[time-nuts] Training period for a Rb clock using GPS
lists at rtty.us
Wed Jun 2 11:08:09 UTC 2010
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.
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:
> 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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