Hi Hal, Thanks so much for the detailed post. I have a follow up question: What is the relationship between the training time and the appropriate value of the time constant (currently set at 18 hours)? The time constant isn't the size of a moving average window is it? Thanks again for your help. We are a bit clueless here but trying to learn... =Abhay
On Wed, Jun 2, 2010 at 2:02 AM, Hal Murray <hmur...@megapathdsl.net> wrote: > > par...@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. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.