[ARC5] Interesting crystal information

[Dennis Monticelli]

Bruce,

I don't know about the other guys, but I'm eating this stuff up.  Thank you
for sharing.

Dennis AE6C

On Fri, Mar 15, 2013 at 9:59 AM, Bruce Long <coolbrucelong at yahoo.com> wrote:

>
>
>
> 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
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