[time-nuts] Worst possible error on a rubidium

[WB6BNQ]

David,

The answer to your question is quite complex.  A number of factors control the
operation of a passive atomic resonator acting as a filter.  Temperature, pressure,
buffer gas mixtures, external magnetic forces, coupled light excitation, the length
of the cavity, the interrogating external RF energy level are but some of those
factors.

The "Atomic" nature of the filter is based upon the extremely narrow resonance of
electron absorption of energy which occurs at a specific frequency and is influenced
by the previously mentioned factors.  That specific RF excitation frequency is
different for each element in the Periodic Table.  For example the Hydrogen Maser is
around 1.45 GHz; the Rubidium is in the 6 Ghz range and the Cesium is in the 9 Ghz
range.

None of the specification sheets even approach trying to answer such a question
directly.  However, with certain qualifying assumptions, it could be inferred from
"certain" specs, if available, as a general idea, BUT, by no means the complete
answer.

For example, looking at Symmetricom's XPRO Rubidium spec sheet, they give a 10 year
spec of ?+/-1x10e-9.  The assumption is the unit was built correctly, was adjusted
to be precisely on frequency at its intended installation site and was left powered
on, in a stable atmosphere, without failures of any kind for the entire 10 years.
If the product truly met those assumptions and specs, then I would say, with some
confidence, that if you picked up a used one working properly that it would be
reasonable to assume after powering up and allowing it to come to a stable
temperature (24 hours) and it indicated a locked condition that it would be within
1x10e-9 of the correct frequency.  Unfortunately, FEI is not so forth coming with
their product literature, but I suspect their units are similar.

Also, be aware that at least one member on this list has reported buying two 5680A's
from China and they were both significantly off frequency by many hertz.  Inspection
and determination of the problem is a project in motion.  This same member has
further stated other reports exist on the WEB of other units exhibiting the same
problem, although I have not seen those reports.

It may prove out that the two 5680A's have a problem.  It is equally possible that
they are deliberately offset for some specified yet unknown reason.  Time will tell.

While a Cesium frequency standard is by definition and without question accurate,
that only applies under a narrow set of circumstances.  It is possible to miss
adjust such a beast and that is why multiple standards labs constantly cross check
themselves.

Remember, to error is human and machines do what we tell them !

Bill....WB6BNQ


"Dr. David Kirkby" wrote:

> If an old random 10 MHz Rubidium oscillator is working (i.e. powers up, and
> eventually locks), what is the maximum possible frequency error it could have?
>
> Could it remained locked with an error of 1 part in 10^7, 10^8, 10^9, 10^10 etc?
>
> I assume there are physical limits which would simply stop it functioning too
> far from the correct frequency, but don't have much clue what they are.
>
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