Ed,
Tuning the cavity should peak everything - it just maximizes the
excitation power at the microwave frequency, so you get the most
output from the Rb light wavelengths. A mechanical cavity resonator
will have a very wide (compared to the modulation frequencies you're
looking for) bandwidth, so unless something happened to it
physically, it should be OK as originally built or adjusted. However,
you may want to look at the multiplier chain and SRD bias circuit
components and adjustments - those could have drifted quite a bit
over forty years, limiting the microwave power due to being
off-frequency, or having poor multiplication efficiency.
I'm guessing that the second harmonic is indeed present, but just
buried in the noise, and the loop still can "lock" because of the
further signal processing, even though you don't see the evidence -
remember it's a lock-in amplifier capable of digging a tiny signal
out of the noise. If you go through the multiplier and check and
tweak things, you may get more excitation power and signs that it's
getting back to normal. Once you get enough power, if the Rb cells
are still good, the second harmonic signal should show up large
enough for the circuit to detect sufficient S/N ratio and provide a
valid lock indication.
Ed Breya
Ed Palmer wrote:
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>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.
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