GB644044A - Improvements in or relating to noise-suppression circuits for frequency-modulation receivers - Google Patents

Abstract

644,044. Automatic noise suppression. PHILIPS LAMPS, Ltd. July 15, 1947, No. 18843. Convention date, July 17, 1946. [Class 40 (v)] A frequency modulation receiver in which slight or no automatic volume control is applied to stages preceding the peak limiter, comprises a thermionic tube situated between the highfrequency input and low-frequency stages, which is blocked when the received signal is below a predetermined amplitude by a voltage equal to the difference between a constant voltage and the rectified output of the limiter. The invention enables the receiver to be muted during no-signal periods without introducing distortion during the reception of signals. Fig. 1 shows part of a frequency-modulation receiver in which the output from peak limiter 1 is fed to frequency discriminator 3, 4, which supplies the low-frequency amplifier 8. A bias source 7 is used to bias diodes 3 and 4 so that these are non-conducting for signals below a predetermined level, but this bias is opposed by the voltage developed across resistance 13 by rectifier 15 which is also fed with signals from limiter 1. The arrangement is such that the receiver is muted during periods of no signal when only noise is present, but when signals above a predetermined amplitude appear the operating threshold level is reduced, thereby reducing distortion. An alternative arrangement is shown in Fig. 2, in which the control grid of a peak limiter 21 is negatively biassed by means of a source 25 connected in series with a resistance 24 which is also fed with an opposing bias derived from rectifier 23 which is fed from the output of the limiter 21. In a modification, the source 25 is replaced by the voltage developed across the resistance in the cathode circuit of an intermediate frequency amplifier (Fig. 3, not shown). In a further modification, the rectified voltage from one of the diodes in the frequency discriminator is used as the varying opposition bias to reduce the effect of the constant voltage bias (Fig. 3a, not shown). Specification 613,952 is referred to.