Bi-Level Squelch

Posted on Feb 7, 2014

As no one wants to listen to discriminator noise, the proper operation of the squelch circuit in any FM radio is vital. In a repeater, the squelch / COR is even more critical. The basic function of a squelch circuit is simple: detect the presence of a radio signal and turn the speaker audio on. That`s really all they do. The first FM squelch circu

Bi-Level Squelch
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its worked by detecting the presence of a signal by measuring changes in the radios AGC (Automatic Gain Control) voltage. This method worked poorly on weak signals so a better method was developed, the noise operated squelch. The noise operated squelch works by measuring the amplitude of the discriminator white noise above the speech band. The phrase "above the speech band" is the critical part. Your receiver`s audio is split into two bands. One (from 300 to 3, 000 Hz) contains the speech. Everything above 5, 000 Hz is noise. No intelligence is carried there. When the radio hears a signal, the discriminator output begins to quiet. That is, the amplitude of the white noise above the speech band begins to drop. If we convert this AC high frequency noise to a DC signal, we can measure it. We can compare it to a known reference voltage as well. When the noise signal becomes weaker than the reference, the squelch circuit turns the speaker on. When the noise is stronger than the reference, the speaker is turned off. In practice we can add some timing as well. By adding a simple resistor / capacitor timing circuit to our squelch, we can keep it from opening on short bursts of noise that would be distracting. This still leaves one problem. You can`t have the squelch circuit close the audio immediately either. If you do, the audio from fluttering signals like mobiles would be badly chopped. This requires the squelch to have a time...

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