[Elecraft] AGC Circuit Affecting IF Signal?

[Darrell Bellerive]

On March 5, 2007 08:07 am, Darrell Bellerive wrote:
> Regardless of where I set the AGC threshold voltage via R1, the K2 always
> has more gain with the AGC off than with the AGC on. The lower the AGC
> threshold voltage the less the difference between AGC off and on.

I removed C181 connecting the IF OUT and the AGC Mixer U1 pin 1, disconnecting 
the IF from the AGC.

I also lifted the end of R4 towards U2 pin 1, and grounded this end of the 
resistor. This puts the MC1350 into full gain.

At this point the AGC circuit is totally isolated.

R1 is set for an AGC threshold voltage of 3.80 volts.

There is no longer any difference in receiver gain between AGC on and off. No 
surprise here!

Measuring the voltage at pin 1 of U2 shows 3.310 volts with the AGC on and 
3.372 with the AGC off. This is not what I expected. I expected the voltage 
to be the same whether AGC was on or off. Instead what I found was that the 
voltage increased with the AGC off. This should have the effect of lowering 
receiver gain when the AGC is off, exactly the opposite of what is happening.

I reinstalled C181 and repeated the voltage measurement on the AGC line at U2 
pin 1. Resistor R4 is still lifted and grounded at the u2 pin 1 end. Receiver 
gain increases by about 0.7 dB with the AGC off. The AGC voltage now reads 
3.377 with the AGC off and 3.520 with the AGC on. Decreasing the AGC 
threshold voltage to minimum 2.776, the AGC line measures 2.383 volts with 
the AGC off and 2.490 with the AGC on. In either case the AGC voltage 
increases with the AGC on. This is the opposite of what happened with the IF 
disconnected from the AGC mixer and would cause the reduction of receiver 
gain.

I don't think that this is a performance affecting difference, but it does 
help explain why the receiver gain is always higher with the AGC off than it 
is with the AGC on. I also don't think that there is anything wrong with the 
AGC circuit in my K2. A less than 1 dB change in audio output is not 
noticeable by the human ear.

So, bottom line is that adjusting R1 for equal receiver gain between AGC on 
and off is not possible. One can get close but never equal. As the AGC 
threshold is reduced the difference between AGC on and off gets smaller. At 
some point one cannot hear the difference.

The problem is that as the AGC threshold is reduced, so is the range of the RF 
gain control.

Next, the VE6DRW AGC mod. I'll report my findings soon.

Darrell   VA7TO   K2#5093

-- 
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada