Re: [Linuxptp-users] Frequency Adjustment

[Vanderpool, C. <cly...@ax...>]

By far the best explanation I have seen.  Thank you!

On Thu, Jun 23, 2016 at 9:13 AM, Richard Cochran <ric...@gm...>
wrote:

> On Wed, Jun 22, 2016 at 08:16:20AM -0400, Vanderpool, Clyde wrote:
> > I guess a little bit of both.  From looking things up on google or
> > different forums all I could find was kind of circular (i.e. 'frequency
> > adjustment is the computer adjusting it's frequency')  I figured it had
> > something to do with the clock pulse of the individual machines but I
> don't
> > want to guess.  I have to present some information and I want to make
> sure
> > I sound reasonably intelligent.  Any info would be appreciated.
>
> In general, there are two important variables when synchronizing two
> clocks.  The "offset" is the instantaneous time difference between the
> two clocks.  If you perfectly synchronize two wrist watches by pushing
> their pins in at exactly the same, then at that moment, the offset
> will be zero.  After that, the two watches will slowly drift away from
> each other.
>
> This happens because the quartz crystals oscillate at slightly
> different rates.  Another word for the rate is "frequency".  The
> difference between the rates of two clocks can be represented as a
> fraction or percentage.  Normally the difference is small, and so you
> will see "parts per million" (PPM) and "parts per billion" (ppb) used
> instead of percentage.
>
> Regarding the ptp4l program, it is not enough for it to simply correct
> the time offset, it also must change the rate of the local clock to
> match that of the master clock.  The program uses the PTP to estimate
> the time and frequency offsets from the master and correct them by
> applying a frequency adjustment to the local clock.  The amount of
> frequency adjustment is shown in the log in units of ppb.
>
> There are tons of papers to read about this topic.  This page and the
> site might interest you:
>
>   http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm
>
> Or try this classic David Mills paper:
>
>   https://www.eecis.udel.edu/~mills/database/papers/time.pdf
>
> Here are all of the Mills papers:
>
>   https://www.eecis.udel.edu/~mills/papers.html
>
> These two were helpful to me:
>
> @TechReport{mills1993precis,
>   author =       {David Mills},
>   title =        {Precision synchronization of computer network clocks},
>   institution =  {University of Delaware Electrical Engineering
> Department},
>   year =         1993,
>   month =        {November}}
>
> @TechReport{mills1992model,
>   author =       {David Mills},
>   title =        {Modelling and analysis of computer network clocks},
>   institution =  {University of Delaware Electrical Engineering
> Department},
>   year =         1992,
>   month =        {May}}
>
> HTH,
> Richard
>