Posted by
Bill Coleman-2 on
Jul 07, 2007; 2:12pm
URL: http://elecraft.85.s1.nabble.com/K3-2-7-kHz-Shape-Factor-tp449609p449610.html
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.
Bill Coleman, AA4LR, PP-ASEL Mail:
[hidden email]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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