Posted by
Bill W4ZV on
Jun 27, 2007; 11:59am
URL: http://elecraft.85.s1.nabble.com/K3-2-7-kHz-Shape-Factor-tp449609.html
K3RIZ:
>Can anyone give me the shape factor for the
Elecraft supplied, 5 pole, 2.7 kHz filter?
I was hoping Wayne might respond since we
are now within 1 month of shipments and we'll soon
need to decide whether to go with the 5-pole 2.7k or
the 8-pole 2.8k. In addition to plots of the 5-pole
filters (like already exist for the 8-poles), I'm
waiting to see some IMD/BDR results for all filters.
Ten-Tec, Icom and Yaesu have had some disappointing
IMD/BDR results published for Orion, the IC-7800 and
the FT-2000 *after* introduction, so I hope Elecraft
does not repeat the same mistake with the K3.
My guess about the 5-pole is that it will have
a 6/60 dB shape factor a little better than 5:1. This
is about what the Ten-Tec 4-pole filters are and a
5-pole should be a little better...perhaps ~4:1.
Let's also remember that the shape factor of a
roofing filter is not so critical as if it were being
used to determine ultimate selectivity. 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"). From the
FAQ of Inrad's paper on roofing filters by W2VJN:
"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."
http://www.qth.com/inrad/roofing-filters.pdf 73, Bill W4ZV
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