At 01:12 PM 7/7/07, Bill Coleman wrote:
On Jun 27, 2007, at 6:59 AM, Bill Tippett wrote:
In rigs like
the K3, ultimate selectivity is determined by the DSP
filters which will have excellent shape factors. In
DSP rigs, the most critical area of a roofing filter
for it's intended purpose of IMD performance is down
to about 30 dB on the filter skirts (i.e. not 60 dB as
measured by the classical "shape factor").
But wouldn't a roofing filter with a small shape factor be "better"
for IMD performance? Isn't 40 dB of nearby signal rejection better
than just 30 dB?
Now, this all is keeping other things equal -- that the group delay
or non-linear characteristics of the filter itself don't become
unfavorable.
I'm basically unclear on why ultimate rejection in a roofing filter
is so unimportant. If you are on 40m in Europe, just a few miles away
from a megawatt shortwave broadcaster, ultimate rejection would seem
to be of critical importance.
Similarly, if you are at a M/M station on the second rig on a band,
ultimate rejection is key, as there is another 1.5 kW signal within a
handful of meters of your receive antenna.
As I've said many times, let the actual IMD/BDR
measurements (yet to be published) be your guide. All
else is simply theoretical speculation.
73, Bill W4ZV
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