GB571404A - Improvements in and relating to radio receiver circuits - Google Patents

Abstract

571,404. Wireless receiving circuits. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 24, 1944, No. 1314. Convention date, Jan. 30, 1943. [Class 40 (v)] In a transmission channel, two amplifiers connected in cascade are arranged to operate as a multivibrator in the absence of an adequate signal so as to prevent the transmission of noise. Fig. 1 shows the invention applied to eliminating noise when tuning between stations in a frequency modulation receiver in which intermediate frequency energy is fed through cascade limiters 19, 30 to the input winding 14 of a disoriminator. The anode of 19 is resistancecapacity coupled to the grid of 30 while the anode of 30 is connected to the grid of 19 through capacity 40 and resistance 41. In the absence of an adequate input to 19, valves 19 and 30 operate as a multivibrator having a frequency somewhere between the audio and intermediate frequencies. As valves 19, 30 are alternately biassed to cut-off and to anode current saturation, no noise voltages are transmitted through the channel in the absence of a signal. When the signal reappears, the negative' voltage developed across resistor 26 by rectification of the signal, stops the multivibrator action and re-establishes thenormal limiter action. The level at which the multivibrator action commences is determined by adjustment of resistor 41 or condenser 39. In a modification, Fig. 2 (not shown), a network is provided for preventing multivibrator hiss from being introduced into the transmission channel. Also a switch is provided in the feedback circuit to facilitate a comparison of the noise output with and without the multivibrator action. The invention may be applied to any two amplifiers, not necessarily operated as limiters.