GB600809A - Improvements in radio receiver circuits - Google Patents

Abstract

600,809. Automatic noise suppression. BURGESS, A. F. (Zenith Radio Corporation). Aug. 9, 1944, No. 15143. [Class 40 (v)] In a radio receiver for signals, other than amplitude-modulated signals, amplitude-limiting means are used, in addition, to derive a control voltage to render the output circuit inoperative when the signal amplitude is below a predetermined' value, and to provide a reduction of the signal output from the limiter under these conditions. As shown, in a frequency modulation. receiver, a first limiting valve stage 6 feeds a second limiter 7, through condenser 22, followed by a discriminator circuit 25 .. 31, including diode rectifiers 8, the output from which feeds an audio amplifier 9. The control grid 21 of valve 7 is connected through a, high resistance 52 to the positive side of the H.T. source 18, its anode circuit includes a resistance 41 and its cathode is earthed. In the absence of a carrier at grid 21 its potential is slightly more positive than the cathode and a large anode current flows through resistance 41 reducing the positive potential of its anode end, 42. This point is connected to a potential divider and filter 43, 44, 45, 51, a portion of the voltage across which is applied to the grid of the audio amplifier 9 whose cathode 47 is maintained at a pre-set positive potential by the potential divider 48 ... 50 of which resistance 49 is variable. In the condition described, the valve 9 is biassed to cut-off and the valve 7 also attenuates the signal passed to the discriminator. When a signal of greater than a predetermined amplitude is applied to grid 21 of valve 7, rectification sends the grid negative and point 42 becomes more positive to render the valve 9 operative. The muting arrangement may be disabled by closing switch 53 which connects grid 32 to its cathode through resistance 46. A diode 54 is provided to prevent grid 32 from going more positive than an adjustable point on resistance 49. In modifications, the muting voltage is applied to the anode and/or screen grid of the first audio amplifier, Fig. 4 (not shown), or operates a relay directly or through an amplifier to short-circuit the loud-speaker and energize or de-energize a signal lamp, Figs. 5, 6 (not shown). Specification 552,943 is referred to.