[Elecraft] filters

[K9MA]

A quick look at the Elecraft filter response plots for the KFL3A-250 and 
KFL3A-400 shows these bandwidths:

dB      -250     -400

-6       400      420
-20      480      530
-40      600      700
-60      780      920


So, the narrow filter won't make much difference close in, but may 
provide more rejection more than 200 Hz from the center frequency.  
(Assuming it hasn't drifted too far.)  Another way to look at it is that 
the narrow filter has about 10 dB more rejection beyond +/-200 Hz.  
This, of course, adds to the DSP rejection, and may prevent overloading 
of the DSP by a strong adjacent signal.  How often this makes any 
difference is hard to say.

I can say that I can't recall ever switching to the narrow filter to 
reduce QRM from a signal more than about 400 Hz away.  I frequently do, 
however, for those less than 200 Hz away, in which case the extra 
rejection must come almost entirely from the DSP.

Scott K9MA



On 1/1/2018 13:35, john at kk9a.com wrote:
> So I guess you're saying that there is not much difference, only 55Hz, so
> maybe it is not worth the cost of having both. My K3S's are programmed to
> switch these roofing filters in closer to their actual passband. The cost of
> the 250Hz filter is a small percentage of the transceiver cost so I do not
> mind having this option.  I see it being far more useful than the 1.0Khz
> filter.  WD8Z said that he ordered the 400Hz by mistake and my initial
> comment was that it is a fantastic CW/RTTY filter. I have two in each of my
> K3S's.
>
> John KK9A
>
>
> -----Original Message-----
> From:ed at w0yk.com  [mailto:ed at w0yk.com]
> Sent: Sunday, December 31, 2017 10:58 PM
> To:john at kk9a.com
> Cc: 'Elecraft Reflector'
> Subject: RE: [Elecraft] filters
>
> The actual passband of these two filters is more important than the
> marketing name.  The "400 Hz" filter is about 435 Hz wide at the -6dB
> points.  The "250 Hz" filter is about 380 Hz wide.  Look at the graphs on
> the Elecraft website.
>
> Ed W0YK


-- 
Scott  K9MA

k9ma at sdellington.us