[time-nuts] Low-long-term-drift clock for board levelintegration?

[Azelio Boriani]

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 <tvb at 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 at 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.
>