David,
On 12/31/2011 08:30 PM, David wrote:
I agree that they are an amazing value but what kind of operating
lifetime do the Rb tubes have? SRS has this to say:
Historically, the lifetime of rubidium frequency standards has been
dominated by rubidium depletion in the discharge lamp. To avoid excess
flicker noise, manufacturers would load less than 100 µg of rubidium
into spherical discharge lamps. The PRS10 uses a lamp with a side arm
loaded with 1 mg of rubidium. This design eliminates rubidium
depletion as a failure mechanism, and provides better temperature
control without excess flicker noise.
http://www.thinksrs.com/products/PRS10.htm
Check out page 192 (page 6 in the PDF):
http://tycho.usno.navy.mil/ptti/1990/Vol%2022_13.pdf
It says:
The tests showed, that the basic aging process of the gas-discharge tube
can be explained by surface conductance, caused by the influence of HF
discharge plasmia on glass surface when alkali metal vapor is used. In
its turn it creates a systematic frequency drift due to a light beam
shift and shortens the lifetime of the gas-discharge tube. To decrcase
the influencs of these factors, the rubidium standard utilizes the
reduction mode. When the instrument is turned on, the gas-discharge tube
is warmed up in a short time up to the temperature (200-250)°C. The
conductivc film, formed on the internal tube surface, is broken and
gas-discharge is ignited in the norrnal way. The reduction mode allowed
us to increase a gas-discharge tube lifetime and improve the
metrological parameters of the rubidium standard.
It should be mentioned that this comes out of a lab being quite serious
about their materials etc. and they build impressive hydrogen masers.
The CH1-78 rubidium lamp looks like nothing I've seen elsewhere. It's a
one of a kind.
So, they essentially did what I did with the heat gun, heat it up
properly. This technique was not "invented" by me, I in turn had picked
it from Gerald Molenkamp VK3GJM for the FRS-C:
http://www.vk3um.com/Rubidium%20Standard.html
Cheers,
Magnus
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