I am trying to work out how much the thing drifts per year measured in time. Datasheet says it has long term stability of 2 x 10^-11. But 2 x 10^-11 of what? 1 hz? 10mHz or whatever the physics pacakage is running at? Or something else?
Thanks.M
If 10MHz (10 Mega Hz) have 2x10Exp-11 error, say example it is this amount too fast.
Its frequency is 10 000 000.000 2 Hz. (10.0000000002 MHz)
In rough this stability means error. But "long term stability" - and I ask "how long"
How about short term..
Then, in what situation. In stabilized outside temperature, pressure, magnetic field etc.
If compare example cheap Efratom LPRO-101. (DATUM LPRO-101) these are factory adjusted inside +-5x10exp-11 error, same accuracy what example I have done as I have sometimes calibrated these to someone.) It means that frequency is (with given enviroment variables) between 9.9999999995 - 10.0000000005 MHz.
Of course if same error in one Hz. It means 0.99999999995 - 1.00000000005Hz.
Now it can ask how many seconds need that it have (accumulated) one second error.
But in practice year level drift is more than +-2x10exp-11.
I have checked tens of LPRO-101 and some these FEI models also after they have take out from active original use.
After different time of use and diffrent sometimes also very hard enviroments used. Then I have checked (before my adjustments) they have all been typically well inside +-10x10exp-10 error window. (10MHz under 10mHz error) Mostly drifted to up some mHz. (why?) No one can exactly tell how long they have used and how long after any calibration so this do not give much of information.
Also most of Rb need be 48-72 hour or even more on before check. (but well.. <30 minutes they are typically very near real working freq)
C-field setting may also drift. (It is sensitive to magnetic field strenght and direction and it is also method for adjust it) Even earth magnetic field affect it.
Rb physic package have many mechanism for drift. It is NOT primary standard as example Cesium or Hydrogen maser.