GB699592A - Squelch circuit for radio receivers - Google Patents

Abstract

699,592. Radio receiving circuits. BENDIX AVIATION CORPORATION. Dec. 7, 1951 [Dec. 29, 1950], No. 28770/51. Classes 40 (v) and 40 (vi). A muting system for a frequency-modulation receiver comprises means for amplifying and rectifying noise, the rectified product being applied to a muting device to mute the system, and is characterized by means for applying rectified signal to the noise rectifier so as to prevent muting and, in addition, means for deriving the envelope of the audio output of the system and applying it to reduce the gain of the noise amplifier. A noise amplifier 15 is coupled to the discriminator 11 by a network 14, 40 passing only the higher audio frequencies, and feeds noise to the cathode of a grid-controlled rectifier 17. In the static state the rectifier 17 passes current and a positive D.C. potential is applied from its cathode, via a low-pass filter 33, 43, to the grid of a D.C. amplifier comprising a pair of valves 34, 36. The valves 34, 36 are in series connection and the anode circuit of the valve 36 contains the winding 39 of a muting relay. The valve 17 being conductive, its rectification efficiency is low and also its cathode impedance, so that the gain of the amplifier 15 is also low and the circuit is insensitive to noise. When a signal carrier appears, a negative potential is produced by rectification at the grid of the limiter 12 and is applied to the grid and cathode of the valve 17 via resistances 18, 20 and 18, 19 respectively; circuit elements are so chosen that these paths have different timeconstants and the potential finally applied to the cathode of the valve 17 is a fraction of that applied to the grid. The appearance of a sufficient negative potential across the resistance 21 biases back the D.C. amplifier and causes the muting relay 39 to be released. The arrangement is such that, in the region in which the grid and cathode of the valve 17 are at about the same potential, the rectification efficiency of this valve is high and its loading on the amplifier 15 largely removed. The sensitivity to noise is then high, so that if noise accompanies the signal production of a sufficient negative potential across the resistance 21 to release the relay 39 is delayed, due to rectification of noise in the valve 17, until a higher signal level is reached. In addition, the audio output from the final stage 25 of the receiver is applied to a diode rectifier 28, which is provided with a delay bias by means of resistances 29, 42. Thus if the audio output is adequate, a negative potential appears across the smoothing network 30 and is applied via switch 44 to bias back either the amplifier 15 or the rectifier 17. In this way muting as the signal level falls is delayed until the audio output is unsatisfactorily small.