[Elecraft] K3 - 2.7 kHz Shape Factor

[Bill Tippett]

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