Posted by
Jim Dunstan on
Feb 03, 2010; 12:58am
URL: http://elecraft.85.s1.nabble.com/Attenuator-and-RF-Gain-Settings-tp4503505p4504978.html
At 07:10 PM 2/2/2010 -0500, you wrote:
>Brian,
>
>That is an illusion - yes the K3 gets more quiet (on noise) if the AGC
>Threshold is reduced.
>BUT what that indicates is that the AGC is being activated by the noise
>and the K3 is reducing the gain due to AGC action.
>The lowest AGC Threshold that is usable for any given level of
>atmospheric noise is the threshold setting just higher than the setting
>which begins to reduce the noise. In other words, listen to a spot on
>the band where there are no signals, then start at the highest Threshold
>setting of 008.
>Reduce the value of the AGC Threshold until you sense that the noise
>level starts to go down - then move the Threshold up to the next level.
>
>That setting will produce the most usable sensitivity for that band,
>that antenna, and at that particular point in time (atmospheric noise
>levels do change with time). If the resulting noise level is
>bothersome, then take other steps to reduce that response - Preamp off,
>ATT ON, and reduce the RF Gain (in that order) until you can just barely
>hear the noise. That will result in the most sensitive setting for the
>receiver.
>Remember (as Jim Brown just posted) - that atmospheric noise is just
>another 'signal' to the receiver that is to be amplified (it is coming
>in on the antenna port). One must condition the receiver to place that
>noise level at (or just above) your threshold of perception to reduce
>your fatigue level when listening to any band. That is what the preamp,
>attenuator, and RF Gain controls are for.
Hi,
I am not an Elecraft owner ... yet ... However I have built (homebrewed)
many receivers over the years ... and building a good AGC system is one of
the hardest parts (biggest challenge in modern parlance). One of the
things I learned after many years is how to make the AGC sensitive to
noise. Most of the receivers I built and commercial ones I have used
basically relate the AGC level exclusively to received signal. However in
designing the AGC detector time constants I found it very useful to have
receiver gain controlled by noise level to some degree. As background
noise goes up ... receiver gain goes down to a degree ... and as the
background gets quieter gain should go up a little. I never did get it
just right ... and the general front end attenuator and RF gain control
always got used a lot hi hi.
I find the topic very interesting .... and I find it very encouraging the
effort put into AGC design in the K3.
Jim,
VE3CI
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