[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.

More information about the questions mailing list