Topics

�BITX ... AGC or AVC?


 

Loris,

I had experimented with a AGC which was very simple. Disconnect R23 and R43 from ground and introduce
a varicap which I had MV2109. A series resistor to the varicap through which you varied the capacitance
and thereby the gain. This method worked very well with both IF stages and even on the TX branch.

I wish some members would experiment on this approach.

Raj

At 28/10/2020, you wrote:
On the CCARC (Carroll County Amateur Radio Club, Inc.) website I found a very interesting article on automatic gain control that is worth reading!
Without many frills Carol makes us understand that an AGC (automatic gain control) on the IF of the uBITX, due to the nature of the amps used (bidirectional), is very complex to make at this level of the circuit ...
therefore ? ...
Much better and less invasive to act directly on the volume control, implementing a simple but effective automatic volume control (AVC)!
Think man ... think ...
A little less software and a little more electronic experimentation and, with much less effort, very complex problems can be solved!
Think man ... think !
Greetings to all, Loris IW4AJR


 

On Wed, Oct 28, 2020 at 12:31 PM, Raj vu2zap wrote:
I had experimented with a AGC which was very simple. Disconnect R23 and R43 from ground and introduce
a varicap which I had MV2109. A series resistor to the varicap through which you varied the capacitance
and thereby the gain. This method worked very well with both IF stages and even on the TX branch.

I wish some members would experiment on this approach.
Hello Raj ...
Interesting solution, if I have some free time I could try it!
I would like to have an explanation from you:
what is the function of AGC  transmission for uBITX? ... perhaps as a compressor instead of an external speech-processor?
Thank you, 73 Loris - IW4AJR


Jerry Gaffke
 

Loris is right, Raj's R23 and R43 are both on the transmit side of the uBitx bidi amps.
So perhaps not AGC for the receiver, which is what most would be interested in.
But could also work on the receive side of the bidi amps.

A clear and complete description from Raj would help others to try it, 
"introduce a varicap" is not terribly specific.

A search in the forum posts for "Raj AGC" finds no hits on this AGC scheme.


Don, ND6T, had a couple blog posts about easy hacks to implement manual RF gain
on the Bitx40, should be possible to make these work on the uBitx.
http://bitxhacks.blogspot.com/2017/01/nd6ts-rf-gain-control-with-tombstoning.html
http://bitxhacks.blogspot.com/2017/02/another-approach-to-rf-gain-control-on.html


Dozens of threads in the forum on adding AGC to the Bitx40 and uBitx.
The "automatic" part of AGC may not be needed, but a simple RF gain control
to avoid receiver overload would be good.
That and back-to-back diodes in the audio chain somewhere
to prevent blown eardrums when using headphones.

Jerry, KE7ER


On Wed, Oct 28, 2020 at 06:10 AM, IW4AJR Loris wrote:
On Wed, Oct 28, 2020 at 12:31 PM, Raj vu2zap wrote:
I had experimented with a AGC which was very simple. Disconnect R23 and R43 from ground and introduce
a varicap which I had MV2109. A series resistor to the varicap through which you varied the capacitance
and thereby the gain. This method worked very well with both IF stages and even on the TX branch.

I wish some members would experiment on this approach.
Hello Raj ...
Interesting solution, if I have some free time I could try it!
I would like to have an explanation from you:
what is the function of AGC  transmission for uBITX? ... perhaps as a compressor instead of an external speech-processor?
Thank you, 73 Loris - IW4AJR


Evan Hand
 

AmatuerRadioKits sells an AGC that, per instructions, connects to the receiver input first IF at R10.  It does appear to work, though I have not done extensive testing.

I am only offering up this option to show that others have done the IF gain adjustments, though not in the 2nd IF stage as is suggested in this topic.

Here is the link to the files for the kit:
https://amateurradiokits.in/downloads/

73
Evan
AC9TU


 

Sorry guys,

On the TX side it is R23/43 on RX side is R13/33. It worked as AGC for RX and ALC on TX.
On RX I controlled the gain from received audio on TX was a DC gain control pot.

I can't provide specific circuit details from here as I am holed up at the farm. All these
experiments were done on a bitxv3 board. To experiment with this idea you will need
to be knowledgeable on the subject and not for beginners.

Raj


At 28/10/2020, you wrote:
Loris is right, Raj's R23 and R43 are both on the transmit side of the uBitx bidi amps.
So perhaps not AGC for the receiver, which is what most would be interested in.
But could also work on the receive side of the bidi amps.

A clear and complete description from Raj would help others to try it,
"introduce a varicap" is not terribly specific.

A search in the forum posts for "Raj AGC" finds no hits on this AGC scheme.


Don, ND6T, had a couple blog posts about easy hacks to implement manual RF gain
on the Bitx40, should be possible to make these work on the uBitx.
http://bitxhacks.blogspot.com/2017/01/nd6ts-rf-gain-control-with-tombstoning.html
http://bitxhacks.blogspot.com/2017/02/another-approach-to-rf-gain-control-on.html


Dozens of threads in the forum on adding AGC to the Bitx40 and uBitx.
The "automatic" part of AGC may not be needed, but a simple RF gain control
to avoid receiver overload would be good.
That and back-to-back diodes in the audio chain somewhere
to prevent blown eardrums when using headphones.

Jerry, KE7ER


On Wed, Oct 28, 2020 at 06:10 AM, IW4AJR Loris wrote:
On Wed, Oct 28, 2020 at 12:31 PM, Raj vu2zap wrote:
I had experimented with a AGC which was very simple. Disconnect R23 and R43 from ground and introduce
a varicap which I had MV2109. A series resistor to the varicap through which you varied the capacitance
and thereby the gain. This method worked very well with both IF stages and even on the TX branch.

I wish some members would experiment on this approach.

Hello Raj ...
Interesting solution, if I have some free time I could try it!
I would like to have an explanation from you:
what is the function of AGC  transmission for uBITX? ... perhaps as a compressor instead of an external speech-processor?
Thank you, 73 Loris - IW4AJR