Aha. It sounds like the "desense" problem arises when the DSP filter
is narrower than the roofing filter. A strong station within the
roofing passband but outside the DSP passband will shut down the
receiver through AGC.
I think we all misled ourselves by the impression that desense was
impossible in the K3. It's working just as it ought -- but operators
need to widen out the DSP to the full width of the roofing filter,
when there are strong stations every few Hz. A bit counter-intuitive,
perhaps.
Peter N8MHD
On 2/23/09, K2MK <k...@comcast.net> wrote:
> Hi Wayne and all:
>
> My filters are 200, 400, 1.0, 1.8 and 2.7 and they are all CW enabled. I
> was using headphones and my sub RX was off. My serial number is 2208 and the
> radio was factory assembled (including the sub RX).
>
> I did not hear key clicks when this was happening. The off frequency
> transmissions were clean. I also made a point of observing the signal
> strength of the offending stations and they were usually S8 or S9. Not the
> 20db or 40db over S9 that I was expecting.
>
> I thought it was interesting that recovery was noticeably slow on the DX
> station's frequency when this was happening. However, under normal
> conditions when US stations were calling directly on top of the DX station,
> recovery was fast. Hence the ability to copy the DX station through the dits
> when the QRM was on frequency.
>
> Maybe a passband shift could of helped. I would like to see the shift limits
> changed to something less than 50Hz. Maybe 10Hz on CW and 50Hz on other
> modes.
>
> I appreciate the interesting ideas from K6LL and W4ZV. Unfortunately I'll
> have to wait for the next busy contest to try them out. This just isn't the
> type of condition you can experience under normal band conditions.
>
> 73,
> Mike K2MK
>
>
>
> wayne burdick
> Sun, 22 Feb 2009 18:26:03 -0800
>
> Mike,
>
> The K3 is virtually desense-proof, with a BDR of ~140 dB. But to take
> advantage of this, you need a narrow crystal filter -- the closer to
> the DSP bandwidth the better. This is exactly the situation that we had
> in mind when we designed the 200-Hz 5-pole filter. For CW pileups, you
> can't beat it.
>
> What crystal filter were you using at the time?
>
> Of course if the transmitting stations are "wide" due to key clicks,
> there may be situations where no amount of filtering can help (for any
> receiver). The DSP noise blanker and NR may be useful sometimes -- you
> might give this a try.
>
> Wayne
> N6KR
>
> On Feb 22, 2009, at 4:45 PM, K2MK wrote:
>
> > I had a great time with my K3 during the ARRL DX contest. I do S&P and
> > I was
> > trolling around with my filter width at 50Hz. Absolutely outstanding.
> > The
> > auto spot is equally outstanding.
> >
> > At 50Hz width it was quite clear that many stations call off frequency.
> > Using RIT, I could see that it was typical for them to be 70Hz or more
> > away
> > from the DX station but I could not hear them in my 50Hz passband. The
> > real
> > problem was when one of them was S9 or greater. They completely swamped
> > weaker DX stations.
>
>
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