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And........very narrow filters, due to group delay, will take a
noise pulse or spike and it will go SPLAT across the passband.
Hence the filter makes the spike much wider than the original
width of the spike. For the most part, random or broadband
noise is unaffected. Just opening up the passband to the point
where the narrow filter switches out can make noticeable
improvement in receiving conditions. I prefer to use a narrow
DSP passband and manually select a wider roofing filter. My K3S
makes for a super sweet receiver in this regard.
On 11/26/2018 6:23 PM, Jim English
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.
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
filter but is there something else that people are using?