AGC Circuit Affecting IF Signal?

[Darrell Bellerive-2]

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.

In accordance with the MC1350 datasheet, to find the maximum gain of the
MC1350, I grounded pin 5 of the MC1350 via R4 on the Control Board. I lifted
the end of R4 on the U2 side and attached a ground.

The gain increased slightly. With the AGC on the AF output averaged 12.9 mV
about 1 mV higher (0.7 dB) than with the normal AGC circuit. What was
interesting was that with the AGC off, the AF output averaged 13.9 mV,
about 1 mV higher (0.6 dB) than with the AGC on. With the AGC off the gain of
the K2 was about 0.3 dB higher.

With the output of U2A pin 1 disconnected at R4, I would have expected no
difference with the AGC switched on or off. My guess is that the AGC mixer U1
(SA612) is loading down the IF OUT lead connected to pin 8 of the MC1350
which is reducing the level of the IF signal sent to the product detector.

What else could be causing this? If indeed the AGC mixer is loading down the
IF signals when the AGC is on, would this not change the impedance at the
input to the 2nd crystal filter and affect the filter shape?

Darrell   VA7TO   K2#5093

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Chris Kantarjiev K6DBG]

Darrell,

This is a shot in the dark, but when I was tracking down a problem
with low audio on my K2, I found that C10 in the AGC circuit was leaky,
allowing some signal to appear at U1 pin 2, and causing all kinds
of oddities when AGC was turned on.

73 de chris K6DBG
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Darrell Bellerive-2]

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
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com