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

Bob Camp lists at rtty.us
Wed Jun 2 07:08:09 EDT 2010


Hi

The only thing I would add to that is:

If your training starts from power up, part of your hour is spent learning something wrong. Most Rb's drift a bit as they stabilize after being turned on. 

Bob


On Jun 2, 2010, at 5:02 AM, Hal Murray wrote:

> 
> 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:
>  http://www.gpstime.com/files/PTTI/PTTI_2006.pdf
> 31 pages, lots of good stuff, aka time sink.
> 
> More here:
>  http://www.leapsecond.com/pages/vp/heater.htm
> 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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