Posted by
Bill W4ZV on
Sep 18, 2007; 7:28pm
URL: http://elecraft.85.s1.nabble.com/K3-2-7-k-Filter-vs-2-8-k-tp453169.html
>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 its 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.htmlFilter 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. unwanted) ~S9+30 signals from entering the IF chain.
This means the shape factor of the filter is relatively
unimportant beyond about 30 dB down on the filter skirts,
so there is little advantage of an 8-pole over a 5-pole
filter as far as the receiver is concerned. If your filter
eliminates unwanted S9+30 signals from propagating to the
DSP stage, then the DSP can provide the actual final bandwidth
selectivity (without unwanted IMD products). As you can see
in the above measurements, there is essentially no difference
in IMD performance (2 dB being well within measurement
uncertainty). So, for receiver performance only, I would
say there is NO difference in the filters other than the
extra cost of the 8-pole.
One potential difference has to do with the K3 transmitter.
Since it transmits SSB through these same filters, the 8-pole
might be preferable since it would more effectively attenuate
unwanted products, but I'm sure the 5-pole will meet published
specs since it is the standard K3 filter. In other words,
if you got an 8-pole, it would make little noticeable
difference in the receiver, but it would make your transmitted
signal a little cleaner.
I hope this helps you. I'm ordering the 5-pole because
I don't see the advantages of the 8-pole are worth the cost.
73, Bill
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