Hi Bob,
Bessel filters do have wonderful group delay characteristics, and a bandpass
version is entirely mathematically realisable...but not especially useful.
The skirts of such a filter are so gentle that there is little point in using
it for communications audio filtering unless you used many, many stages. If
you were to do that, however, the inevitable minor defects in the physical
realisation would likely result in phase characteristics little better than a
simpler Butterworth design of comparable steepness.
It is not an absolute requirement to have linear phase over the entire band
of interest; only enough of it to allow the shape of the more-or-less square
wave envelope to be reproduced without excessive overshoot. (This could be
as little as 50 Hz of the overall bandwidth for normal CW.) We have been
using brick-wall highpass and lowpass filters in broadcast audio paths for
some time now, exhibiting virtually no ringing or overshoot. This is done
partly by incorporating active phase shift networks to compensate for the
normal huge group delay excursions in the elliptical designs needed to
achieve the required rolloff characteristics. Alas, design formulae for some
of the fancier implementations are hard to come by.
73,
John KD4IDY
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