[time-nuts] Training period for a Rb clock using GPS

Hal Murray hmurray at megapathdsl.net
Wed Jun 2 09:02:55 UTC 2010

parekh at berkeley.edu said:
> I am a newbie at this, but have been playing around with 2 prs10s. For our
> application we need to run the clocks without gps, but we do get to sync it
> to gps *initially* for as long as we want. However, what we've noticed is
> that when we train it for short periods of time (< 1 hour a day) the clock
> drifts for a few microseconds a day once we've disconnected gps, but when we
> train it for say 12 hours, its drift seems to be much less (sub sub
> microsecond/day). We were wondering why this should be so!

Look at it the other way.  How long should it take to train it?

Let's use rough numbers.
  There are 1E5 seconds per day.
  Your "few" microseconds is 1E-6 seconds.
    That's an accuracy of 1 part in 1E11.
  Your "sub-sub" is 1/10 microsecond or 1E-7 seconds.
    So that's an accuracy of 1 part in 1E12.

The data sheet says:
  Aging (after 30 days)  <5E-11 (monthly)
5E-11 is 50E-12, so that's 2E-12 per day which is what you saw.

The data sheet also says:
  The PRS10 can time-tag an external 1 pps input
  with 1 ns resolution. These values may be reported
  back via RS-232, or used to phase-lock the unit to an
  external reference (such as GPS) with time constants
  of several hours.

There are 4E3 seconds in an hour and 1E9 nanoseconds per second.  So in an 
hour, you can get close to 1 part in 1E12.  But that's assuming that the 
input PPS signal is right-on.

There are two types of GPS receivers.  Most use a free running clock and 
generate the PPS pulse with the closest clock edge.  They typically have 
noise on the order of 15-50 ns.  Fancy ones will tell you how far off they 
think it is.  The really fancy ones will have a VCXO so they can slew the 
clock to the right offset.

One magic word is "hanging bridges".  It comes up in discussions occasionally.

For lots of info on that area:
31 pages, lots of good stuff, aka time sink.

More here:
2 or 3 screens, good stuff, a quick read.

So with only an hour, it's not unreasonable that you are off by a factor of 
10, but you might have to get unlucky for a hanging bridge to get you.

But there is another factor to consider.  What sort of filter is the software 
using between the PPS input and the knob that adjusts the frequency?

More from the data sheet:
  When tracking an external input, the time constant can
  be set from 5 minutes to 18 hours.

I think the manual says the default is 65K seconds.  That's 18 hours.  Unless 
you changed it, that explains why 1 hour wasn't enough.  It might get better 
if you give it more time and/or tweak the time constant if you can only get 
12 hours.

These are my opinions, not necessarily my employer's.  I hate spam.

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