[time-nuts] Antique Rubidium Standard Questions
Ed Palmer
ed_palmer at sasktel.net
Tue Apr 24 23:14:11 UTC 2012
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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