Selection and Testing of Electronic Components for LM

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Fig. 2 Cable groups with the conventional lay-up Top, 10-palr group Bottom, 25-palr group Fig. 3 Cross stranding lines Top, line lor manufacturing 10-palr groups Bottom, 25-pair line with drum twister take-up The principle of the cross stranding technique Cross stranding differs from the other stranding methods inasmuch as the pairs or other elements in question are assembled to form a group with the pairs continuously changing their relative positions during the assembly. This can be carried out in different ways. The elements can be assembled in groups either in accordance with a set pattern, systematic cross stranding, or at random, randomized cross stranding. Systematic cross stranding has the disadvantage that two elements meet at fixed intervals. In high frequency systems the intervals can correspond to wavelengths in the frequency range concerned, which can give rise to a resonance phenomenon that is difficult to eliminate. With randomized cross stranding the elements are crossed at random, which eliminates this resonance phenomenon. Randomized cross stranding Capacitance unbalance has a predominant effect on the value of the crosstalk, particularly at low frequencies. If we consider a conventional, concentric 10 or 25-pair group, fig. 2, it is well known that unbalances arise mainly between adjacent pairs, 1—2, 2 — 3, 3 — 4 etc. Unbalances also occur to some extent between the center pairs and the pairs in the first layer, sometimes also between pairs in neighbouring layers. Unbalances between any other com binations are almost non-existent. It has also been established that at least the highest unbalances increase approximately in direct proportion to the length of the cable. In cross-stranded cables the random mixing ensures that two pairs are adjacent only for a limited part of the cable length and thus the high capacitance unbalance values are reduced. The ten pairs in a 10-pair group (fig. 2) occupy ten different positions. If, for example, we consider pair no. 1, wefind that two other pairs can be considered as adjacent. Two other positions (in the centre of the group) are slightly further away but can still be considered as adjacent. If the positions of the pairs in the group are changed at random along the length of cable, we find that two arbitrarily chosen pairs will be adjacent for only about 4/9 of the cable length. The capacitance unbalances contribute to the crosstalk mainly during this minor part of the cable length. Cross stranding of groups that contain more than ten pairs gives an even greater reduction of the unbalances. In a cross-stranded 25-pair group two pairs are adjacent for only 3/24 to 4/24 of the total cable length, which gives a corresponding reduction of the unbalances. This calculated reduction is approximate, but the tendency is that a random mixing of an increasing number of pairs gives a corresponding reduction of the capacity unbalances
Fig. 4 Cross stranding device for a 10-pair line Fig. 5 Random pulse generator Fig. 6 between the pairs. For example, in a 100-pair group the unbalances would hardly reach measurable values. On the other hand the 10-pair group must be considered as the smallest unit for which cross stranding gives a reasonable reduction of the unbalances. For practical reasons, such as colour coding, the cross stranding technique is considered suitable for groups with between 10 and 30 pairs. Cross stranding, both randomized and systematic, gives the group a certain mechanical flexibility. Thus in this respect it can be compared with such processes as the braiding operation used when making flexible cables. The most suitable mixing ratio for cross stranded 10-pair groups is approximately two crossings per metre, and these crossings have proved to make the cable core looser. This results in greater separation of the pairs and thus a lower mutual capacitance compared with the conditions prevailing in a layer stranded cable. When changing over to cross-stranded cable the insulation thickness can therefore be reduced, for the same value of capacitance, which means a reduction in cost. This will be illustrated later on in the article. Process and machines 107 The most common pair cable specifications prescribe groups containing between 10 and 25 pairs. Small fixed groups are used when the cross stranding technique is applied. It is then possible to carry out pair twinning and stranding of groups in one and the same operation. A cross stranding line, fig. 3, consists of the following main components: 1. group twinner 2. random pulse generator 3. mixer, the cross stranding device 4. binding head 5. length measuring device 6. take-up stand Group twinner The group twinner is basically a number of twinning machines assembled to form a unit, fig. 6. The design of the twinning machines has intentionally been kept uncomplicated. The reason for this is that the process is duplicated 10, 12, 13 or 25 times in each machine. There is greater risk of faults in sophisticated machines and the efficiency is reduced because of the greater number of repairs.
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Selection and Testing of Electronic Components for LM

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