[time-nuts] question Alan deviation measured with Timelab and counters

[Magnus Danielson]

John,

On 01/20/2015 01:55 AM, John Miles wrote:
>> John,
>>
>> Your new three-corner hat feature is really cool. Already tried it, even
>> if I did not spend quality time on setting source labels correctly.
>>
>> Whenever I have a setup capable of running it, I will test-spin it again.
>>
>> I suspect that when you make separate measurements, the noise of the
>> individual sources does not match up perfectly which may reduce quality,
>> especially systematics like hum.
>
> True, and Bill Riley's documents emphasize that simultaneous measurements are best.  That's easy to do with the SMA jacks on the TimePod but more of a challenge with counters, unless you buy several of them and run them from separate GPIB adapters or serial dongles.
>
> IMO, if you can keep correlation, tonal artifacts, and other systematic errors under control and make long-enough measurements to generate a lot of data points in the tau range of interest, then the technique should work well with non-simultaneous acquisitions.  At the end of the day it's no better or worse than the quality of the individual ADEV measurements.  You have to be wary of measurements at taus where any of the traces have large error bars or are wandering up and down over time, or where some of the traces are suspiciously close to each other.

Agree.

For oscillators, they should have been turned on long enough such that 
any drift is negligible. Alternatively you process out the quadratic 
trend out of it. The later should be accompanied by some quality measure 
of how much remaining systematics there is (see Jim Barnes PTTI paper on 
Drift Estimators).

Also, identifying systematic noises (such as in phase-noise plot) and 
notching them out before processing should improve the quality of the 
3/N-cornered hat processing, since it really is for solving power-sums 
of random noise, but not for systematic noise.

Until such processing of each measurement is in place, we have to make 
as clean measurement runs as possible, which is the basic quality of the 
ADEV.

Cheers,
Magnus