[time-nuts] Grinding crystals...
Magnus Danielson
magnus at rubidium.dyndns.org
Fri Jun 21 10:04:08 UTC 2013
On 06/21/2013 06:11 AM, Jim Lux wrote:
> On 6/20/13 4:57 PM, Gary wrote:
>> A common scheme in metal deposition measurement is to measure the
>> frequency of a crystal prior to starting the deposition process, then
>> monitoring the frequency shift of the crystal as the metal is sputtered.
>>
>> I was told crystals are tuned this way at the factory, but don't know
>> this for a fact.
>
>
> it's also how particle counters based on "piezobalance" work. The
> particles hit the crystal and stick, lowering its frequency.
Which is how one mechanism for frequency drift works, and hence is
avoided. Whatever junk is on the crystal, it get's thrown off by the
heating and operation, and that balance take a long time to shift, until
you turn it off and it deposit back on. Then as you turn it on you
experience the same thing again. By cleaning the crystals and even bake
them out, this can be significantly reduced. BVAs for instance is baked
out with a turbopump operating, and at the end the "stove pipe" is
pinched of as in normal vacuum processes.
Cheers,
Magnus
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