[time-nuts] Some long-term data

John Ackermann N8UR jra at febo.com
Sun Dec 24 10:51:59 EST 2006


Magnus Danielson said the following on 12/23/2006 07:21 PM:

>> Magnus Danielson said the following on 12/23/2006 05:22 PM:
>>
>>> Such as temperature and humidity. One of the Cesiums seemed to have a rather
>>> large frequency offset. Did you drift-compensate your ADEV measures or not?
>> No, I ran the ADEV in Stable32 without removing drift.  My understanding
>> is that linear drift shouldn't affect the calculation.  I was also a
>> little concerned whether drift removal would work very well given that
>> both Cesiums had periods where the offset changed for quite a long
>> period (CS1 near the end of the data, CS2 at the beginning) and I wasn't
>> sure what impact that might have.
> 
> I know next to nothing about Stable32, so I don't know what that black box
> actually is doing.
> 
> From a theory standpoint, it is trivial to show (I've done the exercise to
> convince myself, but it is already covered in literature) that linear frequency
> drift does affect the result. Time/phase offset and frequency offset cancels.
> 
> From a practical standpoint, linear frequency drift may or may not be small
> enought not to interfere with the measure of the noise power. To be sure,
> measure the drift rate.

I think I've been sloppy in my terminology above and in other messages
-- I meant to say, as Magnus does, that linear offset doesn't affect the
stability measurement.  Drift such as aging certainly would.

In the data I've been working with, there is a component of linear drift
in the CS2 results; if you chop off the first 1250 records where the
offset was essentially 0 and look at the rest of the data, the offset is
about +7x10e-13 and a quadratic fit shows a drift of 6.4x10e-15/day.

What's interesting about CS2 (a fairly old 5061A upgraded with the later
model 10811A OCXO and a replacement FTS tube) is that it ran for over a
year prior to this experiment with very, very low offset; there were
certainly wobbles but the long-term offset was in the low 14s.  That
shows at the beginning of the current plot, but about 20 days into the
run something changed (and I don't know what) that caused the offset to
increase.

Similarly CS1 (a later model 5061A also with late OCXO and replacement
FTS tube) ran at about -2.5x10e-13 for the first 70 days of the run, and
for quite a long time prior to that.  But starting at the 70 day point,
it went into a series of slow oscillations with a mean offset of about
zero; recent data shows that it may be returning to the historic offset.
 Looking at the first 70 days with the offset removed, there's very
little discernable linear drift; instead most of the instability appears
to be various cycles ranging from daily to a couple of weeks.

And, for anyone who's interested in playing with the data, I've uploaded
it to http://www.febo.com/time-freq/uploads/.  There are three files in
the zip, which is about 327k.  Each consists of MJD and phase data with
a base tau of 600 seconds.

John


John




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