[time-nuts] Surplus Guidelines, was: Rubidium Standard

Brooke Clarke brooke at pacific.net
Sun Dec 10 19:34:12 UTC 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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