On Sat, 2007-03-03 at 14:51 -0800, Roger wrote:
> I did a lot of experimenting with the crystal filters
> 15 to 20 years ago on the Argosy. I have all the
> paperwork on my experiments somewhere but Can not find
> them now. I believe that I ordered my crystals from
> Jan's or one of the other crystal company's. I did
> find that I could modify the 4 pole up to the 6 or 8
> pole and could change them to the bandwidth that I
> wanted, I kept many pages of info plotted but it is
> among piles of records from the last 30 years.
> KO4O
There have been articles in QST, ARRL Handbooks, Ham Radio magazine, and
MFJ has a book on the design of crystal ladder filters. A classic Filter
Design Handbook by Zverev also allows designing to many different
amplitude response curves, all with math.
The ham magazine articles tend to be centered on crystals of matched
center frequencies. Typically for a wide filter there has to be series
capacitors on the crystals to set their frequency and there are shunt
capacitors to set the coupling between the crystals. One starts with a
gaggle of crystals, measures the series resonant frequency of each, the
equivalent series resistance, and the 3 dB bandwidth (or Q). Then one
can apply these values to create a crystal filter with the desire
characteristics. Often 9 MHz crystals can be bought (Mouser, Digi-Key,
Dan's Small Parts, Jameco, and major distributors) for less than 50
cents each making selection relatively easy limited by the test
equipment.
Its an interesting exercise to compare the transient response of a
Tchebychev filter curve to that of a Butterworth and then a Bessel.
Steep skirts are the antithesis of good transient response and freedom
from ringing from static.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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