Re: Audio AVC

Jerry Gaffke
 

I'm with Jim1 and Jim2, looks fine to me.
Under a buck in parts.
The sort of thing you could mount on the back of the volume control
with the only additional wire being +12v to power it.

For all my talk of the extra dynamic range you get with RF or IF attenuation,
this simple AF attenuation AGC circuit is good enough for most uBitx owners.
Solves the problem of tuning in a weak station, then having to tear off the phones
when the big gun across town opens up.  Only disadvantage is that the
big gun might be distorted due to overloading the audio pre-amp.
You already know if that will be an issue in your particular circumstance
by listening to the stock rig.

We've had a half dozen different circuits to do audio attenuation like this
get posted to the forum.  I suspect any one of them would work well enough.
Primary consideration is, does it allow you to hear background noise when
there is no signal, yet still sufficiently attenuate the strong signals.
Without distortion, of course.
Attack and decay times are also of concern, but that should be just a matter
of fudging a couple resistor values.

Regarding Scott's comment:
  "And because I’m a dinosaur I want to believe a decent IF-derived system is better,
    though I have to admit the ubitx audio-derived stuff is pretty good. :)"

IF derived AGC signal level sensing must be done after the 12mhz filter,
or it will de-sense on strong signals elsewhere in the band.
So that sensor will be right up against the demodulator, where we have a 7dBm BFO
hitting those diodes.  On the uBitx (and Bitx40), it would be very tough to avoid
sensing anything other than the 12mhz BFO, would require a bunch of shielding
and some sort of isolation stage between filter and demodulator.
I'll live with the occasional brief pop, and stick with audio derived AGC.

Of course, if you're a true dinosaur, you're operating AM phone.
No BFO, and thus no such trouble. 

Jerry, KE7ER


On Sat, Apr 20, 2019 at 08:03 PM, jim wrote:
Meets my criteria for a mod
 
(1) works
(2) minimally invasive
(3) dirt cheap, easily obtainable parts
(4) easy to remove (when it don't work)
 
Jim

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