Old school stuff: Squelch

I was talking to a counterpart a while back and discovered that they didn't know what squelch was really. After some discussion, I learned that they don't teach RF stuff, as part of the Electrical Engineering curriculum, at most tertiary institutions anymore. So my (much younger) colleague can be forgiven for not knowing this.

Squelch is a technique used in FM* two way radios to prevent white noise coming through on the speaker of a receiving radio during the absence of a signal. Old analogue two-way radios employed a squelch circuit that could be adjusted so that only signals above a certain level would be heard by the user.

If there was no squelch circuit/function, the user would only hear (very annoying) noise.

Older two-way radios had a squelch knob, that allowed the user to adjust the squelch to suit their own preference. This presented a headache for field technicians, as turning the squelch up too high (as tusers often did) would render the radio "deaf" in that a relatively strong signal was needed to open the squelch.

Two-way radios made after the 1980s allowed the squelch to be set on a computer using the programming software. Some radios still required the squelch to be adjusted via a trimmer potentiometer inside the radio.

How it worked.

These old radios used a circuit like the one shown above. The quadrature detector would receive RF (usually at 455kHz) from the IF filter and produce unfiltered audio. This unfiltered audio would be sent to the audio amplifier where it would be filtered and amplified. Additionally, the unfiltered audio was sent to the squelch circuit where it would pass through a +-5KHz high pass filter which would basically remove all the audio.

If you measured the point just before the diode with an oscilloscope, you would see noise when there is no signal and close to nothing when there is a strong signal. This high-pass-filtered signal would go through a simple recitifier that would convert it into a DC voltage (the voltage being inversely proportional to the received signal strength).
This voltage is then fed to a comparator. If the voltage drops below a certain level, the comparator output would go high, thus unmuting (turning on) the audio amplifier and presenting whatever is being received, on the speaker.
The level at which the comparator unmutes the audio amplifier is set by the potentiometer on the other input to the comparator.

In radios which allowed the squelch to be set via a computer, the potentiometer would be replaced by an output from a digital-to-analogue converter.

Hysteresis.

The above circuit usually had around 6dB hysteresis. This meant that a received signal; which is increasing in amplitude; will unmute the receiver at XdBm and if the signal decreases the receiver would mute again around around (X-6)dBm.

Squelch in MOTOTRBO

Many modern two-way radios make use of a Software Defined Receiver (SDR). These do not use a squelch circuit but rather rely on an algorithm to determine whether a valid received signal is present. If a valid signal is present on an analogue channel, the recovered audio will be presented on the speaker.
In a MOTOTRO radio, the analogue signal follows the same path as a digital signal - only the way in which the signal is processed varies - this happens in the software of the radio.