Implementing squelch in AM...

Thread Starter

Externet

Joined Nov 29, 2005
1,412
Hi.
Could pin 5 AGC be used to implement squelching on an AM receiver ?
Is it an input pin, output pin, both, timing, filtering...?
upload_2019-2-8_16-35-9.png
It is part of a TA2003 integrated circuit running on less than 3V.
upload_2019-2-8_16-36-21.png
Any advise is welcome.
 

sghioto

Joined Dec 31, 2017
1,038
Put a voltmeter on pin 5 and read the voltage when receiving a station and when not receiving a station to compare. If there is a difference then it may be possible.
SG
 

MisterBill2

Joined Jan 23, 2018
3,998
The Short answer is YES. But making squelch work well on an AM receiver is a bit more complex.
A simple comparator circuit using the AGC voltage and an adjustable reference voltage, along with an analog switch, and there is your squelch system. Or it can be a bit more complex.
 

Dodgydave

Joined Jun 22, 2012
8,467
Last edited:

MisterBill2

Joined Jan 23, 2018
3,998
Probably the IC shown in the first post can be used to provide a squelch function, BUT it is also likely that a fair amount of amplification of the AGC voltage will be required. In addition, if there is a delayed AGC function so that gain is not reduced for weak signals, the squelch function will not be available except for stronger signals. That is why a squelch function is much less common in AM receivers, since it is more complex to implement. Using a different IC that has an RSSI (received signal strength indicator) output will make the task simpler.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,412
Thanks, gentlemen, good guidance.

Dodgydave : To use pin 7 I would need a 10.7MHz IF presence detector. For unmodulated carrier, there will be no audio embedded there.
 

MisterBill2

Joined Jan 23, 2018
3,998
Thanks, gentlemen, good guidance.

Dodgydave : To use pin 7 I would need a 10.7MHz IF presence detector. For unmodulated carrier, there will be no audio embedded there.
Pin 7 is indeed an IF signal pin, and connecting a detector circuit would be rather complex. The audio signal is only available on pin #11. And an audio driven squelch circuit that rejects noise is quite complex, and does not use 3.3 volts supply.
So now I am wondering if this is an existing circuit, or a design stage of a project?
 

Audioguru

Joined Dec 20, 2007
11,251
The audio from an AM radio is nothing but noise so what would you detect to apply a squelch?
The AGC simply adjusts the audio output level to be the same for weak distant and strong local signals and interference.
 

MisterBill2

Joined Jan 23, 2018
3,998
AM squelch systems are based on carrier levels, which any system with adequate sensitivity can easily distinguish from noise. But sensing speech makes it a bit more complex. Those circuits exist and have been around for many years. Check out the "DJ Killer" circuit in some old archives.
 
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