[time-nuts] Simple questions on GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Sep 27 22:03:47 UTC 2009

Joseph Gray wrote:
> >From looking at the EFC graph, I assume that even after almost 48
> hours, the GPSDO hasn't yet reached it's best performance (correct?).
> The PU at this time 3.4uS. I understand that this is a prediction for
> the next 24hrs if GPS lock is lost. The question is, what does the
> 3.4uS apply to? I assume it is the PPS. Since the PPS and the 10MHz
> are phase locked, then the 10MHz drift when unlocked should be
> proportional, right? What do the numbers for the EFC correspond to? I
> don't know what units are being used for these numbers. I understand
> that LPF is Low Pass Filter, but what exactly is that number telling
> me?
It means that the estimated phase error after 24 hours in holdover is 3.4us.
Since the accumulated phase error is the integral of the
fractional frequency error with respect to time this means that the
fractional frequency error averaged over that 24 hours is 3.4E-6/86400 =
Since (among other possibilities) either an offset of 3.9E-11 or a
linear frequency
drift of 7.8E-11/day will both result in the same average frequency
error one
cannot make a reliable estimate of the frequency error after 24 hours
from the
accumulated phase error alone.
> Sorry for so many questions. Any additional expanations are encouraged
> and welcome.
> Joe Gray


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