Patent GB-405897-A (GB)
Title: Improvements in or relating to radio and like receivers
IPC: H03G5/26
Filed: 1933-07-31
Abstract: 405,897. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.-(Assignees of Beers, G. L. ; 119, Summerfield Avenue, Collingswood, New Jersey, U.S.A.) July 31, 1933, No. 21532. Convention date, July 29, 1932. [Class 40 (v).] Selectivity and audiofrequency fidelity are automatically regulated in accordance with the level of signal strength to maintain the same standard of quality under varying conditions. An associated manual control is used to minimize sideband " beat " interference by reducing the high-note output on the L.F. side when the H.F. circuits are not operating at maximum selectivity. Sensitivity is also regulated by varying the grid bias applied to the input valve amplifiers. Selectivity is altered by varying the effective impedance of auxiliary valves shunted across certain of the tuned circuits. The L.F. response is adjusted by varying the relative transfer of signal energy as between high and low note channels feeding one or more loud-speakers. The Figure shows a superheterodyne circuit comprising a high-frequency amplifier 1, local oscillator 5, and first detector 3 coupled to intermediate-frequency amplifiers 7, 9 and 11. The output from the valve 11 is split between two channels, one comprising an intermediatefrequency amplifier 27 coupled to a second detector 31, feeding a low-frequency output 37, 41 having a high-note response. The second channel comprises an intermediatefrequency amplifier 29 coupled to a second detector 33 and a low-frequency output 39, 43 having a low-note response. An auxiliary valve 59 controls the sensitivity by varying the grid-bias applied to certain of the earlier valves, and also controls the selectivity response by varying the effective impedance of other auxiliary valves 75, 77, 79 which are shunted respectively across the tuned inputs of the valves 1, 7 and 9. Another auxiliary valve 81 is shunted across the input to the valve 11, and also, through the resistance 93, varies the grid-bias for the intermediatefrequency valve 27. The output from the valve 59 is connected through a slider 63 to a potentiometer 65, 67, 69 connected at one end to a point 71 on the supply 49 more positive than the cathode of the valve 59, and at the other end through a conductor 73 to the grids of the valves 1, 7 and 9. With the slider 63 in position B the full voltage developed across resistance 69 is applied to reduce the sensitivity of the valves 1, 7, 9. Simultaneously the impedance of the shunt valves 75, 77, 79 and 81 is reduced in order to broaden the tuning. When the slider 65 is changed to position A, the voltage drop across the resistance 69 is decreased whilst that across resistance 67 is increased. The main control valve 81 is so biassed that in the absence of signals a substantial plate current flows, causing such a voltage drop across resistance 93 that the auxiliary control valves 75, 77, 79 are cut off. The grid of the intermediate frequency valve 27 being connected to resistance 93, at maximum selectivity the output from that valve 27 is reduced, and the bulk of the signal energy is diverted into the second detector valve 33 which feeds the low-note ampliner 39, 43. This serves to minimize high-note interference. When tuned to a strong signal the output from control valve 59 increases and, if the slider is still in position A, the voltage across resistance 67 increases and throws the grid of valve 81 more negative. The voltage across resistance 93 is thus reduced and increases selectivity by rendering the shunt path through the valves 75, 77, 79 ineffective. Simultaneously the higher impedance of the valve 81 increases the effective coupling between the valves 9 and 11, thereby improving the quality of the lowfrequency output. Also since the resistance 93 reduces the effective bias on the grid of the valve 27, the L.F. amplifier chain 31, 37, 41 is brought more fully into action. The two independent L.F. channels shown may be replaced by the customary single channel, the high-note response of which is similarly controlled by the slider 63. Further the automatic valve 59 may be replaced by a variable resistance 96 which serves to control the sensitivity response, whilst the slider 63 is used to ensure co-operation between the radio and audio circuits to maintain fidelity of reproduction. Specification 399,068 is referred to.