Patent GB-476799-A (GB)
Title: Improvements in or relating to electric signal transmission systems
IPC: H04B3/14
Filed: 1936-03-03
Abstract: 476,799. Impedance networks. MANIFOLD, M. BOWMAN-, and COLLARD, J. March 3, 1936, Nos. 6483/36 and 4855/37. [Class 40 (iii)] An equalizer network for a band of frequencies extending from 0 to 100 kc./sec. or up to 2 or 3 Mc./sec. comprises series resistance-capacity arms and shunt inductance-capacity arms so dimensioned as to compensate for the variation with frequency of the effective resistance, inductance, capacity and leakance of a cable or like transmission line. In order that distortion may be prevented, the attenuation constant has to be made independent of frequency while the phase constant has to be proportional to frequency, and these conditions hold provided that the frequency be sufficiently high and that the effective resistance R, leakance G, inductance L and capacity C per unit length of the line be independent of frequency. Over a wide frequency range, however, these requirements are not met by an uncompensated transmission line, and the actual resistance and inductance of the line comprise constant parts represented by R0, L0, together with variable parts represented by meshes A, B, C. The characteristic impedance, approximately #(L /C), is then nearly invariable while the attenuation constant and time delay vary with frequency. For neutralizing this variation, series arms D, E, F and shunt arms H, I, J are so dimensioned that the variation with frequency of their effective inductance and resistance is equal and opposite to that of corresponding sections A, B, C, the required relations being R1 = R and C1 = L/R<2>, or L2=LZ<2>0/R<2> and R2=Z<2>0/R, where Zo is the characteristic impedance. Frequency variation of leakance and capacity is similarly neutralized. In practice the equalizer comprises a balanced or unbalanced ladder network having series and shunt arms dimensioned as above, the sections being differently dimensioned but all having the same characteristic impedance. The Specification describes a method of curve-fitting whereby the line can be equalized for successive portions of the frequency range by means of differently dimensioned artificial lines of the type shown, which are then connected in cascade. Each section DH, EI, FJ may be replaced by an equivalent section having one of various forms, Figs. 5 to 8 (not shown), or by a constantresistance section of the form indicated in Fig. 10. For dealing with the highest frequencies a section of the form shown in Fig. 11, resonant at a frequency above the working range, may be used. The equalizer may be finely subdivided, the series arms D, E, F having small impedances and the shunt arms H, I, J large impedances. In cases in which the attenuation of the natural line varies either as the pulsatance # or as ##, the equalizer may comprise sections which are lumped instead of being finely subdivided, but the characteristic impedance of the equalizer then varies with frequency and matching has to be effected by means of a terminal impedance of the kind shown in Fig. 9. The attenuation characteristic for each lumped section has its maximum at zero frequency, and the frequency at which the attenuation has fallen by half is called its " centre frequency " ; then according to a feature of the invention the centre frequencies corresponding to successive lumped sections are identified with the frequencies corresponding to equal increments of attenuation in the characteristic of the natural cable, and formulµ for choosing the optimum values of the increments are given in the Specification. If the attenuation characteristic of a cable is of the form (A# +B##), two equalizers having sections whose centre frequencies are spaced appropriately for the two terms may be connected in series. The equalizer may be adapted for use with varying lengths of cable by providing component equalizers corresponding to , ¢, 1, 2, 4 ... miles for selective connection in series, and smoothness of the overall characteristic is insured by the above-mentioned spacing of the centre frequencies. The equalizer may be used in conjunction with series capacity loading of the kind described in Specification 455,492 for the purpose of eliminating the distortion that arises from failure to satisfy the Heaviside conditions at low frequencies, and the Specification describes means for matching the impedances under these conditions. Amplifiers may be interposed between sections of the equalizer. Specification 455,492, [Group XXXIX], is referred to.