[time-nuts] timelab
Magnus Danielson
magnus at rubidium.dyndns.org
Thu Jul 5 10:41:20 UTC 2012
On 07/05/2012 07:14 AM, Don Latham wrote:
> So if the 5370 determines the data rate, then in calculating adev or
> other such products, we're assuming the time differences are ergodic
> because we're not getting them sequentially, but rather selecting the
> delay intervals at long inetrvals between them?
It's trying to determine the tau0. It's actually determining the T
factor, the time between measurement results. The difference T-tau0 will
be your deadtime and it will bias your measurements. This bias function
was first shown by Dr. Allan in his Feb 1966 paper. Since we have
modernized the analysis somewhat. Please see
http://en.wikipedia.org/wiki/Allan_variance#Bias_functions
and in particular the B2 bias function.
There is two ways to approach it for propper values:
1) Set up the measurement for no (apparent) deadtime. I use a PPS (or
other rate) trigger out of something like the TADD-2 either for start or
for measurement arming.
2) Compensate for it using the bias function.
If you ever "skip over" measurements, toss the measurement result or at
least stop. The series ends when a "skip" occurs and a new series
begins. You can combine the results, but it is tricky, error prone and
you loose data on long taus which is what is hard to get much of anyway.
Hence, just averaging the sample arrival time without critically analyze
the pauses can lead you into incorrect values as things will be biased
in all kinds of ways. Inferring the time-base on arrival time should be
avoided whenever possible.
Cheers,
Magnus
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