Universal Squelch

016 RADIO & TV This squelch circuit is simple, universally usable and provides a large enough gain to enable it being incorporated in the auto- matic-gain control (ACC) circuit of a variety of receivers. The input signal derived from the AGC circuit in a receiver is attenuated by network R1-R2-Pl. The signal at the wiper of P1 is taken to the inverting input of opamp Al, which is connected as a comparator. The non-inverting input is provided with a reference voltage of 200 mV by potential divider R9-R10. The output signal of Al is applied to a Schmitt trigger circuit, A2, via low-pass sec- tion R2-C2. This filter ensures that small noise and other interfer- ence signals do not affect the correct fucntioning of the squelch. Capacitor C3 removes the steep skirt of the output signal of A2, which renders the operation of the AGC rather more pleasant to the ear. The output of A2 is then taken to the base of output transistor T1 via potential divider R7-R8. The open-collector output of ...