Thanks for reading this,
I'm doing some study on audio bandpass filters before building a
different variant of the PA0LQ stagger tuned filter. Texts suggest that
the type of multiple feed back (MFB) op amp filter should not use a Q of
greater than about 5, for performance stability, but the PA0LQ design
has a Q of just over 30 per stage. I'm going to try a much lower Q and
pass band width of 150 Hz or so, and also 800 Hz centre frequency. My
thinking is that several sidebands are needed to give reasonable rates
of rise and fall times of the keyed signal, and a square wave has odd
harmonics. I have also read the SM7CMY thesis and there is mention of a
"critical bandwidth" that seems to me to be typically around 150 Hz.
The human brain is part of the overall aural filter. It seems that
selected stagger tuning is the "answer" to minimising ringing in the
(electronic) filter. I guess I will find out in due course if a 150 Hz
stagger tuned version "sounds better".
For anyone considering active filter variants, there is design
information on page 16.28 of the 1999 ARRL Handbook, for a single
stage. For the desired application, the mulit stage stagger tuning
still needs to be somehow worked out!
But the intended thrust of this email is to ask about if anyone can find
a reference to a BESSEL BANDPASS filter. Text books all say that the
Bessel low pass filter has the most linear phase response (best group
delay) of the basic range of filters (Butterworth, Chebyshev, elliptic,
Bessel). But I can not find any reference to a BANDPASS variant of the
Bessel filter. I have a feeling that it is not realisable
mathematically, and that is why it is obvious by its absence as a text
book band pass filter, but if there is a filter theory guru on the
reflector I would dearly like to hear a response.
73, Bob ZL2CA
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