[ntp:questions] Re: ntpd PLL and clock overshoot

Richard B. Gilbert rgilbert88 at comcast.net
Mon Oct 9 13:36:28 UTC 2006


Bart Van Assche wrote:

> As known the PLL implemented in ntpd/ntp_loopfilter.c can overshoot. In 
> the source code of ntpd 4.1.2 it is documented that this overshoot is 
> less than 5 percent. Measurements I performed confirm this.
> 
> The PLL algorithms in ntpd 4.1.x and are 4.2.x different. The source 
> code of ntpd 4.2.0 specifies that document UDel TR 97-4-3 documents the 
> ntpd 4.2 PLL algorithm. Anyone any idea where I can find that document ? 
> I could not find it via Google, and it's not in David Mills list of 
> papers (http://www.cis.udel.edu/~mills/papers.html). I'm asking this 
> because for ntpd 4.2.0 I measured an overshoot of about 100 percent. Can 
> anyone confirm this ?
> 

I can't confirm the 100 percent but the current version doesn't work too 
well with my GPS reference clock at startup!  I had something like a 90 
millisecond offset when I started ntpd.  Over the next few minutes it 
corrected that offset but didn't stop, or even slow down, when it hit 
the zero line.  It kept right on going until it had a -9 millisecond 
offset.  It took about thirty minutes lock in tightly!   I could have 
figured it out with pencil and paper in less time than that and, as a 
mathematician, I can't count to twenty with my shoes on!




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