[time-nuts] Gravity and OCXOs

Magnus Danielson magnus at rubidium.dyndns.org
Fri Aug 14 08:02:58 UTC 2009


Murray Greenman wrote:
> Rick is right.
> 
> The effect you see when turning your GPSDO upside down will be
> predominantly the direct gravitational effect on the OCXO crystal and
> its mountings. You see this with any OCXO, and the good ones will have a
> '2G turnover' or other G rating quoted.
> 
> For example, the well known HP 10811A specification says 'Gravitational
> Field: <4 x 10^-9 for 2g static shift (turnover). That's fairly typical.
> 
> I just measured the effect on a good C-MAC 10MHz OCXO (similar to those
> used in many recent GPSDOs) and measured 40mHz p-p, or exactly 4e-9.
> 
> It's complicated in the GPSDO because the correction mechanism (via GPS
> 1pps) is much slower than the rate at which you can change the
> gravitational effect. Rb references simply have a faster loop and will
> correct the G effect more quickly.

If you care about that speed, you use GPSes which has higher speeds and 
you don't use a PPS since as you pointed out is unfit for it. Also, if 
you care about it, you'd use sensors to sense the gravity and use that 
for the bulk adjustment, if a low sensitivity crystal isn't enought or 
available in matching specs.

The PPS is just an arbitrary limit.

Cheers,
Magnus



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