Selection and Testing of Electronic Components for LM

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Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Insulation: Foamed polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Insulation: Solid polyethylene Conductor diameter: 0.7 mm Cable length: 500 m Fig. 13 RMS values of capacitance unbalance distribution for cross stranded cable Each cross represents the RMS value of the 45 capacitance unbalance values within a 10-pair group Fig. 12 Distribution curves, showing the capacitance unbalance for 25 and 10-pair groups Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Curve A represents the capacitance unbalance values within 25-pair groups In cross stranded cable Curve B represents capacitance unbalance values within 10-pair groups in cross stranded cables The characteristics of completed cables are naturally also dependent on the quality of the individual pairs as regards the uniformity of conductors and insulation, lay lengths etc. As can be seen from fig. 11 the unbalances between groups is much less than the unbalances within groups. Fig. 12 shows that the unbalances in 25-pair groups are lower than the corresponding values in 10-pair groups. The quality of a cable as regards capacity unbalance is given as the RMS (root mean square) value. The distribution diagrams in fig. 13 represent the RMS values obtained for different types of cables. As can be seen from the diagrams, the spread is relatively large and thus a reasonably large number of measured values will be required in order to be able to establish differences in the quality of cables that have been manufactured in different ways. Mutual capacitance In cross stranded cables there is no systematic difference in mutual capacitance between pairs, caused by their positions in different layers. There are, however, some small differences in mutual capacitance because of the different lay lengths and manufacturing tolerances of the pairs. This is shown in table 1. Cross stranded, PE insulated 10-pair group cables without jelly filling have a lower mutual capacitance than the corresponding 10-group layer cables (2 + 8). The reason for this is that the cross stranded cables contain more air because of the stranding method. A reduction in mutual capacitance of about 3 % has been noted. The mutual capacitance relationships are different for cables with other types of insulation material. The cables are generally specified for a fixed mutual capacitance and hence the conductor insulation in cross stranded cables can be reduced with a consequent reduction in material consumption. High frequency characteristics The high frequency characteristics of symmetrical cables are becoming increasingly important. This applies wherever the cables are situated in the network and particularly when they are to be used for PCM systems. Typical crosstalk values for cross stranded cables, given as the mean value m and the standard deviations, are shown in table 2. The standard deviation, o, for near-end crosstalk is of particular interest. The
PE insulated, solid cable 10-pair 10-pair groups. groups. Standard |aid up in cross deviation. |ayers stranded .1, of the (2 + 8) mutual capacitance. percentage 1.4 0.75 Table 1 Near-end crosstalk 25-pair 10-pair at 1 MHz groups groups m o m o dB dB dB dB Within groups 63 6 58 6 Between adjacent groups 80 6 68 6 Between groups separated by one group 97 6.6 78 6 dB dB Far-end crosstalk at 150 kHz RMS 78 70 Table 2 Typical crosstalk values for cross stranded cables m Mean value a Standard deviation RMS Root mean square Fig. 14 The number of permissible 30-channel PCM systems, N, can be read off from the diagram for cr-value for conventional cables with 10 or 25-pair groups laid up in concentric layers is 8 - 10 dB and, as shown in table 2, the corresponding value for cross stranded groups in approximately 6 dB. The suitability of a particular cable for PCM transmission is dependent on the following four parameters: m Mean value, crosstalk a Standard deviation, crosstalk L Attenuation over a repeater section N Possible number of PCM systems The possible number of 30-channel PCM systems can be determined with the aid of fig. 14 with a certain degree of statistical reliability at given m, L and a values. The diagram is based on singlefrequency measurements. Example Two cables with two 10-pair groups each, one cross stranded and the other with concentric layers, have the following typical near-end crosstalk values. m = 68 dB a = 6 dB for the cross stranded cable a = 9 dB for the cable with concentric layers L = 29dB(m - L =39 dB) Fig. 14 shows that the cable with o = 6 dB can be filled (10 systems) whereas the second cable permits only one system although the mean value of the crosstalk attenuation is the same (68 dB) for both. As can be seen the lower 111 spread of the cross stranded cable is of great importance. The crosstalk level between 10-pair groups in cross stranded cable is 10 dB better than the level within groups and this ratio is very constant. For groups separated by one group there is an additional improvement of 10 dB. In the case of 25-pair groups the corresponding difference is 17 dB. These good and well defined values and the well organized cable lay-up make cross stranded cables very suitable for PCM systems. Summary The method and machines for telecommunication manufacture which have been described above have resulted in — improved cable quality Owing to the cross stranding of the groups the number of high capacitance unbalance values between pairs is reduced. The capacitance level and standard deviation of the cable are also reduced. - reduced production costs Since twinning and group stranding are carried out in one single operation, the amount of space required is reduced and also the investment, operation and maintenance costs. In addition the planning and supervision of the production are simplified.
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Selection and Testing of Electronic Components for LM

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