GB816463A - Improvements in or relating to electric wave filters - Google Patents

Abstract

816,463. Band-stop filters. STANDARD TELEPHONES & CABLES Ltd. July 5,1957, No. 21376/57. Class 40 (8). A bandstop filter comprises a network of reactive elements selected and arranged to form an all-pass constant-resistance delay equalizer, having a parallel-resonant circuit or circuits in one or more arms, and further comprises a series-resonant device connected to shunt each of these arms. Figs. 1 and 2 show respectively the lattice and bridged-T forms of the network, the relative values of the elements being indicated. The additional series-resonant circuit comprises elements L 2 , C 2 in Fig. 1 and 2L 2 , <SP>1</SP>/ 2 C 2 in Fig. 2. In order to obtain more convenient values, part of <SP>1</SP>/ 2 C 4 (Fig. 2) may be associated with the series-resonant circuit which then takes the form of a parallel-resonant circuit in series with a capacitor (Fig. 3). A crystal may be used to replace the LC seriesresonant network and may be coupled to the parallel-resonant arm by means of a transformer which provides the inductance 2L 1 . The impedances of 2L 2 and <SP>1</SP>/ 2 C 2 may be transformed in a similar manner. In one form, one of the cut-off frequencies of the filter is chosen to be tne lower of the two frequencies of infinite reactance of Z 1 (the reactance of arm 1-3 or 2-4, Fig. 1) and the upper cut-off is at the frequency of zero reactance. In another form the lower cut-off is chosen to occur at the frequency of zero-reactance of Z 1 and the upper cut-off at the upper frequency of infinite reactance. In both these cases an attenuation pole occurs between the cut-offs where Z 1 is equal to Z 2 (the reactance of arm 1-4: or 2-3, Fig. 1). In a further case, the cut-offs occur at the upper and lower infinite reactance frequencies of Z 1 and the pole at an intermediate frequency chosen such that Z 1 =Z 2 =O. The Specification discusses the image-impedances of these filters and gives numerical examples. The circuits may be used to provide delay equalization and bandstop operation simultaneously.