[time-nuts] CSAC Project(was CSAC purchase)

Ronald Held ronaldheld at gmail.com
Fri Jan 26 07:19:10 EST 2018


Bob:
      Sounds reasonable.  You suggest to let it age a year and reset
often during the year?
      No way to compensate for a linear frequency drift?
                 Ronald


 If you *don’t* correct the *frequency* offset, then you ultimately
have a device
 that is off by quite a bit per year. The key here is that it is
frequency (and not time)
 error. Once you get a significant frequency error, the amount of time
you gain or loose
 goes up. You no longer are in a 0.1 second region, you are now into a
“second per
 year” sort of situation.

 Some math:

 If the CSAC is at zero frequency error at the start of the year and
drifts by 10 ppb
 over that year, you have an average error of 5 ppb. Keeping things
simple, you get
 1/6 second error that year. (5 / 30 = 1/6).

 If three years later, the CSAC is at 30 ppb and drifts another 10 ppb
in frequency,
 you now are at 35 ppb average frequency error. You will gain / loose
more than a
 second in that year.

 The real numbers are slightly different. You need to look at when
over the year the
 aging happens. A device that ages a lot early on in the year will do
worse than a
 device that ages linearly over the year. A device that does all it’s
aging only on the
 last day would do better than either of the other cases.

 Bottom line:

 Your CSAC wrist watch is very much *not* a millisecond per year sort of device.
 Best guess is it is in the 50 to 150 ms per year vicinity in the
first year after calibration.
 Based on previous posts, that is in the same vicinity as a WWVB sync’d wrist
 watch and not quite as good as an typical Apple Watch.

 Bob


  Jim:
  Likely wait for your data before ordering one.  Too bad the chip
price is so high, compared to a few years ago.
             Ronald



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