Hi That will give you the "best" answer with a simple loop. The problem is that "best" may not be good enough to actually get your Rb on time / on frequency. Something more sophisticated than a simple loop may be needed.
Bob -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Abhay Parekh Sent: Thursday, June 03, 2010 12:28 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Training period for a Rb clock using GPS Ok, great. So if we can train for h hours we should set the time constant somewhere between h/10 and h/5. It would be safer to pick something closer to h/10 since when the clock powers up it might "start" in the wrong place so a smaller value helps the clock move quickly into the right area, but h/5 will act as a better buffer against hanging bridges. Is my reasoning correct? Thanks =Abhay On Thu, Jun 3, 2010 at 9:07 AM, Bob Camp <li...@rtty.us> wrote: > Hi > > If you have an 18 hour time constant you would need a training period of 5 > to 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 > minute range. > > Bob > > -----Original Message----- > From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On > Behalf Of Abhay Parekh > Sent: Thursday, June 03, 2010 12:02 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Training period for a Rb clock using GPS > > 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. > > > > > _______________________________________________ > 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. _______________________________________________ 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.