GB527902A - Improvements in or relating to frequency selective circuit arrangements - Google Patents

Abstract

527,902. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., RUST, N. M., and GOODENOUGH, E. F. Jan. 12, 1939, No. 1108. [Class 40 (v)] A frequency-selective circuit arrangement comprises one or more valves associated with a feedback impedance network including a piezoelectric crystal and a manual or other adjustment for controlling the effectiveness of the feed-back in such a way as to provide a smooth and gradual broadening of the selectivity of the circuit. In one arrangement, Fig. 1, applied to the intermediate frequency stage of a superheterodyne receiver for use, for example, in radio direction finding, the feed-back network in the cathode leg of a valve V comprises a crystal P resonant to the mid-band frequency, an inductance L, a capacity C and a variable resistance R all in parallel, the values of the inductance and capacity being such as to secure a symmetrical inpedancefrequency characteristic with reference to the intune frequency. The signal input or control grid is connected to the earth end of the network through a tuned input circuit 1 and the anode circuit includes the usual tuned output circuit 2 feeding into the next stage. That the sharpness of selectivity may be smoothly controlled by varying the resistance R is illustrated by a diagram representing the variation of the attenuation in decibels with the number of kilocycles "off tune" for values of R ranging from 0 to 500,000 ohms. In a modified arrangement the feed-back network is in the anode circuit of the second of two valves having their grids connected or coupled in common to an input circuit, the resistance R being a potentiometer of which the adjustable tap is connected to the control grid of the first valve. In another arrangement, Fig. 4, the crystal P, in circuit between the cathode and the input circuit 1 is shunted by condensers C 1 , C 2 of which the junction point is earthed and by partially magnetically coupled inductances L 1 , L 2 , earthed through a resistance R<SP>11</SP>; the effectiveness of the feed-back and hence the selectivity is varied by varying the mutual conductance of the valve either manually, as by a potentiometer control of a source of negative potential connected between the grid leak and earth, and/or automatically, as by application of A.V.C. voltage to the grid. The arrangements claimed in Specification 509,398 are disclaimed.