[time-nuts] Gravity and OCXOs

[Murray Greenman]

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.

Murray