Posted by
Corboy - Poteet on
Sep 18, 2007; 9:12pm
URL: http://elecraft.85.s1.nabble.com/K3-2-7-k-Filter-vs-2-8-k-tp453169p453174.html
I have a question. The standard IMD test is a two tone test. In a
contest type situation with multiple strong signals in close
proximity, wouldn't greater ultimate rejection by the crystal roofing
filter be a significant advantage?
Mike W5FTD
>>Do any field testers care of offer comments on the 2.7 k filter vs.
>>the 2.8
> k?
> Dick I'm not a beta tester but am familiar with
> roofing filters because Orion had essentially the same
> front-end as the K3 (main difference being its 1st IF was
> at 9.0 MHz instead of 8.2 MHz). N4LCD recently asked a
> similar question and I did not respond thinking someone
> else would. They didn't so here goes for both of you.
> The following are words by George W2VJN of Inrad
> on page 6 of his excellent article on roofing filters:
>
http://www.qth.com/inrad/roofing-filters.pdf> ***********************************************************
> 5. If 6 poles work so well, why not 8 poles?
> The most important part of the filter
> characteristic is from the pass-band on down
> to about –30 dB on either side of center. Eight poles would provide much better
> stop-band isolation, but it’s not required in a
> roofing filter and would make no
> noticeable improvement in IMD performance.
> ***********************************************************
> Indeed his statement is borne out in the IMD numbers
> Eric posted previously:
>
http://mailman.qth.net/pipermail/elecraft/2007-September/073442.html> Filter 20kHz 10kHz 5kHz 2kHz
> 2.7 kHz, 5 pole 100+ 98 92 n/a
> 2.8 kHz, 8 pole 100+ 100 93 n/a
> The major role of a roofing filter is to prevent adjacent > (i.e.
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