[time-nuts] Phase microsteppers

[Brooke Clarke]

Hi Tom:

Here is some data taken today.  First coul is clock time when I read 
counter and second coul is time interval in ns.
I may have missed one or more of the readings.  Is this enough to get a 
1,000 second Allan variance?  If so how does the FTS4060/S24 compare to 
other Cesium clocks?

9:04 	209.960
9:09 	203.016
9:44 	202.892
10:03 	206.380
10:28 	206.758
10:49 	206.574
11:07 	210.268
11:22 	208.814
11:43 	210.123
12:03 	205.945
12:13 	196.303
12:29 	198.971
12:46 	191.788
13:05 	194.967
13:40 	196.669
13:57 	196.454
14:15 	196.792
14:34 	202.643
14:54 	202.977
14:59 	204.740


Have Fun,

Brooke


clock time when I read the counter:Tom Van Baak wrote:

>Before I answer the microstepper questions let me
>first describe the alternatives.
>
>Let's suppose you have the following:
>- a working cesium (or rubidium) standard,
>- a government reference (such as Loran or GPS),
>- time interval counter, and perhaps
>- some sort of PC logging (GPIB, RS-232, etc.)
>
>Within hours, or certainly days, you notice your Cs
>1 PPS is diverging from a UTC 1 PPS. So what to
>do about this?
>
>You can compute the average Cs frequency error
>with just two data points since frequency is delta
>phase over delta t. So you really don't even need
>PC logging for this. Just look at your TI counter
>display, wait a day and look at it again. 864 ns a
>day is 1e-11, 86 ns a day is 1e-12, etc.
>
>Or you can get fancy and plot phase and look at
>the slope. If the slope is ragged, wait longer until you
>see a clear trend. This could be hours or even days.
>
>If you don't like amount of frequency error you can
>try to adjust the C-field. It will take several tries but
>you should be able to get your Cs accurate to within
>1e-12; maybe better depending on your eBay luck.
>
>If you want to get closer than this you may want to
>first check the stability of the standard. After several
>days or weeks of data you can compute the Allan
>Deviation of your standard for, say tau 1 day. Even
>just one data point per day for a week is enough to
>give you a rough ADEV result.
>
>If you find the ADEV doesn't get much below 1e-12
>then there's no point in trying to set the accuracy
>of your Cs below 1e-12. Stability is not affected by
>accuracy but accuracy is limited by stability.
>
>No clock is perfect. Clocks have noise floors. At the
>1e-12 or 1e-13 level frequency will wander, phase
>error will, over time, grow unbounded. The same is
>true for wristwatches (just move the decimal point
>over far enough).
>
>The reason I stress computing the ADEV is so that
>you can adjust your expectations. I'm pretty sure no
>amount of C-field adjustment of a standard 5061A will
>get you down to 1e-13 or the 14's.
>
>All Cs standards, once set, will show accumulated
>phase errors, no matter how carefully you set the
> rate or control the environment. So you should
>expect over days, or weeks, or months, that your
>Cs will be off by many nanoseconds, even many
>microseconds, eventually.
>
>It then becomes humbling that a $25 surplus GPS
>receiver will easily beat your cesium as source of
>a UTC tick ;-)
>
>So the first point I wanted to make is don't expect
>your Cs to ever be perfectly accurate.
>
>The next question is, after you get the C-field set
>to your liking, how to make further time or rate
>adjustments.
>
>/tvb
>
>
>
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>
>  
>

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