GB241972A - Improvements in telephone transmission lines - Google Patents

Abstract

241,972. Rosen, A. July 2, 1924. Measuring the unbalance of telephone and like circuits; induction, preventing and minimizing.-In a method of measuring the impedance unbalances between the side and phantom circuits of one or more quads with a view to reducing cross-talk and overhearing, the unbalances of each section are measured by means of an alternating current bridge, the distant end of the section being terminated by a network simuiating, as regards interference between the circuits, a section of infinite length. Fig. 2 shows such a network in which, for a loaded line at speech frequencies, the elements may be taken as plain resistances such that W = X = Y = Z and M = N, the values being calculated from the characteristic impedances of the side and phantom circuits. Fig. 4 shows the form of alternating current bridge used. This has fixed and variable condensers 3, 4, and in parallel therewith equal resistances 5, 6 and a variable resistance 7 which may be put in series with either of the equal resistances by means of a switch 8. The connections to the quad ABCD are varied by means of a switch 12, and four determinations are thus made, from which the impedance unbalances may be calculated. The condensers 3, 4 may be replaced by a differential condenser, and the resistances 13, 14 may be omitted, the fourth wire of the quad being disconnected. When measurements have been made for each section, the line is joined up in the most advantageous manner (1) by arranging the order of the sections, and (2) by crossing the wires of successive sections. It is shown that, on account of attenuation, the interference values of a particular section depend upon the position of the section in the line, so that every large unbalance should be neutralized by an opposite unbalance located as close to it as possible. For short lengths of line it is preferable to consider the effect of crossing from each end of the line. The line may be divided into parts comprising a few sections, the sections of each part being balanced at a high-frequency and the parts then being balanced at a lower frequency; or the balancing may be carried out in three stages. For short lengths of line, only a single frequency need be used.