[Elecraft] K3 - 2.7 kHz Shape Factor

[Bill Coleman]

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: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901