GB646920A - Improvements in or relating to a noise reduction system for radio receivers - Google Patents

Abstract

646,920. Radio receiving apparatus. COLONIAL RADIO CORPORATION. April 24, 1947, No. 10963. Convention date, June 29, 1944. [Class 40 (v)] A noise reduction circuit for an amplitude-modulation receiver comprises a first condenser 24 connected through a first electron tube 20 to the signal demodulator 12, and through second electron tube 29 to a second condenser 27 which is changed to a value related to the peak amplitude of the modulation envelope, and means for rendering both electron tubes non-conductive during noise impulses exceeding the 100 per cent modulation level, to isolate the audio output circuit. The arrangement reduces the effect of large noise impulses to a value below that of the 100 per cent modulation level. In Fig. 1, the carrier or intermediate frequency is supplied from transformer 10 to signal demodulator 12, the output from which is supplied via potentiometer 13, 14, diode 20 and volume control 23, to the audio frequency stages of the receiver. In the absence of noise, diode 20 conducts since the potential at B, which is supplied to the cathode of diode 20 via diode 29, is more negative than that of point A. Point C is stabilized at a negative potential by condenser 27, and in the event of a noise impulse which drives point B, and to a lesser extent point A, still more negative, diode 20 is rendered non-conducting. Noise impulses in the audio output are thus limited to a finite value, which by adjustment of the ratio of resistances 13 and 14, may be made equal to the hundred per cent modulation level. In order to reduce the interference effect still further, an additional diode 26 is provided, which is normally biassed to be non-conducting, since the anode is connected to point C. The incoming high-frequency energy is applied to the cathode of diode 26 via a capacity potentiometer 16, 17. In the event of a steep noise impulse of excessive amplitude, diode 26 conducts and produces a positive impulse which is supplied via condenser 28 to the cathode of diode 29, thus cutting off diode 29 and thereby preventing the noise impulse in the audio output circuit from reaching the hundred per cent modulation level. The Specification specifies suitable values for the resistors and condensers.