there is no master oscillator in a regulated-line section, then beyond the junction between the tworegul

13、ated-line sections considered, reintroduce the lower line-regulating pilot coming from the previoussection, after its level has been stabilized.Fascicle III.2 - Rec. G.343 3Generally speaking, it is possible for one pilot to have two or more functions if the Administrations concernedso decide.Plan 2

14、 - The same recommendation as for the 12-MHz system (Recommendation G.332, 2.2).2.3 Additional measuring frequenciesPlan 1 - Frequencies that may be used are the following:60, 308, 556, 808, 1056, 1304, 1552, 1800, 2048,2296, 2544, 2792, 3040, 3288, 3536 and 3784 kHz.The recommended accuracy for the

15、 frequency of these signals is 40 Hz. The power level of these additionalmeasuring frequencies should be adjusted at the output of the transmit amplifier to have a nominal value of -10 dBm0.The harmonics of the additional measuring frequencies below 2.1 MHz should each have a level at this pointnot

16、higher than -70 dBm0.Plan 2 - The additional measuring frequencies recommended for the 12-MHz system in the same frequencyband should be used (Recommendation G.332).3 Hypothetical reference circuitsThe hypothetical reference circuit depends on the line frequency arrangement.With the supergroup arran

17、gement, the first hypothetical reference circuit for the 1.3 MHz system, described inRecommendation G.341, 3.1, is to be used.With the mastergroup arrangement, the circuit to be used is:- either the second hypothetical reference circuit for the 1.3 MHz system, described in RecommendationG.341, 3.2,-

18、 or the first hypothetical reference circuit for the 12 MHz system, described in Recommendation G.332, 3.1.4 NoiseRecommendation G.341, 4 applies.5 Matching of the coaxial-pair impedance and repeater impedancesFor an elementary cable section above 4 km in length the sum N of the three terms defined

19、inRecommendation G.332, 5 must be at least equal to the following:- 50 dB at 60 kHz,- 57 dB above 300 kHz,with linear variation from 50 dB to 57 dB in the 60-300 kHz band, in the case of a linear frequency scale.4 Fascicle III.2 - Rec. G.343Note - These values are based on the assumption that the at

20、tenuation/frequency characteristic does not show anyripple exceeding 1 dNp (about 1 dB) at the end of a homogeneous section 280 km long. A relaxed condition wasapplied at 60 kHz, as it may be difficult at low frequencies to obtain a reflection coefficient for the repeater input andoutput impedances

21、which is sufficiently small in relation to the impedance of the cable.6 Relative levels and interconnection6.1 Relative level at amplifier output- at 4028 kHz: - 9 dBr, or- at 4287 kHz: - 8.5 dBr.6.2 Pre-emphasis characteristicThis is defined by the formula:in which the constants are so selected as

22、to give between 9 and 11 dB of pre-emphasis.Both of the sets of values below meet this requirement:1) a = 10 b = 3 fr= 4.7 MHz2) a = 11.25 b = 1.56 fr= 4.4 MHz6.3 Interconnection in a frontier section of two systems in which the elementary cable sections are of the samenominal length (this is true o

23、f two 4 MHz systems, and also of two 6 MHz systems)As the relative line levels and the pre-emphasis characteristic are already covered by recommendations, theinterconnection of two systems in a frontier section will not give rise to any great difficulty in this case. TheAdministration on the receivi

24、ng side can receive the other Administrations line levels provided minor adjustments aremade in the first main repeater station (for details, see Recommendation G.352).6.4 Interconnection of a 4 MHz and a 6 MHz system in a frontier sectionIn the absence of a special agreement between Administrations, the method described in RecommendationG.352 should be applied in this case.6.5 Interconnection at a main stationSee Recommendation G.213.7 Power-feeding and alarm systemsRecommendation G.341, 7) also applies to systems conforming to the present Recommendation.