Hi Magnus,
Please note, I goofed when I said that the drift was 30 ppt. It's 30
ppb, i.e. 3e-8.
On 4/23/2012 2:08 PM, Magnus Danielson wrote:
Ed,
On 04/23/2012 07:54 AM, Ed Palmer wrote:
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.
OK. A bit different to some of the rubidiums, but looking around it is
not as uncommon as one might think. This technique is used in some of
the FEI 5680 and also the later GPS rubidiums.
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.
What is the OCXO issue?
Due to drift, the AT crystal is so far off frequency that the EFC can't
correct it. There is no frequency adjustment on the oscillator, but
there is an adjustment for oven temperature. In order to bring the
frequency back into the EFC range, it's running at a temperature below
it's turning point.
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.
You should see second harmonics regardless, it's an effect of the dip
itself. I would check if you can observe the second harmonics on the
signal from the detector by some other means. If you seems to lock up
but does not see second harmonics, it just could be something in the
second harmonics detection which needs some LTC.
I had the same thought so I replaced the Rb lamp with an LED that was
modulated at the 2nd harmonic frequency. The signal showed up at the
2nd harmonic test point. I changed the modulation to the fundamental
frequency and saw approximately the same amplitude at the fundamental
test point. Since the signal paths are almost identical (I have the
full manual with schematics), I concluded that the signal chain was
working and would display the second harmonic if it was present.
There are many sources for shifting the frequency, including the power
supply (check voltage and clean-ness), the trimming of the C-field,
the leakage of helium, resonator tuning, temperature trimming of lamp
and resonator may shift amplitude and hence frequency through light
pulling.
It would be good if you could hook up the modulation sine on X and
return signal on Y on a scope, that would give you a clear display of
the resonance dip.
I just tried that, but had no success. I thought the actual dip was so
small that you couldn't see it and that's one of the reasons why the
modulation is used?
Oh, and check if you have leakage problems around the integrator cap,
that would also shift the frequency.
I'll check for that, but wouldn't that type of leakage just be corrected
by the loop?
Thanks,
Ed
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