GB590366A - Improvements in and relating to noise suppression circuits for radio receivers - Google Patents

Abstract

590,366. Radio receiving circuits. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). April 17, 1944, No. 7039. Addition to 555,403. [Class 40 (v)] The parent Specification describes means for muting a radio receiver by means of biassing potentials obtained from rectified noise voltages. In the present invention the biasing potentials are opposed by a potential of adjustable value to enable the bias to be removed when it falls below a predetermined value, and means are provided to prevent the opposing potential from providing a positive bias. As shown in Fig. 1, in a frequency modulation receiver, the I.F. output is fed to a first limiter 15, second limiter 16, discriminator 17, audio amplifier 64 and loud-speaker 20. The time constant of the self-biassing network 31, 32 of limiter 16 is such that it acts as a grid-power detector for amplitude modulation and thus audio-noise voltages may be taken at point 50 on the anode decoupling network 38, 42. The noise voltages are fed through low-pass filter 56, 57 and blocking condenser 55 for rectification by diode 58 to produce at the upper end of load resistance 59 a negative voltage which is applied to the grid of audio amplifier 64. In the absence of a received carrier signal, the rectified noise voltage biasses valve 64 beyond cut-off, thus muting the output. Receipt of a carrier signal reduces the passage of noisethrough limiter 16 and thus allows valve 64 to operate. An adjustable positive bias is applied to the grid of valve 64 from H.T. source 37 and potentiometer 75 so as to set the signal level at which the output becomes unmuted. To prevent the resultant applied bias becoming positive, i.e. in the absence of noise, a diode 70 is provided which shunts away the applied bias when it becomes positive. Instead of the positive bias being supplied directly from H.T. source 37 it may be taken from point 50 in the anode circuit of the limiter 16 by moving switch 45 to the other position. In this case the positive bias increases simultaneously with a decrease in negative noise bias, thus rapidly unmuting the output on the receipt of a weak carrier signal. In ß further modification, Fig. 2 (not shown), the noise voltages are taken from the input of the first limiter through a low-pass filter to a noise amplifier before rectification to produce the muting bias. The receipt of a carrier causes cut-off of the noise amplifier and thus unmutes the output. In this arrangement the input to the noise amplifier includes a resistance shunted by a condenser. Thus, an increase in negative bias due to rectification of noise voltages in the first limiter is offset by the additional noise voltage fed through the condenser to the noise amplifier.