GB434632A - Electric wave transmission networks - Google Patents

Abstract

434,632. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Assignees of Bode, H. W. ; 278, West 4th Street, New York, U.S.A.) Nov. 8, 1934, No. 32124. Convention date, Jan. 31. [Class 40 (iii)] A dissipative arm is added to a filter section of the ladder type in order to diminish variation of the attenuation constant throughout the pass band. Owing to dissipation in coils and condensers the attenuation is not zero in the transmission range, but rises in value as the cut-off frequencies are approached. In the case of a low-pass constant-k pi-section, Fig. 4, the half-sections C, L may be regarded as being terminated mid-series on their inner sides and in this case a shunt conductance G is connected to the middle point 15 of the series arm. In the case of a tee-section, Fig. 8, the half-sections L, C are regarded as being terminated mid-shunt and in this case a series resistance R is inserted as shown. It is shown in the Specification that the resulting insertion loss, provided it be small, is proportional to the mid-series image impedance in Fig. 4 and to the mid-shunt image admittance in Fig. 8, the networks of Figs. 4 and 8 being electrically equivalent to one another. The best values for G and R are deduced by showing that the above arrangements affect the attenuation characteristic in the same way that an m-derived modification affects the impedance characteristic of the prototype filter. Band-pass filters are similarly modified, Figs. 9 to 16 (not shown) ; and Fig. 23 shows an m-derived low-pass filter in which the series inductance L1 is tapped to accommodate the shunt conductance G. Mutual inductance may be introduced between the inductances L of Fig. 4, Fig. 7 (not shown). Mutual inductance may also be introduced in order to avoid the necessity for tapping a series impedance such as L, L, Fig. 4, at the point 15 ; thus the series impedance Z1 of a pi-section, Fig. 18, may be shunted by a pair of close-coupled coils l, l, having a shunt conductance G. A tee-section may be modified in an analogous manner, Fig. 19 (not shown). The section shown in Fig. 4 may be modified by the introduction of an inductance or anti-resonant shunt l, c tuned to the cut-off frequency in series with G, Fig. 26, so that the added branch l, c, G attenuates in the middle of the pass range but not in the neighbourhood of the cut-off. Fig. 28 is an analogous modification of Fig. 8. The parts l, Fig. 26, and c, Fig. 28 may be omitted, Figs. 29, 30 (not shown).