Squelch circuit for frequency modulation receiver

Claim 1

In a frequency modulation receiver including means for receiving a modulated wave for converting the same to an intermediate frequency wave, and including low frequency amplifying means for the derived modulating signal, the combination including, discriminator means for deriving from the intermediate frequency wave the modulating signal and noise signals extending in a frequency range substantially greater than the modulating signal, which noise signals increase substantially in amplitude in the absence of the Modulating signal, limiter means, circuit means coupling said discriminator means to said limiter means for applying substantially the complete amplitude and range of frequencies of the modulating and noise signals produced by said discriminator means to said limiter means, said circuit means including means decoupling said limiter means from said discriminator means so that said discriminator means is not loaded thereby, frequency selective means coupled to said limiter means for selecting from the output thereof noise signals extending in a frequency range above the modulating signal, said limiter means producing a limited output in response to all modulating signals coupled thereto from said discriminator to thereby reduce the amplitude of noise signals in the presence of modulating signals at the output of said discriminator means, noise detector means for detecting and amplifying the noise signals selected by said frequency selecting means, said noise detector means including circuit means for developing a signal which represents the addition of the rectified signals produced by the negative and positive half-cycles of said selected noise signals, and second circuit means coupling said detector means to the low frequency amplifying means of the receiver for squelching the low frequency amplifying means in response to detected noise signals which reach a predetermined amplitude, said second circuit means isolating said noise detector means from said low frequency amplifying means when said low frequency amplifying means is not squelched.