Atomic references are in general, short lived. Cs beam's run
out of vacuum, or out of Cs (some debate about that!). Rb
references plate out the Rb metal on the lamp's glass bulb,
and get to a point where not enough light can get past the
plated out Rb metal film for the detector to operate.
The Rb can very often be fixed when the lamp goes dark by using
a heat gun to evaporate the Rb on the inside of the bulb where
the light is emitted, and let it plate out somewhere else.
The biggest problem is gaining access to the Rb bulb. Many
of the compact units are essentially assemble only devices.
When either a Cs beam standard, or a Rb reference runs out of
"physics" to operate, its disciplined VCXO will end up against
its rails too high, or too low in frequency (depending on how
it was designed).
Also, crystal ovens are a reliability problem. They are
electrically heated devices, and as such must be protected
against thermal runaway situations... lest they catch fire.
[I have seen oven failures where the entire oscillator board
melted its solder, and parts dropped out.]
In many cases, the thermal fuses installed inside of the oven
will fail, even though there is nothing otherwise wrong with
the oven. But again, they are usually assemble only devices,
and replacing the fuse will require serious removal efforts.
-Chuck Harris
Dr. Frank wrote:
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I fully agree to Ed Breya. Your Rb standard very probably is not locked, 17ppm off is a typical sign for that.
All such OCXOs (10544 or 10811) can never be out that far, if they are operating correctly (i.e. getting warm, please check).
As both OCXO agree to each other, they seem to be working.
Please check the LOCK signal of your Rb clock.
After you identified the root cause, further hints how to correctly adjusti either of your clocks can be given.
Frank
