Yes, the relation frequency_drift-> time_error seems difficult to figure out. I see this misunderstanding daily here at work and haven't yet found a way to explain to my colleagues. I have already used: integral, area, count accumulation but none worked.
On Mon, Feb 20, 2012 at 10:37 AM, Tom Van Baak <t...@leapsecond.com> wrote: > I think a box that can't get some external source of time in three years >> is one that we can pretty well write off as lost. Thank you (several of >> you, actually) for the clear explanation of the math. >> >> http://www.msc-ge.com/en/news/**pressroom/manu/1241-www/3567-**www.html<http://www.msc-ge.com/en/news/pressroom/manu/1241-www/3567-www.html> >> http://www.thinksrs.com/**downloads/PDFs/Catalog/SC10c.**pdf<http://www.thinksrs.com/downloads/PDFs/Catalog/SC10c.pdf> >> >> So if I'm reading those specs right, they both offer 2E-10, or 100 >> microseconds per 500,000,000,000, or 121 microseconds per week. So, if >> those are affordable (and I haven't yet called to check), that's telling me >> that in order to be useful in the long term, these boxes need to be getting >> some reference time from somewhere at least once a week. >> > > Hi Bill, > > Not quite. The 2E-10 isn't a time or frequency *accuracy* spec; it's a > *frequency drift* spec. > What this means is that the frequency may change by up to 2e-10 per day, > day after day... > > Let's say the oscillator is keeping perfect time now. > Then 24 hours from now it may be fast or slow in frequency by 2e-10. > If the oscillator is fast by 2e-10 it will be gaining time at the rate of > 0.2 nanoseconds per second. > That doesn't sound like much but since there are 86400 seconds in a day, > that's equivalent to gaining at a rate of 17 microseconds a day. But that's > just the first day. > > The second day the oscillator may be fast by yet another 2e-10. By the end > of the day it's now 4e-10 fast so it's now gaining at a rate of 35 > microseconds a day, in addition to all the time error from yesterday. > > Think of frequency changing like an upward *ramp*. The time error > accumulates like the *area* under that growing triangle. > Hence the quadratic growth of time error (1/2 * drift * t^2). > After a week the total time error is over 400 microseconds; you hit your > 100 microsecond limit in about 3.5 days. > > The SC-10 starts at $250, presumably for a low-grade version, not the one > you want. > The DX-170 looks interesting. Let us know when you get a price quote. > Note also the temperature spec; can you maintain the temperature of your > device to +/- 1 C? > > /tvb > > > ______________________________**_________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<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.