I've been using a homebrew 8 pole active audio filter for over 20 years.
It's bandwidth is 90 Hz. At the time I designed it, I decided on that
bandwidth because of the available tolerances of the resistors and
capacitors. Regarding ringing, it is a "non-problem" here. The type of
filter that I designed is noted for minimum ringing. By "type", I mean
Butterworth, Chebyshev, etc. - sorry that I don't remember which one it is.
My memory is really hazy on this, but if I remember correctly, the ringing
is due to the pulse distortion caused by non-linear phase delay. In any
case, the point is that it is possible to design a narrowband filter that
has negligible ringing.
I also have an outboard Timewave DSP-59+. When I use it by itself as a
bandpass filter, the audio sounds horrible - very tinny, etc. I find that
when I put my homebrew 90 Hz filter after the DSP, I'm able to clean up the
audio so that it sounds normal. When I need maximum sensitivity, I set the
DSP between 25 to 75 Hz and of course have it cascaded with the homebrew
analog filter that cleans up the audio.
The radio here is a Yaesu FT-1000D. I find the tuning rate much too quick
for such narrowband operation, so I've added a commercially available kit
that gives a 10:1 reduction in tuning rate.
Regarding not hearing stations calling off frequency, this can be a problem
when using such narrowband reception, and especially in contests, it is
necessary to always "wiggle" the dial back and forth after each QSO to pick
up stations that may be calling off frequency.
In summary, after getting the narrowest IF filters for your radio, having a
good narrowband AF filter can often be the determining factor in copying the
weakest signals. My comments are entirely based on experience. The theory
behind this is well known and has already been discussed in this string.
73,
Riki, 4X4NJ
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