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George,

An AGC detector must be placed after the the desired signal has been separated out
with the narrowest bandwidth filter in the radio, so after the 11mhz crystal filter is correct.
Unfortunately, when trying to detect that 11mhz signal on the uBitx,
you will likely see nothing but the very strong BFO, also at 11mhz.

It's possible to do, but you will need a carefully designed stage for isolation
between the crystal filter and the demodulator, it is currently a direct connection.
And likely need a well shielded BFO and demodulator.

There's a reason all the various AGC implementations for the uBitx
detect the audio signal, not the IF signal.


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Building an amplifier to go from mV DC to Volts is fairly straightforward..
I would probably do it with two op amps configured as inverting amplifiers,
one after the other.? Use three if you really want an inverted output.



Trying to do it all in one op amp stage might be possible, but that much gain
approaches the limits of what typical op amps can do? ? And while not
terribly important in this case, it would drastically reduce the possible bandwidth.
Lots of gain in a single stage can be unstable.

Op amps are cheap, 2 or even 4 in a single package for well under a dollar.

This could also be done with transistors instead of op amps.

Jerry, KE7ER


On Fri, Oct 30, 2020 at 09:51 AM, George Metaxas wrote:

Hello, I am a uBitx v5 owner and modder. I am opening this thread again as I am considering placing a p-channel JFET in parallel to the signal entering the 1st IF amplifier as a means to control the RF gain. I am currently trying to figure out a circuit to be added right before the product detector and perform envelope detection on the 2nd IF signal, then use the DC voltage to do AGC.
I have found out by simulation - not built any test jig yet - that the gate voltage to the JFET should range between zero and 5 V in order to control the RF gain as such. However, the envelope detector produces a voltage that is both much smaller and ranges over much smaller values, of the order of mV.
What do you think of that approach? Could that be a good way to have IF-derived AGC on uBitx? How can I use the output of an envelope detector as a way to produce a 0...5 V AGC voltage signal? I'm really stuck on that.
73, George, SV1DEQ