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

Bob kb8tq kb8tq at n1k.org
Thu Jan 25 18:11:25 EST 2018


> On Jan 25, 2018, at 5:47 PM, Ronald Held <ronaldheld at gmail.com> wrote:
> Nigel;
>   Will read the PDF carefully.
>      Ronald
> "if it runs for years (or even months)" sounds like an informed comment:-)
> When searching for some data recently I came across a report which
> might be relevant.
> "A Second Look at Chip Scale Atomic Clocks for Long Term Precision
> Timing", written by
> Alan T. Gardner and John A. Collins of the  Woods Hole Oceanographic
> Institution, details their
> experience with a number of earlier and more recent CSAC modules and
> their findings make
> for very interesting reading.
> At the time of writing a copy is available here....
> www.obsip.org/documents/Gardner_IEEE_Oceans_2016.pdf
> Tim:
>   Only three times worst then CSACs?
>       Ronald
> y those Woods Hole guys.
> Their temperature-compensated 5 milliwatt crystal oscillators can be
> back-corrected (linear drift model) to a few tens of milliseconds over a
> year and they make a convincing case they know how to do this.
> Their similar graphs for CSAC oscillators are maybe a factor of three
> better.
> Tim N3QE
> Bob:
>  Does it make sense to reset yearly if the offset is milliseconds?  Tenths?
>       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. 


> Actually it was not quite what it sounded like. What I was trying to
> say was “free runs”
> for years or even months. Any device that is re-calibrated will have
> the aging drift zeroed
> out in that process. As noted in another post, CSAC’s have gone
> through some growing
> pains. The Woods Hole paper came out sort of at a low point in the process. The
> current crop of CSAC parts seem to be more reliable than the ones Woods Hole
> reported on. I’ve seen failures over the years, but not a lot of them ….
> Bob
> Tom:
>   That PDF was interesting.  Not certain I would've one.
>     Ronald
>> www.obsip.org/documents/Gardner_IEEE_Oceans_2016.pdf
> Also see the very nice presentation:
> "Challenges of precise timing underwater"
> http://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resources/Gardner_OBS_Timing_ATG_20150427.pdf
> /tvb
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