Squelch trigger circuit

A circuit for use in an RF receiver for receiving a modulated carrier signal having a predetermined information band and producing a squelch trigger signal, the receiver including an AGC circuit including an input terminal and means for receiving an input signal at the input terminal and amplifying the input signal to produce an IF output signal having a noise component that is outside of the information band, the level of the noise component being directly proportional to the gain of the AGC circuit and thus inversely proportional to the level of the modulated carrier signal, the AGC circuit including an AGC detector connected to receive the IF output signal and to produce a detection signal having a characteristic indicative of the level of the IF output signal, the circuit comprising:

• a bandpass filter connected to receive the modulated carrier signal and to produce a bandpass filtered signal, the bandpass filter having a passband that includes the information band, whereby substantially all frequency components within the information band of the modulated carrier signal pass through the bandpass filter, but noise components having frequencies outside the passband are substantially attenuated by the bandpass filter;
• means connecting the bandpass filtered signal to the input terminal of the AGC circuit;
• and
• a trigger circuit including means for receiving the detection signal and means for producing the squelch trigger signal when the level of the noise component in the IF output signal outside of the band passed by the bandpass filter crosses a threshold.