[time-nuts] Most accurate small crystal

Bob Camp lists at rtty.us
Thu May 27 11:43:31 UTC 2010


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. 


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
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