On 11/09/2014 07:11 PM, Bob Camp wrote:
Hi
On Nov 9, 2014, at 11:18 AM, Chris Albertson <albertson.ch...@gmail.com> wrote:
On Sat, Nov 8, 2014 at 11:13 PM, Neil Schroeder <gign...@gmail.com> wrote:
At one point I was considering phase locking all of them together - but
again that seemed less than straightforward. You can do it PLL back to
back, but is there a way to have a loop that contains multiple clocks? I
would think the "telephone game" would apply.
NTP does this but on a MUCH lower frequency and longer time scale. But I
think NTP's general method could apply. NTP will accept any number of
reference clocks. (Yes sone people run NTP using just one GPS receiver as
a reference but best practice is to use five references.) NTP compares the
set of ref. clocks with each other and first tries to find the subset of
clocks that track each other, assuming the outliers are "wrong". It
continuously checks this and maintains a set of "true tickers". From these
it computes a consensus time using a weighted average of the "true"
clocks. The weights are based on the jitter and other quality measuring
statistics. Using this method reference clocks can be taken on and off
line without need to re-start NTP.
That may (or may not) give you the best ADEV on the output. My guess is that
the filtering algorithm will need to be a bit more complex. NTP’s aim is mainly
to throw out bad clocks and pick one as best. We would more likely want to
combine the outputs and use all of the good clocks we have. The idea is to
improve on the ADEV of the *best* source you have available.
The aim is to remove false-tickers and then build the best ensemble of
the remaining sources and weigh them according to stability.
It seems this goal is not very well met in practice, but the theory
foundation is pretty good.
Cheers,
Magnus
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