[Elecraft] K3 receiver desensing on CW during contest

[Bill Tippett]

Hi Mike,

On Mon, Feb 23, 2009 at 11:00 AM, K2MK <k2mk at comcast.net> wrote:

> Here are my CONFIG settings related to the AGC. The only ones I played with
> during the contest were SLP and THR. I always use AGC-F but did occasionally
> try AGC-S.
>
> AGC-HOLD   000
> AGC-PLS   NOR
> AGC-SLP   010
> AGC-THR   005
> AGC-F   120
> AGC-S   020

These all look reasonable.  I set my AGC-S to the max 040 but I
actually never use AGC-S.  If I were an AMer or 75m SSB ragchewer, I
might.

> The differing conditions I described were the situation of everyone calling
> the DX close to his frequency versus one strong station calling 70Hz off
> frequency. When even strong stations called the DX right on frequency I was
> able to hear signals through the QRM. When strong stations called 70Hz off
> of the DX frequency the receiver went quiet and took about 1/4 to 1/2 second
> to recover after he stopped sending.

This is what I'm having problems understanding:

"When even strong stations called the DX right on frequency I was able
to hear signals through the QRM."

1.  If two stations are exactly on frequency, the best ears in the
world will not be able to separate them.  I can distinguish pitch
*differences* (i.e. one after the other) of about 0.4 Hz repeatably
(see link below), but there's no way I could copy two simultaneous CW
signals dead zero beat, so I don't understand your comment.  The human
ear's DSP has an effective bandwidth of ~50 Hz, so I could probably
distinguish two signals 70 Hz apart if their amplitude was not too
different (the human ear has AGC limits also!)

http://tonometric.com/adaptivepitch/  (a fun test to take!)

2.  The way the K3 and similar front ends (i.e. Orion, FT2000/9000, IC
7700/7800) work, strong signals inside the roofing filter passband
will desense *all* signals within the passband...i.e. they will affect
zero beat signals just as much as those 70 Hz away (which would be the
case even if you were using the 200 Hz filter).  So I cannot
understand how signals off zero beat would be affected more than those
zero beat.

Anyhow, I'm clearly not understanding what you're saying.

73,  Bill