Selective signal receiver for telephone equipment - has monostable blocking input to zero-point counter to give noise immunity

摘要

The selective signal receiver, for telephone equipment, avoids signal suppression as a result of noise pulses. The receiver identifies frequencies by counting the number of passages through zero and measuring the time between them. The zero counter is preceded by a circuit that blocks the input to the counter for a given time after a zero has been detected. This blocked time is slightly less than the expected time between passages through zero of the frequency being detected. Noise pulses cannot therefore affect the counter during the blocked period. The blocking circuit consists of a monostable multivibrator.