[ntp:questions] Loop Filter Gains vs. Polling Interval

Mischanko, Edward T Edward.Mischanko at arcelormittal.com
Wed May 18 14:21:28 UTC 2011


Miroslav,

I'm a little confused.  I set the Allan intercept, as you suggested, but
when you say FLL didn't you mean PLL?  Thanks for your help.


On Behalf Of Miroslav Lichvar
Sent: Tuesday, May 17, 2011 11:09 AM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] Loop Filter Gains vs. Polling Interval

On Sat, May 14, 2011 at 12:23:56PM -0500, Mischanko, Edward T wrote:
> Can anyone tell me, does the sensitivity for frequency adjustment
lessen
> as the polling interval increases?  I ask because I'm observing that
my
> offset increases and the frequency adjustment decreases to the point I
> fall out of sync at polling intervals above 256.  What am I doing
wrong?

As I have recently learned, the Windows ntpd works with daemon loop
(instead of the kernel loop) which is optimized more for stable clocks
and noisy networks. So if your clock's frequency changes rapidly, ntpd
won't be able to keep up, as it would with the kernel loop.

Fortunately, you can improve that significantly by enabling the FLL
part of the loop by setting a shorter Allan intercept, in 4.2.6 it's
11 by default (set in log2(s)), i.e. FLL is active with poll 11 and
above.

For example:

tinker allan 7

HTH,

-- 
Miroslav Lichvar
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