[time-nuts] Surplus Guidelines, was: Rubidium Standard
Brooke Clarke
brooke at pacific.net
Sun Dec 10 14:34:12 EST 2006
Hi Time Nuts:
It would be helpful to to have some guidelines about what goes wrong
with surplus time standards. I know the most common problem with older
equipment is that any electrical connection can oxidize and often just
cycling all the switches and sockets of all kinds will bring equipment
back to life. For more on that and other failure modes common to all
electronic equipment see:
http://www.pacificsites.com/~brooke/HaT.shtml
But what I'm looking for here are problems that are specific to
precision oscillators.
For example in Cesium standards the tube (physics package) wears out.
The test for this is the magnitude of the beam current. Therefore if
after going through the normal setup procedure the beam current is zero
or very small the tube probably is dead. As far as I know it's not
practical to do a home repair of the beam tube.
I think there are two things that go wrong with Rubidium standards.
1. The Rubidium in either the lamp or the absorption cell (I forget
which) gets used up and that cell needs to be replaced. The PRS10,
designed for telcom use, will last maybe a order of magnitude longer
than the military surplus FRS Rb sources.
2. They age to the point where the PLL can no longer lock. This
requires resetting the divider numbers which may or may not be possible
depending on the design of the standard. What test will differentiate
this problem from a used up Rb cell?
In the case of crystal oscillators they can age to the point where the
coarse adjustment will no longer bring the frequency back to nominal
(guess how I know this). A short term fix is to add padding caps in
parallel with the coarse tuning cap, but after a year or two even that
will not work. I now know that I should have just let the frequency be
off and use it to drive a clock where the divisor allowed for a 1 PPS
output.
Have Fun,
Brooke Clarke
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com
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