[time-nuts] Predicting clock stability from the various characterization methods

[Stephan Sandenbergh]

Hi,

For a while now I've got a growing interest in clocks, stability and
specifically GPSDOs (mostly quartz). My education started out with the NIST
Tech Note 1337. After studying the largest part of the document I felt that
this was probably not a bad starting point. This document explained the
various issues involved with characterizing clocks (Alan deviation and phase
noise) and I bet that most of you are familiar with it.

>From latter I've learnt that clocks can be characterized in both the time
and frequency domain. Also, that the Alan deviation and phase noise plots
are the recommended performance measures to compare clocks in the respective
domains. Another cool thing is that there is an empirical link between the
two and that one can switch between the two with reasonable accuracy.

If one took two Allan deviation (or phase noise) plots one can compare two
clocks directly. Both the phase noise and Allan deviation measures are
statistical in nature and are respectively frequency and time dependent. I
also know that one can get a grip on the phase noise plot by integrating
under the curve for a specific frequency range. This answer can then be
scaled to seconds to give the integrated jitter over the frequency range
involved. What I don't understand is how does one use the Alan deviation to
predict how much a clock will drift on a certain time scale. For example,
500ps over a period of 1us. Or 1s for each day.

Initially I was convinced that the Alan deviation is very nifty because one
can easily identify the different noise types. And, of course to directly
compare clocks in the time domain. However, there seems to be an easy to
read off by how much a clock will drift after a certain period in time? It
would be much appreciated if someone could elaborate a bit on this topic. Or
point me to a previous thread that already did.

Regards,

Stephan Sandenbergh