Message-ID: <3938ED38.36F4@xtra.co.nz>
Date: Sat, 03 Jun 2000 23:34:16 +1200
From: "vernall" <vernall@xtra.co.nz>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Bessel bandpass filter?
References: <3b.5b0ebd6.2669f0ef@aol.com>
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WarmSpgs@aol.com wrote:
> 
> Hi Bob,
> 
> Bessel filters do have wonderful group delay characteristics, and a bandpass
> version is entirely mathematically realisable...but not especially useful.

I was not planning to use one.  My query was about apparent lack of a
band pass version being mentioned in text books.

> 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.

Yes.  It seems the stagger tuned Butterworth is the design to go for.
 
> 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

What I am not sure of is how to stagger the stages of bandpass active
filters.  However, they can easily be tweaked, and applying "input
beeping" and using a scope to minimise ringing in the output seems to be
a good experimental way of adjusting it for the desired characteristic.

Thanks for the comments.

Bob ZL2CA