Squelch Opration

A signal from a discriminator in coupled into C4 RV1 is used to set the squelch level. U2 configured as an inverting amplifier gives a gain of -10 the output of the amplifier is coupled into U1 a highpass filter with a cutoff below 12kHz this is high enough to allow the noise to pass but not be effected the received signal. The output of the HPF is coupled into a rectifer to genrate a DC signal to drive the switching transitor Q1.