Re: K3 - 2.7 kHz Shape Factor
Posted by
Bill W4ZV on
Jul 07, 2007; 2:31pm
URL: http://elecraft.85.s1.nabble.com/K3-2-7-kHz-Shape-Factor-tp449609p449611.html
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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