[Elecraft] Nulling QRM

[Brett Howard]

I had a thought about your comments on the preamp.  Being that many
simply leave it on (personally I don't think I've ever used mine) but
anyway wouldn't the hardware AGC being at max gain be another indicator
that it may be useful to use?  Perhaps the PRE icon could flash if use
of that feature was not helping you much like the NB icon does if one
has high settings and tunes to an area where its no longer needed.

~BTH

On Thu, 2009-09-03 at 06:41 -0700, Lyle Johnson wrote:
> 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
> 
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