GB813962A - Improvements in or relating to electric wave filters - Google Patents

Abstract

813,962. Bridged-T filter networks. STANDARD TELEPHONES & CABLES Ltd. May 22, 1957, No. 16215/57. Class 40 (8). A filter having a narrow band-pass comprises a symmetrical T-network of three inductors L 7 , L 7 , L 8 , Fig. 1, the two series inductors being bridged by a series resonant network or device 5 (e.g. a piezo-electric crystal), and by a capacitor 6, two further series capacitors C 1 being arranged between the inductors and the filter terminals. Alphabetical reference symbols also denote values of corresponding elements. The capacitor 6 is adjustable and the total effective capacity (including that of the crystal 5) shunted across the inductors is C 2 . A shunt resistance R 3 is included to balance the resistive components of the elements. The circuit is designed by finding the component values of the known m-derived filter of Fig. 2 which give the required characteristic assuming an image impedance R o which is much greater than the image impedance R 1 desired for the final circuit. The intermediate circuit of Fig. 4 is then arrived at by deriving the network inverse to Fig. 2 (Fig. 3, not shown) and transforming the resulting #-network of three inductors into the T-network L 7 , L 7 , L 8 . The shunt elements C 1 are now transformed to series elements (see Fig. 10) of the same value, with the result that the impedance of the filter becomes R 1 = 1/# 02 C 1 R 0 instead of R o . Although the value of R 1 varies with frequency, the pass-band is narrow enough to make the effects of this negligible. The effect of adding R 3 in shunt with L 8 is to cause effective resistances R (not indicated) to appear across each pair of terminals, these representing the dissipation of the network. For convenience, the resistances R may be considered to be series elements of value R 4 #1/#0<SP>2</SP>C 1 R, where several filter sections 11, 12, 13, &c., Fig. 11, are connected in cascade, the effect of resistances R 4 may be compensated by the addition of shunt resistances R 5 which over the pass-band form T-networks of image impedance R 1 with the effective resistances R 4 . The combination L 6 , C 3 is replaced in Fig. 1 by the crystal 5.