[time-nuts] OCXO shock protection

Tom Van Baak tvb at LeapSecond.com
Mon May 20 10:35:18 EDT 2013


> We will be moving to southern California in the near future.
> 
> My question if for people who live in that area is do you need to do
> additional shock mounting for OCXO’s because of the ongoing,
> usually minor, earth tremors that take place?

Hi Perry,

Your OCXO will be fine, unless it falls on the floor.

"Shock" mounting won't help because earthquakes are more slow rocking, rolling, or shaking than shock. The bandwidth of the acceleration is quite low, around 1 Hz. Your OCXO, and the shelf holding it, not to mention your house and neighborhood will all enjoy the same ride. I suppose a helium balloon suspended OCXO would be immune.

Here's a back-of-the-envelope calculation:

The typical OCXO has an acceleration sensitivity on the order of 1e-9 per g. You can easily measure your OCXO by placing it on each of six sides, computing the frequency difference.

If the quake is strong, the temporal acceleration is on the order of 0.1 g. See:
http://en.wikipedia.org/wiki/Peak_ground_acceleration

This means the short-term frequency *modulation* is down at the 1e-10 level, or below (if you orient the OCXO along a less sensitive axis). Most OCXO already have this much short-term noise, so you'd never notice the quake. Even with a low noise OCXO, it takes a very high-resolution phase meter to detect rapid frequency changes at the 1e-11 level. Note there is little or no effect on timing, since the sum of the +/- acceleration over the duration of the quake is essentially zero. The same goes for net change in frequency, unless the OCXO is gets moved or tilted as a result of the quake.

Pendulum clocks are much more fun to measure during a quake:
http://www.bmumford.com/mset/tech/quake/

My home lab survived a 6.8 quake. If you do sub-cm GPS you may want to double check your coordinates after a strong quake, as your house (and neighborhood) may have moved:
http://leapsecond.com/pages/quake/

/tvb



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