Method and apparatus for providing a very fast acting noise squelch

Abstract

A fast acting noise squelch control circuit for FM radio receivers samples the noise above the audio band to determine the presence of an RF carrier. The detected noise signal is integrated by a low-pass frequency gain integrating filter and compared, using a high gain differential amplifier, against a preset squelch threshold voltage to provide a signal indicative of the presence or absence of a received carrier. The output of the integrating filter is initialized at a reference voltage that is only slightly below the threshold voltage setting for the comparator to minimize integration time and increase sensitivity of the squelch. In this manner, a long time constant in the noise integrating filter can be used without having to wait for a long time to establish whether a carrier signal is present. After a predetermined observation interval, the comparator output is checked and the comparator threshold voltage setting is lowered if a carrier of sufficient signal strength has been detected to increase the squelch control circuit hysteresis. The improved squelch control system can be utilized in intermittent or scanning receiver systems while using the normal noise filtering as would be employed in continuously operated receivers without having to use a long 'turn-on' period that would result in unacceptably slow cycling rates.