GB426893A - Improvements in or relating to radio receivers - Google Patents

Abstract

426,893. Valve circuits for wireless reception. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Assignees of Carlson, W. La V.; 726, Clinton Avenue, Haddenfield, and Kirkwood, L. R. ; 6220, Wayne Avenue, Merchantville, both in New Jersey, U.S.A.) Dec. 22, 1933, No. 36134. Convention date, Dec. 22, 1932. [Class 40 (v)] A noise-suppressor circuit comprises means for reducing the amplification of an amplifier preceding the detector when the received signal strength is below a predetermined level. The invention is shown applied to superheterodyne receiver comprising a radio frequency amplifier 1, local oscillator 3, first detector 2, intermediate frequency amplifier 4 and second detector 5. The noise suppression and automatic volume control potentials are developed by a double diode triode valve 18, the diode anodes 19, 20 of which are fed from an independent intermediate frequency amplifier 16 which receives its input through condenser 17, from the output of the first detector 2. The load circuit of the diode portion 20, 19<1> comprises resistances 21, R2, R3 from tappings on which A.V.C. potentials are led to the valves 1, 2 and 4. The volume control action is delayed by the positive voltage applied to the cathode 191 from the cathode resistance of valve 4. The latter is also included in the plate circuit of the triode portion of valve 18 and is adapted to block the grid of valve 4 when current flows in said plate circuit. This occurs when the diode 19, 19<1> is receiving signals of below a predetermined value and its load resistance R12 connected to the triode grid is carrying a correspondingly small current. The sensitivity and noise suppression threshold valve are controlled by a manually variable cathode resistance R18 which carries current, through a resistance R14, from the H.T. supply and biasses valves 1 and 2 to a variable extent. The knob 34 controlling resistance R18 also controls a switch 28 adapted to break the plate circuit of the triode portion of valve 18 to disable the noise suppressor. The requirements as regards selectivity of the various intermediate frequency circuits are discussed. In a modification, Fig. 2, (not shown), the A.V.C. amplifier and rectifier are comprised in a single pentode valve and a further auxiliary amplifier feeds a single diode triode noise suppressor valve. A tuning lamp is coupled to the anode circuit of a valve supplied from an A.C. source and whose grid is controlled by the noise suppressor valve so that the lamp is extinguished when no signals are being received. Specifications 417,284 and 424,779 are referred to.