I wish to thank Jim M0BMU for his good summary comments on filter
design, including Bessel types.
I do not have either of the two reference books Jim named and I will
request the public library to seek them on interloan.
While I happen to have quite a good selection of salvaged high Q audio
inductors, as for the PA0LQ filter, any confirmed good design is of
wider applicability to others if it is an active filter type (and using
good op amps). While a coupled passive filter may give good results, it
is closer to a special one-off result if the inductor types are hard to
acquire. Also if it needs a high impedance load, then it could need an
op amp anyway, so it would not be an entirely passive circuit as an
accessory. I'm going to include an audio PA chip so the accessory
filter can have a volume control and feed a loudspeaker or low impedance
headphones or a PC sound card.
Suggestions of others resulted in some PC CAD software being downloaded,
and this is mostly of the "demo" or "knobbled" variety as it is "free".
I have found it to be very interesting where group delay curves are
available. However, as at present I have yet to find out what results
from selected stagger tuning (of isolated resonators, using active
filters).
73, Bob ZL2CA
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