Patent GB-545313-A (GB)
Title: Improvements in electric signal transmission systems
IPC: H04B3/32
Filed: 1941-05-02
Abstract: 545,313. Reducing cross-talk. STANDARD TELEPHONES & CABLES, Ltd. May 2, 1941, No. 5723. Convention date, June 6, 1940. [Class 40 (iv)] [Also in Group XXXVI] In a cable comprising two co-axial conductor pairs and an external sheath or other conductors symmetrically disposed with respect to the co-axial pairs, cross-talk, particularly far-end which is generally predominant with this type of conductor pair consists of that due to direct action between the two pairs and an opposing cross talk due to underset-action over an intermediate circuit comprising the external surfaces of the outer conductors of the pairs and the sheath or other symmetrical conductor. According to the invention, an approximate balance of these two cross talks is effected to reduce the total cross talk to a tolerable value by providing the high conductivity, e.g. copper, outer conductors of the pairs with con-, tiguous over layers of material, such as iron or steel tape, having a surface impedance large compared with that of the high-conductivity conductor and by suitably proportioning the inner to outer surface transfer impedance of such composite outer conductors depending on the thickness' of the layers thereof, the impedance of the sheath &c. being relatively low. The Specification contains a mathematical determination of the total crosstalk in terms of the various parameters of the cables which can readily be measured or calculated with the exception of the transfer impedance. To determine this' a measurement of the indirect cross-talk is made on experimental short lengths of cable in which case the cross-talk formula may be reduced to a much simplified form enabling the transfer impedance to be calculated. The cross-talk over any required length of cable is then calculated and adjustments are effected in the.thicknesses of the copper and ferrous over-layers of the' outer .conductors to being the cross-talk within the toleration limit. A formula is furnished for predicting the change of thickness of the copper tube or its ferrous layer to produce a desired change in the transfer impedance. Similar considerations may be applied if near-end crosstalk is the limiting factor. Examples of cable construction are shown in Figs. 4 and 5 in which the outer conductor of each pair is provided with a ferrous over-layer 5 which may comprise one or more layers of helically wound tape, the lead sheath 3 in Fig. 4 being in contact with the over-layers whereas in Fig. 5 it is insulated therefrom by a paper &c. liner 6. The cable in Fig. 5 also includes quads 7 which may be used for low-frequency signals. In a repeatered system, the conductors of one co-axial pair are transposed in each repeater section to prevent build up of cross-talk from section to section, or the equivalent to transposition may be effected by phase reversing transformer or amplifier stages. If the interacting circuit (sheath 3 and conductors 7) were continuous at the transposition points the effectiveness of transposition would be reduced, the flow of current in said circuit at these points is therefore prevented by earths or open circuits thereat or in the case of conductors 7 by the use of filters, chokes or shunting condensers arranged so that the flow of low frequency signalling currents is not affected. The sheath may be connected to the outer of the co-axial conductors at repeater and mid-repeater points. Cross-talk measurements, Fig. 6.-The indirect cross-talk due to the intermediate circuit comprising the outer surfaces of the conductor pairs 1, 2 and the sheath 3 and further cores 7 is measured by a test on short lengths connected as shown. A test-frequency is applied to the "disturbing" pair 1 and a measuring instrument to the " disturbed " pair 2, the disturbing and disturbed sections being terminated at their adjacent ends by characteristic impedance Z0 and the intermediate circuit being terminated by its characteristic impedance Z4.