Topband: RE: Omni VI+ A/B Test results

[Michael Tope]

Yes, I agree that the group delay outside the low
attenuation range of the filter won't have much effect
on what you hear. But look at the group delay
characteristics of a typical chebyshev or elliptical
bandpass filter and you will see that the "ears" of
the group delay characteristic climb pretty rapidly
as you approach the corner of the filter passband.
Or in other words, on a typical filter without delay
equalization, the flat group delay bandwidth is more
narrow than the flat amplitude bandwidth. The problem
generally gets worse as you add poles or tighten the
shape factor. Thus I would expect a bandpass filter
with a really tight skirts to have more group delay
distortion "in band". I have never swept any IF filters
for ham rigs on a vector analyzer, so I don't know if
they do any delay equalization or not. My experience
comes from filters that I have simulated, measured,
and or built at work. Anyway, it would be interesting
to compare different filters to see how they look in
terms of group delay. But again, without equalization,
I would expect the tighter filters to show more in-band
delay distortion.

73 de Mike W4EF...........................................

>
> Group delay changes should be as small as possible within low attenuation
> area of the filter because that is the only area that contributes to
> significant desired signal or significant noise, while delay changes
> at -15dB or -20dB points or higher attenuation would be for all practical
> purposes meaningless.
>
> I'd think it quite possible to build a steep skirt filter with low group
> errors in the nose of the passband.
>
> 73 Tom
>
>