I'm playing with a Tracor 304-B Rubidium Standard from 1969. I'm using
it as a learning exercise to find out more about the guts of a Rubidium
standard and how it works.
This thing is a beast! Rack-mount, 3U high, 39 pounds (~18 kilos), 9
plug in circuit boards. The OCXO is an oddball frequency that is
multiplied directly to 6.8 GHz. There's no synthesizer in that chain.
A synthesizer is used to convert the oddball frequency to a 5 MHz output.
It's sort of working. The error signal isn't up to spec, but it's
strong enough to give a stable lock although there's no trace of a
second harmonic signal. Allan Deviation is in the Xe-12 range from 1K
to 10K seconds. The OCXO has a not-yet-resolved issue that is probably
degrading the results.
The lock frequency suggests that the Rubidium cell has drifted down by
~30ppt over the 40+ years since it was built. Is that reasonable?
That's much more drift than the specification states, but I doubt if the
spec was intended to be valid for 40 years!
Could the drift be at least partially responsible for the lack of second
harmonic? A message on the list (
http://www.febo.com/pipermail/time-nuts/2006-April/020562.html ) said
that you could peak the second harmonic by adjusting the cavity tuning.
If the cell and the cavity are out of sync would that kill the second
harmonic? How close to they have to be? If this thing has a cavity
tuning adjustment I haven't found it.
FYI, I checked my counter (Racal 1992 referenced to an Efratom FRK-H
Rubidium) against my Z3801A and Tbolt. Both measure 10.000 000 000 MHz.
so I'm confident that my numbers are good.
Thanks,
Ed
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