[Elecraft] Nulling QRM

[Lyle Johnson]

AD6XY wrote:
> I know this is not possible right now. But, is there any way one can use the
> DSP on SSB to allow adjacent stations to be actively rejected and thereby
> arranged to not pump the AGC?

You have several tools at your disposal, but perhaps an explanation of 
what is happening might help.

Any signal within the roofing filter passband that results in a certain 
strength (nominally -50 dBm after the roofing filter selectivity) will 
activate the hardware AGC to prevent over-range of the analog to digital 
converter.

The MDS of the radio is near -140 dBm, so this provides some 90 dB of 
dynamic range before the hardware AGC activates.  I find people often 
turn the preamp on even when it isn't needed.  This adds noise and 
reduces the effective in-band dynamic range. So I suggest turning off 
the preamp.  It's easy to tell if you need it: tuned to a dead spot on 
the band, disconnect the antenna.  If the noise level change is 
detectable without the preamp, you don't need it on and it isn't helping 
even with the weakest of stations.  Similarly, turn on the ATTenuator 
and see if you can still detect a difference in noise level. If you can, 
leave the ATT on.  Turning it off won't help even with the weakest signal.

These two tools: PRE and ATT: allow you to increase the effective 
in-band dynamic range by as much as 20 dB!  And misadjusting them can 
cost you as much as 20 dB in resistance to strong signals pumping the AGC.

If you are still bothered by very strong signals inside the roofing 
filter passband but not in the passband of the signal of interest, it's 
time to consider a narrower roofing filter.

The rest of the AGC is done by the DSP after additional filtering.  It 
will only respond to energy inside the passband set up by use of the 
SHIFT/WIDTH (HI/LO CUT) controls.

73,

Lyle KK7P