Hi If you dig into the way they "certify" things like ships chronometers and wrist watches, they focus on averaged errors over a long period of time. That's pretty much what you do when you check your watch. You check it today and you check it a week from now. It's off by a second (or not) and that's what you pay attention to. It's rate could be off quite a bit over some portion of the period. That's fine as long as it averages out.
This sort of thing is *very* algorithm friendly. You can correct for a lot of things after the fact. The results will be a bit variable since it's a "how lucky did you get" sort of thing. They also will tend to degrade over time, as the data you initially loaded in on temperature performance, aging, and G sensitivity departs from reality. Bob On May 27, 2010, at 12:22 AM, Jim Palfreyman wrote: > Hi All, > > I have a RSA SecurID device which I use to log in to my work's VPN. For > those of you not familiar with these, they show you a 6 digit number that > you use (combined with a PIN) to log in. This number changes every minute. > The changing of this number lines up with the servers at the other end and, > as I understand it, they do take into account gradual drift. These devices > have a limited programmed lifetime of three years as well. So the internal > clock on one of these needs to be decent. > > I have timed the accuracy of this internal clock and have found it to be > pretty good so far. 17 days ago it was ticking over at 21.8 sec past the > minute and a quick visual inspection today and it was still *very* close to > that. I will confirm it properly tonight. > > Now this device travels around with me, like a wristwatch, but is not on my > person. It is usually in my laptop bag. So it is subject to quite varying > temperatures. > > So it's been nearly 3 weeks and this device has barely changed 0.1 to 0.2 > secs tops. Now that's not bad compared to digital watches. > > I presume it has a quartz crystal in it, obviously no oven, but my question > is: > > What is the best crystal you can get on the market today that would work in > a watch type device with very little power available? What's the best > accuracy that can be expected? > > I know straight out of the factory they can be pretty spot on (as my Casio G > Shock was) but it has aged markedly and now gains a couple of seconds a > week. So what is the best we can expect long term too? > > Regards, > > Jim Palfreyman > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
