[time-nuts] question Alan deviation measured with Timelab and counters
magnus at rubidium.dyndns.org
Sun Jan 18 10:00:12 EST 2015
On 01/18/2015 02:44 PM, Bob Camp wrote:
> Ok, I didn’t think I’d seen the plots before.
> I agree that the plots look like “counter limited” data. That’s a fine explanation at the shorter Tau’s. I also agree that some sort of periodic “stuff” is getting into one of the signals and creating the ripple.
Yes, that much is expected.
> What I’m wondering about (and what makes me question the setup) is the fact that the data is still “counter limited” at the mid to low parts in 10^-13 level at just a bit over 100 seconds. A telecom Rb is doing pretty well to be at 1x10^-12 at 100 seconds. Most GPSDO’s are doing well to be mid parts in 10^-12 at that tau. Simply put, the data continues to be counter limited to a pretty low point.
In a setup where you feed both start and stop the same signal, the
measurement noise of the counter setup will behave like that. Since the
time-difference between the start and stop is very small, there is
almost no lower-frequency noise included into the measurement, so the
scaled variant of the noise will just keep going downwards.
If you actuallly measure two sources, the initial slope will be that of
the counter noise, but then pan out into the sum of the two sources.
This is why I was considering that it was wired up as a instrument noise
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