Hi Ed,
On 4/24/2012 4:13 PM, ed breya wrote:
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 was able to insert an attenuator into the SRD drive circuit and found
that I could drop the level by 6db without affecting the maximum
amplitude of the error signal so there seems to be lots of power. I
will be checking the multiplier chain, but first I have to build an
extender board. There was supposed to be one in the unit, but it
evaporated.
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.
Nope. The reason it locks is that there's a switch that bypasses the
2nd harmonic and sweep circuitry and connects the error signal to the
oscillator. Once I adjust the error signal to zero (i.e. resonance) and
flip that switch, the 2nd harmonic is irrelevant.
I just realized that I haven't mentioned that there's a partial manual
for this thing online. It's missing a few schematics, but is otherwise
complete. The URL is http://sundry.i2phd.com/ServiceManual_304b.pdf .
Ed
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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