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

[Bob kb8tq]

Hi

The CSAC (like any vapor cell standard) has a drift (aging) process. That’s 
just the way it works. It is at a *much* lower rate than a crystal oscillator, but
it is the same sort of idea. It is one of their basic differences from a Cesium 
beam tube. 

Can you “estimate” aging in advance? Everything I’ve seen suggests that you
are simply guessing when you do. You will be right in some cases and wrong in 
other cases. You can do better estimating warmup drift and short term effects. 
Working out what will happen over months (or years) is not very easy. 

A *very* basic example: 

I observe a couple of units and they all go positive by 0.8 ppb / mo. I put in
some code to work with that. The unit I happen to have goes 0.8 ppb negative
a month. My code has actually made the unit 2X worse than it would have 
been if I just stayed away from it. 

Yes there are papers on aging estimation. They mainly focus on coming up 
with a “worst case” number. If they guess that 0.8 ppb will go on forever and
it drops off a bit from there, they did ok. For correction purposes … not so much.

On a practical basis, you *will* have to dock this beast up with a charger on 
a regular basis. A solar powered WWVB watch is not unusual. A solar powered
Apple watch or CSAC watch … not so much. When it goes to the charger, 
sync it up with GPS.

Bob

> On Jan 26, 2018, at 7:19 AM, Ronald Held <ronaldheld at gmail.com> wrote:
> 
> 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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