Hi, the simple stupid answer is: 24/365
Ernie. -----Original Message----- From: Bob Camp <li...@rtty.us> To: 'Discussion of precise time and frequency measurement' <time-nuts@febo.com> Sent: Thu, Jun 3, 2010 6:07 pm Subject: Re: [time-nuts] Training period for a Rb clock using GPS Hi If you have an 18 hour time constant you would need a training period of 5 o 10 X 18 hours to get the system to settle. For a one hour training period the time constant should be in the 5 to 10 inute range. Bob -----Original Message----- rom: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On ehalf Of Abhay Parekh ent: Thursday, June 03, 2010 12:02 PM o: Discussion of precise time and frequency measurement ubject: Re: [time-nuts] Training period for a Rb clock using GPS Hi Hal, hanks so much for the detailed post. I have a follow up question: What is he relationship between he training time and the appropriate value of the time constant (currently et at 18 hours)? The time constant isn't the size of moving average window is it? hanks again for your help. We are a bit clueless here but trying to earn... Abhay n 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 s > 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 hen 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 f 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 et 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. ______________________________________________ ime-nuts mailing list -- time-nuts@febo.com o unsubscribe, go to ttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts nd follow the instructions there. ______________________________________________ ime-nuts mailing list -- time-nuts@febo.com o unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts nd 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.