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.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
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