Re: Confused about how AGC works?

Joe Puma
 

Jerry I have had great success in hooking up Sunil’s AGC. I could be the poster child. I even determined a better location to grab audio instead of Vol H which is on the relay side of the R70 resistor. 

I do admit I had a hard time figuring things out in the beginning because of my ‘I’m still learning skillset’ but also because the CD that came with my radio kit could not be read and I only got half of the images and info off of the disk.  Then Sunil sent me some links that helped. Tuning the AGC meter was tricky but just follow the instructions again. The AGC is pretty simple. Just hook up the attenuation where Sunil stated in his instructions and you should be good to go, tune the 2kohm variable. Actually you know what.  I put a 10k because I thought it wasn’t attenuating enough I even changed one of the resistors before the AGC pot to 15kohm more then what was there. I can get specifics if anyone needs I’m going off memory.  

I ended up using coax for my attenuation hookup as that area is sensitive on the board and I was getting RF in from that wire as well as my IF tap for SDR. 

 Here is a pic before the coax 

image1.png


Joe
KD2NFC 





On Apr 19, 2019, at 2:57 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

From 
    https://en.wikipedia.org/wiki/Automatic_gain_control
"Automatic gain control (AGC), also called automatic volume control (AVC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, ..."
A google search for "AGC vs AVC" shows they are considered synonomous in radio design (but not for power grids), and that AGC is the more modern term.

In this forum, all such circuits for receivers have been getting referred to as AGC.
For some reason, the term AVC has only been used in this forum for automatic level control while transmitting
(Except for these posts from MRM.)
Works well for me, as few of us care that much about AVC while transmitting.
I suggest we keep it that way. 


Anyways, regarding the various AGC schemes:

There is this AGC kit from Sunil, it detects audio, and uses that to somehow attenuate the signal in one of the IF amps.
    https://amateurradiokits.in/product/agc-kit-for-bitx-and-ubitx/
Unfortunately, nobody seems to know how to hook it up:
    https://groups.io/g/BITX20/topic/4809028#65586
I'm sure it's easy enough to resolve with a quick email to Sunil (he monitors this forum).
And is definitely a contender, especially if you are already ordering one of the excellent Bitx40/uBitx enclosures from Sunil.

There have been several other AGC kits made available.
Let us know if any are still active, other than Sunil's and the one from   https://shop.kit-projects.com/

An AGC scheme that attenuates audio is a perfectly fine solution.
As stated in my previous post, one that attenuates RF (or IF energy) instead of audio
can give better dynamic range, but is in some ways more complicated and error prone.
An audio only AGC scheme could be mounted on the back of the volume pot, with the only
additional wire required being one to supply power.  No worries about coax and such. 
If you never hear stations so strong that they are still distorted after you turn down the volume knob
on your stock uBitx, you don't need the extra dynamic range offered by an RF (or IF) attenuated AGC.

And if you occasionally do, that could be taken care of with a manual RF gain control,
which could be just a 1k pot in the received RF from the antenna.

Jerry, KE7ER



On Fri, Apr 19, 2019 at 11:21 AM, MadRadioModder wrote:

As discussed before…

 

“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”

 

That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.

 

MRM

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