8-pole vs. 5-pole roofing filters?

[William Hein]

What's the performance difference between the 8-pole and 5-pole K3  
roofing filters?

For crowded-band CW performance am I better off with a 5-pole 200-Hz  
or a 8-pole 250-Hz filter?

Also, do I need separate roofing filters for the main and sub-RX?

73,
Bill NT1Y
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[Darwin, Keith]

I had 2 CW filters in my TenTec OmniV and did a lot of A/B comparing
with them.  One was an 8 pole Inrad 400 Hz filter and the other was a 6
pole TenTec 500 Hz filter.  I had them both in the rig at the same time
specifically to decide which one to keep.  I was able to detect the
steeper skirts of the 8 pole filter as well as the narrower passband,
but the factor that eventually tipped the scales was ultimate rejection.
The TT filter, had better ultimate rejection.  Once I tuned past a
signal, it went away and didn't come back.  With the Inrad filter, the
signal would drop quicker, hit bottom and then come back up.  I could
hear strong signals 6-8 KHz away.

So, I went with the TT filter because for me, ultimate rejection was
more important than the slightly narrower passband.

Now, the question is whether or not the 5 vs 8 pole filters for the K3
have different levels of ultimate rejection.  If they do, then the
choice has become tougher.

- Keith N1AS -
- K2 5411.ssb.100 -

-----Original Message-----
From: William Hein, NT1Y

What's the performance difference between the 8-pole and 5-pole K3
roofing filters?

For crowded-band CW performance am I better off with a 5-pole 200-Hz or
a 8-pole 250-Hz filter?
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[Vic K2VCO]

Darwin, Keith wrote:

> Now, the question is whether or not the 5 vs 8 pole filters for the K3
> have different levels of ultimate rejection.  If they do, then the
> choice has become tougher.

My guess is that the better ultimate rejection that you experienced when
you compared Tentec and INRAD filters had more to do with the details of
the installation of the filters rather than the quality of the filters
themselves. The "blow-by" you experienced with the INRAD filter was
probably taking place AROUND the filter as a result of stray
capacitance, shielding, and grounding issues.

These factors will probably be more or less the same with different
Elecraft filters. Probably the ultimate rejection of all of the filters
will be limited by the outside factors -- the filters will be 'better'
than the wiring.  And in the K3 the DSP will improve the ultimate
rejection of any filter, since we assume that residual signals will be
too small to actuate the AGC or cause IM products.

So I do not think that ultimate rejection will be a consideration for
choosing the K3 filters.

Having said this, I'm still debating whether I should get the 5 or 8
pole CW filters!

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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