[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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