Posted by
Darrell Bellerive-2 on
Mar 05, 2007; 4:07pm
URL: http://elecraft.85.s1.nabble.com/AGC-Circuit-Affecting-IF-Signal-tp444493.html
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
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