[ARC5] Interesting crystal information

[Bruce Long]

It turns out if you take a 10 MHZ quartz crystal and remove
one atomic lattice layer you will move the frequency something like 50
ppm.  Twenty years ago I had designed in
production for 10 MHz SC-cut ovenized oscillators that where meeting 20 ppb
maximum first year aging specs.
So we are talking absolute manufacturing cleanliness and consistency
at the atomic level.
Another interesting crystal fact is if you take a fairly
ordinary 3rd overtone 50 MHz crystal operating in a crystal
oscillator the surface acceleration is well over 1 million Gs so contaminates
are physically thrown off the surface of a “dirty” crystal only to return
during periods of in-activity.

Also at this level of aging performance crystal mechanical
stress is a big factor.  If you place the
crystal blank under mechanical stress as you might when you insert the crystal
blank into the mechanical crystal holder- ie the structure that holds the
crystal inside the evacuated enclosure  that stress will slowly relax over weeks months and years creating
crystal aging at the parts per billion area.

Imaging an AT crystal blank inside an evacuated holder at rest.  Even though quartz is stronger than the best steel in tension, the quartz blank will be sagging slightly due to gravity thereby inducing mechanical stress.  If you turn the oscillator upside down the apparent direction of the gravitational force will reverse and the blank will sag in the other direction.   For AT cut crystals this results in about 20 ppb frequency shift.  SC- stress compensated cuts are about 1 order of magnitude better


IIRC the government sold off several hundred tons of
electronics grade natural quartz crystals from the national strategic stockpile
in the mid eighties  I was at Piezo at
the time and after much thought we declined to bid.  The best natural quartz is still used in the
most demanding applications but for the most part cultured quartz grown in a
autoclave is good enough and its regularity and uniformity gives it a very
large production advantage of natural quartz.

Hope i have not bored anyone or move the topic too far off its original directions.  But I'm an ex crystal guy   What can I say.   Bruce