[Elecraft] 8 or5 Pole cw filters

[Jim Brown]

On Tue,1/13/2015 7:52 PM, Mike Zbrozek wrote:
> And how many cw ops use a sharper 200 or 250 Hz filter? 

I'm primarily a contester, but also a DXer. My K3s have all 8-pole 
filters. 250 Hz and 400 Hz for CW, 1.8, 2.1, and 2.8 kHz for SSB. I 
bought the 1.8 kHz before the 2.1, and I like the 2.1 better.

Remember that these are ROOFING filters -- IF selectivity is set by 
turning a knob to control the DSP IF. The roofing filters do two things. 
Most important, they protect the DSP IF from overload by very strong 
signals very close to you frequency. Second, and also quite useful, 
their selectivity CASCADES with selectivity of the DSP IF. That means 
that when, for example, the IF is set to 250 Hz and the 250 Hz filter is 
switched in (it happens automatically) the skirts get nearly twice as 
steep. We have the rejection of the IF plus the rejection of the filter.

We can set the filter to switch in wherever we want, simply by defining 
it's bandwidth in the setup menu. I have my 250 Hz filter set to switch 
in at 350 Hz, and I do most CW operation with a 250 Hz IF setting. These 
are settings you can play with.

I use a 400 Hz IF for RTTY, except under conditions of extreme QRM, when 
I might narrow it down to 200 or 250 Hz.

Another important point. If you're not a contester, you may not need any 
roofing filters other than the 2.7 or 2.8 kHz TX filter. If that's the 
case, I'd hold off buying additional filters until you've decided that 
you need them.

Note that I didn't answer the question about 5-pole or 8-pole. I buy 
8-pole filters because it is my understanding that filters with more 
poles can be made to have flatter response in the passband. I started 
with 2.7 kHz 5-pole, and later bought a used K3 with an 8-pole 2.8 kHz 
filter in it. It is noticeably flatter when transmitting RTTY, and i've 
since replaced by 5-pole 2.7 kHz filter with the 2.8 kHz 8-pole filter.

73, Jim K9YC