GB530693A - Improvements in or relating to band pass filters - Google Patents

Abstract

530,693. Wireless receiving apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., RUST, N. M., BRAILSFORD, J. D., and GOODENOUGH, E. F. June 8, 1939, No. 16850. [Class 40 (v)] The effect of a band-pass filter is obtained by superimposing upon the frequency impedance characteristic of a network in the input circuit of a valve the frequency impedance characteristic of a further network coupled to the first by means including a further valve. The coupling is such that the impedance elements in the second network are, in effect, reversed so that series elements appear as parallel elements and the impedance is, in effect, inverted and also reversed in sign. The arrangement may constitute a wide band amplifier, Fig. 16a, in which the input and output couplings are arranged to have a " double humped " characteristic or are " stagger tuned, and feedback action to an auxiliary grid modifies the effect of impedance Z11 by superimposing the network Z1V in reversed sense. Negative feedback may be used to provide stability and smoothness of adjustment. Fig. 10 shows an arrangement for varying the bandwidth or displacing the passband relative to interference in an intermediate frequency amplifier comprising valves V1, V2 coupled by the network which is superimposed in reversed sense on the input circuit. The displacement of the pass band may be effected by varying capacities by means of a discriminator circuit. With a parallel tuned circuit in the input the further network may alternatively include a series tuned circuit Fig. 12 (not shown) or a piezo-electric crystal Fig. 13 (not shown). Means may be provided to correct for phase shift due to electron transit time, Fig. 22 shows an arrangement in which a negative mutual inductance -M is used. Alternatively voltage feedback through a small condenser may be used or the effect of negative inductance in the cathode lead may be utilized.