[Elecraft] K3 2.7 k Filter vs. 2.8 k

[Corboy-Poteet]

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.