----- Original Message -----
From: "Arthur Delibert" <[email protected]>
To: <[email protected]>
Sent: Sunday, September 18, 2011 5:01 AM
Subject: [Drakelist] So if crystals change frequency ...
...what about crystal filters? We're all using crystal
filters in our rigs that are as old as the crystals that
we're talking about replacing. Do the crystals in the
filters also drift over time, shifting the passband (or
distorting it, if the crystals in it don't all drift at
the same rate)? Is the drift just too small to worry
about in the case of a filter?
Thanks for any insights.
Art Delibert
KB3FJO
I was able to find out quite a bit about crystals by
doing Google searches for both quartz crystals and crystal
filters. I did not find a comprehensive explanation of why
crystals change frequency but it does seem that its a matter
of the crystal lattice structure changing due to:
impurities, mechanical shock, dirt, perhaps oxidation, and
other things. Modern crystals are adjusted by frequency by
chemical etching rather than mechanical grinding which
yields a more uniform surface which is less prone to changes
in the structure of the surface. They are also fine-adjusted
by control of the plating which forms the electrodes. For
many years the most stable crystals used evacuated
containers. Even with all these changes crystals still
change with age. Filters are quite critical of the exact
characteristics of the crystals that make them up. Slight
changes in either series or parallel resonance or the
spacing of the two affect all the filter properties;
bandwidth, shape and flatness of the pass band, spurious
responses outside of the passband. The more complex the
filter (the more poles) the more sensitive it is to changes
in value. According to one article, both ceramic and
mechanical filters are more stable than quartz crystal
filters but neither type of resonator has the extremely high
Q of quartz. In fact, any of these filters could be made
using L's and C's if one could obtain a sufficiently high Q.
It is the enormous Q available from quartz and other
essentially mechanical resonators which makes these filters
possible. But, its also the high Q that makes these devices
so sensitive to small changes in values.
Probably the best sourc of theoretical information on
quartz crystals remains W.G.Cady's old book but its highly
technical and mathematical. There is a ton of literature on
complex filter design, mostly also highly mathematical.
While the Bell System used astonishingly complex filters for
carrier telephone service in the 1950's calculating them
without modern computers must have been extremely difficult
and manufacturing them a tour-de-force.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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