GB478226A - Improvements in signal-selecting systems for use in carrier signalling - Google Patents

Abstract

478,226. Valve circuits; impedance networks. HAZELTINE CORPORATION. Jan. 21, 1937, No. 1863. Convention date, Feb. 12, 1936. [Classes 40 (iii) and 40 (v)] The band-width of a filter composed of two tuned circuits I, III, Fig. 1, is varied by varying a resistance 14 in the coupling circuit II between them, and the volume is kept constant by simultaneously varying a resistance 15 in one of the end circuits I, III. The resistances 14, 15 may be in series or in shunt and may be jointly controlled by hand. The valve 10 may be the intermediate-frequency amplifier of a superheterodyne receiver. In a modification, Fig. 3 (not shown) the resistances may be constituted by valves and varied by means of a potentiometer controlling the grid bias of both valves. In a further modification, Fig. 4, in which the variable resistance for controlling selectivity is the resistance of the valve 32 shunted upon the coupling circuit II, the resistance of the valve 31 is shunted upon the input tuned circuit I. The potentials of the control grid of valve 32 and suppressor grid of valve 31 are simultaneously varied by means of the potentiometer 34 so as to vary the resistances of the valves in opposite directions. Control of selectivity and volume by desired and undesired signals conjointly. In a superheterodyne receiver, Fig. 5, the intermediate frequency is filtered by a network 44 whose selectivity is controlled by the desired signal through control of the resistance of valve 48, which is effected by the output of the rectifier 46 through the resistance-capacity network shown. The same rectifier 46 governs the gain of the intermediate-frequency amplifier 43 through the lead 62, and regulates the output from the filter 44 by biassing the valve 49 (corresponding to resistance 15, Fig. 1), in accordance with the strength of the desired signals. The undesired signals, on the other hand, control the volume of the output from the radio-frequency amplifier and detector 40; thus when interfering signals are strong the transmission through the filter 44 is reduced and the selectivity is narrowed by the action of the rectifier 46 and valve 49. For this purpose the desired and undesired signals together are applied through a second intermediate-frequency amplifier 71, which passes a broad band, to an automatic-volume-control rectifier 72, which governs the volume of the output from the amplifier 40. The amplifier 71 has a characteristic, Fig. 5a, (not shown), showing a peak of amplification for signals that differ from the carrier by Œ10 kc.; thus the presence of interfering signals reduces the output of the amplifier 40, and so indirectly increases the selectivity by means of the parts 46, 49.