[ntp:questions] Tighter regulation?
Mischanko, Edward T
Edward.Mischanko at arcelormittal.com
Sun May 26 04:44:42 UTC 2013
> -----Original Message-----
> From: questions-bounces+edward.mischanko=arcelormittal.com at lists.ntp.org
> [mailto:questions-
> bounces+edward.mischanko=arcelormittal.com at lists.ntp.org] On Behalf Of
> David Woolley
> Sent: Saturday, May 25, 2013 4:02 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Tighter regulation?
>
> unruh wrote:
>
> >
> > Note that on maxpoll 10, the clock will freerun for about 7000 sec
> > between disciplines. 5ms in 7000 sec is about 1PPM. Now if your computer
>
> The time constant for the proportional correction is 16384 seconds, so
> only about 35% of the correction would be made over 7000 seconds. In
> terms of correcting a random step phase change some time in that
> interval, one is only talking about losing an average about 17% of the
> total correction required as a result of the lower effective sampling
> rate.
>
> I think the time for the integral component to remove a frequency step
> is even longer (I think it actually rings), except that the rapidly
> growing offset will cause the poll interval and time constant to drop.
> I suspect, if the poll interval remained constant, the reciprocal of the
> loop frequency would be so much longer than 7000 seconds that there
> would be little difference between 1024 and 7000 second effective
> sampling rates.
>
[Mischanko, Edward T]
Yes, I agree!
> As I said above, the real issue is that of correctly identifying a step
> in the frequency, or its first derivative, and rapidly turning down the
> time constants.
>
[Mischanko, Edward T]
I tried reducing the Allan Intercept to 7 and the result was wild swings in
frequency ppm. I don't know why? Is the FLL broken? Has anyone else
observed this behavior?
> I am also wondering whether setting a lower Allan Intercept would help.
> One problem is that it assumes gaussian behaviour of frequency
> errors, but the situations where high offsets actually indicate a bad
> time generally involve very non-gaussian statistics.
>
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