[time-nuts] Changing ADEV, (was Phase, One edge or two?)
kb8tq at n1k.org
Sat Oct 25 16:15:56 UTC 2014
> On Oct 25, 2014, at 3:39 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> In message <544AD1D1.4040608 at rubidium.dyndns.org>, Magnus Danielson writes:
>> A great way to illustrate the point of degrees of freedom and the number
>> of sample-points needed to get tight confidence intervals is to see how
>> the high-tau end of a curve updates in TimeLab and behaves as the
>> jiggeling end of a long rope, and as more samples comes in, the
>> jiggeling end moves towards higher taus, but for a particular tau, the
>> amplitude of the jiggeling decreases until it almost stops. This is the
>> effect of the confidence intervals becoming tighter, the range within
>> the real value is becomes smaller and eventually is very tight.
> ADEV snakes about at the far right end primarily becuase of the
> phase jitter which dominates at your minimum tau.
> If you have a phase measurement white noise of 1 ns = ADEV(tau=1)
> and you expect your ADEV curve to bottom out around 1e-12 at some
> larger tau, then you have a factor thousand of noise to average
> out, before a valid ADEV comes out of the noise.
> This is not any different from any other "average out the noise"
> situation in any significant way, sqrt(N) rules, and there is
> nothing you can do about.
In the case of the TimePod, the data can be presented when you have *very* few samples to work with. That said, it is interesting to watch it bring up error bars (which are indeed correctly calculated) and then see the trace walk outside those error bars as the run progresses. There are other measurements that are a bit less susceptible to this. None of them have any magic to get around sqrt(N).
> *except* to decrease your phase measurement white noise, which is
> why tuning your 5370 for peak performance is worth days of measurements
> at the other end of the ADEV.
> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG | TCP/IP since RFC 956
> FreeBSD committer | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
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