GB1174734A - Improvements in Squelch Circuits - Google Patents

Abstract

1,174,734. Transistor muting circuits. MOTOROLA Inc. 20 May, 1968 [6 June, 1967], No. 23951/68. Heading H3T. In a muting arrangement in which the muting is derived from the noise present in the signal channel, and which increases as the signal level increases, a positive-going output from noise detector 10 is applied over a short time-constant circuit comprising resistor 25 and capacitor 26, which reduces the ripple, and a potential divider 28, 29 to a D.C. amplifier 14. Amplifier 14, which derives an emitter bias from potentiometer 38, 39 is normally conductive in the absence of a noise bias on its base, and its collector potential biases audio switch 16 to cut off the audio signal from the audio output line. When the noise increases, as the signal level increases amplifier 14 is biased back and the drop in collector potential causes audio switch 16 to close and couple the audio signal to the output line. The output of noise detector 10 is also applied via a filter network comprising resistor 68 and capacitor 66, which removes any high-frequency signals present, to an emitterfollower 13, in the output circuit of which is a long time-constant filter consisting of resistors 62, 63 and capacitor 65. The full output of this filter is applied via a diode gate 64 to the base of amplifier 14. The noise-derived voltage at the base 33 of amplifier 14-or alternatively from the output of noise detector 10-is applied to the emitter of a detector 22, the base of which is connected via a diode 48 to potential divider 38, 39. Detector 22 is non-conductive in the absence of a noise signal, but such a signal at its emitter of sufficient strength to overcome the delay bias applied to its base causes transistor 22 to conduct and its collector current in resistor 51 biases a further transistor 71 conductive. Conduction in transistor 71 causes resistor 69 to form a potential divider with resistor 68, reducing the noise-derived voltage applied to the base of emitter follower 13. Thus, on the commencement -of signals transistor 13 rapidly charges the capacitor 65, the voltage across which takes charge of the removal of muting. On sustained signals of sufficient strength, however, detector 22 causes the noise signal at emitter-follower 13 to decrease so that capacitor 65 discharges via long time-constant circuit 62, 63. Short timeconstant circuit 25, 26 then takes charge, as that on any sudden cessation of signals muting is rapidly effected, so avoiding a noise " tail " being heard.