----- Original Message -----
From: "Neil M Califano" <cchange...@yahoo.com>
To: <Drakelist@zerobeat.net>
Sent: Friday, September 16, 2011 5:05 PM
Subject: [Drakelist] Changing crystal frequency
Someone I know used to write on crystals with a pencil
claiming the frequency could be changed. Is this true or
an urban legend, because graphite is crystalline?
Its true but not because pencil is graphite. A crystal
is a mechanical oscillator! The frequency is determined by
the mass and thickeness of the crystal along with the "cut".
Crystals are cut from large crystals which are grown. the
angle at which the crystal is sliced in relation to its axis
determines the mode of oscillation and many other properties
including temperature coefficient. The nature of piezeo
electric materials is that they generate a voltage when
deformed and will deform if subjected to a voltage
difference across the appropriate face of the crystal.
Anything on the surface will influence the resonant
frequency including the any dirt. Old time crystals were
simply blocks of crystal cut to the right angle and placed
loosely between two electrodes. They could be varied in
frequency a little by the pressure of the electrodes and
some crystal holders were made with an adjustment to get a
short range of frequencies. Crystals can also be varied by
abrasive treatment or adding something to the surface to
increase the mass. A pencil mark does this. In the days when
crystals were quite expensive there were a lot of articals
in ham magazines about how to make your own. After WW-2,
when lots of crystals became available surplus, there were
numerous articles on how to convert military crystals to ham
frequencies and kits with the necessary abrasives were sold.
Modern crystals are usually grown and are made with
cuts not known sixty years ago. For instance, its possible
to make a crystal with zero temperature coefficient over a
very wide range.
I have not done a web search for piezio electric
crystals but I am sure there must be a decent tutorial on
them.
There was some controversey about just who originated
the use of crystals for control of oscillators but the two
most likely were A. M. Nicholson and W.G.Cady, the latter of
Bell Labs. I think Cady got the patent but Nicholson was
probably actually first. Cady's application give much more
detail on how to make an actual oscillator, that is probably
why he was recognized as the inventor.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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