[time-nuts] Moving GPSDO

Attila Kinali attila at kinali.ch
Mon Oct 17 10:44:55 EDT 2016

On Mon, 17 Oct 2016 07:25:34 -0600
"Cube Central" <cubecentral at gmail.com> wrote:

> I had run across this just yesterday:  https://www.youtube.com/watch?v=zILwgQhjC_Q
> He talks about how crystal oscillators are sensitive to movement and changes in gravity.  An interesting watch!

As usual with EEVblog, there are several things wrong with what he presents.

1) As Bob wrote, the acceleration sensitivity of crystals vary a lot more
than the rough number Dave gave.

2) The acceleration sensitivity has very little to do with the cut.
Even though there is some cut related effect on acceleration, it is
very very small. The by far most dominant effect comes from the
mechanical stress of the holder onto the crystal. It is probably
obvious that the crystal and the holders bend upon acceleration.
This bending will cause the crystal to change its frequency due
to stress on the lattice. Now, why are AT cuts worse than SC cuts?
The reason is simple, SC cuts are usually ment for high stability
applications and people tend to take more care with the mounting
and the crystal shape, than for regular AT cuts.

3) Rb vapor standards have very low acceleration effects. As Dave
said, the control loop will kill most of the effects inside the
loop frequency. The loop frequency is relatively fast, in the
order of couple of 100Hz to a couple of 10kHz, depending on the
exact build of the Rb standard and what type of crystal they used.
Contrary to what he says, there are no acceleration dependent
diffusion effects. The vapor cell is is heated and the gas atoms
in there are pretty fast, any acceleration of them will be negligible
compared to what they do upon collision. The relativistic red shift due
to acceleration is way too low to measure (much lower than the intrinsic 
noise). The only mechanical effect that is possible, is bending of
the microwave cavity or the parts inside. But given that the cavity
is several mm thick aluminium and everything is mounted rigidly,
I very much doubt that there is any significant effect.

4) If you think that the tilting your frequency counter will have
a significant effect, you are severly underestimating temperature
and aging effects. Just in comparison, a good SC cut DOCXO has an
aging of a few parts in 10^-9. So each week will affect on your crystal
more than a 1g of acceleration. And that's with a slow aging DOCXO.
Temperature effects are in the order of 1x10^-10/°C to 1x10^-7/°C.
So depending on how good the OCXO is, a single degree of temperature
will cause a larger shift than 1g of acceleration. Etc.pp.

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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