Episode 959 let's look at how the ICOM IC-245 squelch circuit operates. A simple peak detector which controls when to mute the audio. Be a Patron: / imsaiguy
Thanks for pointing out the bias on the detector circuit to overcome the "deadband" from diode voltage drop. The coupling capacitors in this circuit are all < 1 microfarad. So they are listening for ultrasonic noise above the voice frequency range. When sufficient carrier is present, the noise above and below the carrier drops, and this triggers the unmuting of the squelch. Because of the triangular noise distribution of FM, monitoring the ultrasonic noise is an effective method for carrier detection. Oddly, I have noticed that tone squelch will work when the signal is not even audible. We used a 67 hz tone on a phone patch, and found that weak signals that made no difference in the background noise level, much less any discernable audio, would trigger the tone squelch PLL.
Wait, wouldn't the peak detector turn on the squelch transistor when audio came through and let the noise through instead of the other way around? Is it not maybe a pnp?
Hmm, most receivers of the era took the squelch off the AGC - muting the audio when the received signal strength fell below a given threshold. I didn't see the full schematic to see what's going on here, but the circuit would have been very similar - an integrator and a threshold detector, probably (as you observe) based on Ge diodes. I've also seen an intermediate stage of development where instead of an IC op-amp, a long-tailed pair of transistors gave a high-gain stage. The venerable op-amps like the 702, 709, 301 and 741 all used essentially the same long-tailed pair topology, except that the emitter and collector resistors were replaced with active current sources. That way the voltage gain of the differential stage could be extremely high while remaining stable - the 301, if memory serves, had a gain of 5000 at that stage. Could this 'squelch' actually be a noise blanker - muting the audio in response to an impulsive signal such as a static crash from lightning?
Thanks for pointing out the bias on the detector circuit to overcome the "deadband" from diode voltage drop. The coupling capacitors in this circuit are all < 1 microfarad. So they are listening for ultrasonic noise above the voice frequency range. When sufficient carrier is present, the noise above and below the carrier drops, and this triggers the unmuting of the squelch. Because of the triangular noise distribution of FM, monitoring the ultrasonic noise is an effective method for carrier detection.
Oddly, I have noticed that tone squelch will work when the signal is not even audible. We used a 67 hz tone on a phone patch, and found that weak signals that made no difference in the background noise level, much less any discernable audio, would trigger the tone squelch PLL.
I often wondered how squelch worked, Thank you.
Nice and sharp explain..... thanks for posting..Dinos
Wait, wouldn't the peak detector turn on the squelch transistor when audio came through and let the noise through instead of the other way around? Is it not maybe a pnp?
yes, I must have missed something
@@IMSAIGuy i guess it would work if it was a negative peak detector
That makes sense. Thanks for clarification. I was wondering how this worked with a npn.
Looks like a noise squelch. They take the high frequencies of the audio, that are not in normal audio but high in fm noise, and rectify them.
Usually high-passed at 4-6 KHz for narrow-band. And, AM receivers can have squelching too. Being a pilot, I especially appreciate that!
Hmm, most receivers of the era took the squelch off the AGC - muting the audio when the received signal strength fell below a given threshold. I didn't see the full schematic to see what's going on here, but the circuit would have been very similar - an integrator and a threshold detector, probably (as you observe) based on Ge diodes. I've also seen an intermediate stage of development where instead of an IC op-amp, a long-tailed pair of transistors gave a high-gain stage.
The venerable op-amps like the 702, 709, 301 and 741 all used essentially the same long-tailed pair topology, except that the emitter and collector resistors were replaced with active current sources. That way the voltage gain of the differential stage could be extremely high while remaining stable - the 301, if memory serves, had a gain of 5000 at that stage.
Could this 'squelch' actually be a noise blanker - muting the audio in response to an impulsive signal such as a static crash from lightning?
an important part is the coil in the first amp.
enjoyed this/learned from it.
question: would a comparator be useful in this circuit or just overkill? or bad for some other reason i’m not aware of?
comparator is the right choice today.
@@IMSAIGuy Or else do it in the DSP! If you've got DSP anyway, no sense spending board space on an extra op-amp and a bunch of passive components.
This looks like a voltage doubler .