GB462832A - Control of selectivity in radio receivers - Google Patents

Abstract

462,832. Valve circuits for wireless reception. MURPHY RADIO, Ltd., and MOXON, L. A. Sept. 18, 1935, No. 25913. [Class 40 (v)] The signal amplitude of a radio receiver is maintained constant from a certain point of the receiver onward, the selectivity being controlled from that or a subsequent point in dependence on the strength of undesired signals at the point from which the control voltage is derived. Fig. 1 shows a superheterodyne receiver in which the coupling between the frequency changer V1 and I.F. amplifier V2 comprises a network 5, 6 and valves V5, V6 arranged as described in Specification 442,685, the coupling being variable by varying the control grid bias of the valves. The coupling between V2 and the detector stage V3 comprises a second network 7, 8, V7, V8, exactly similar to that referred to above. Coupled to the anode circuit of V2 is an interference selecting network 10, 11, 12, V10, circuits 10 and 11 being tuned respectively 9 kc. above and below the intermediate frequency; when interference is present with the signal there appear across the load resistance 12 of the diodes V10 voltages of 9 kc. frequency arising from the beating of the interfering station with the wanted carrier. This frequency is amplified by V9 and passed to a filter 13 which is so designed that its characteristic, together with circuits 10 and 11, provides no response over the side band range of the wanted signal and a sharply attenuated peak response to frequencies 9 kc. on each side the wanted frequency. The output from the filter is rectified at V11 from the load resistance 14 of which control voltages are applied over conductor 15 to vary the grid bias of valves V7, V8 thereby varying the selectivity in accordance with ratio of interference to signal strength. To provide adequate control range, a further control voltage is taken from part of resistance 14 over conductor 16 to vary the coupling between V1 and V2 by the network 5, 6, V5, V6. The double-diode triode V3 has in its anode circuit a filter 4 to remove heterodyne whistle ; A.V.C. potentials for valves V1, V2 are applied over conductors 9 and 23. In the alternative system of deriving the control voltage shown in Fig. 3, the interference selector circuit is taken from V2 in parallel with the signal path and comprises circuits 17, 18 tuned to the I.F. but overcoupled to have resonant peaks 9 kc. each side that frequency. After rectification in V12, the harmonic thus produced is amplified in V13, the output being passed through an 18-cycle filter 19, 20. The double frequency output is rectified by V11 and applied to vary the couplings as described in Fig. 1. In a further modification of Fig. 3, the output from the circuits 17, 18 is applied to two pentodes V14, V15, Fig. 4, the anode circuits of which are coupled to circuits 19, 20, 21, 22, the circuits of each pair being timed one 9 kc. above, the other 9 kc. below the I.F. After rectification, the output is applied as in Fig. 1. Specifications 448,716, 450,081, and 451,227 also are referred to.