The problem with very narrow CW filters is that they generally have very steep skirts which means the generally they have to be designed without regard to the group delay variation throughout the passband.  Most are based on Chebychev coefficients or some modification thereof.  As such, there is a significant amount of energy stored at the passband edges which cause ringing to be heard in the demodulated signal.  Receivers based on DSP in the signal processing stages generally don't suffer from this as the filters are a linear phase type that are constant group delay which do not have the ringing issue.  Where the AGC pickoff point is located may affect the performance but I must say that I love my (by now) old iron flex 5k transceiver.  Last month when everybody was trying to work the Ducie island station I had an issue where there was some lid operator QRMing.  However, he was close in frequency but not close enough.  I just cranked the bandwidth of the flex DSP filter down to about 50 Hz and that turkey was GONE.  Then I worked him of course.  I love my 5k so much that I bought a spare since they are nor being built anymore and are cheap now.  They do require a windows computer that can get out of its own way however and they are definitely not for the computer squeamish but getting past that I think they are the bomb!  The newer ones are probably better but being a HAM which stands for "haven't any money" I am too cheap to buy one.  At least now.

73,

Jim WO7V

On Mon, Nov 26, 2018 at 7:46 AM john ni0k <jasimmons@...> wrote:

Operating the CW WW CQ contest makes me wish for something narrower than
the 400 Hz filter in my rig currently. I know Elecraft has a 200 Hz
filter but is there something else that people are using?

73,
-John NI0K