Hi John,
Don't know if I address your real issues, but I want to add some more
pieces to the puzzle.
On 10/27/2012 09:16 PM, John Ackermann N8UR wrote:
Since I recently got an HP Cs degausser (thanks, Stijn!), I though I'd
go through the whole setup routine for my 5061B/004 and see how close
the C-field-via-Zeeman setting would bring me to GPS-derived frequency.
It turned out to be an interesting and puzzling exercise.
This 5061 seems to be in perfect working order -- quick lock, good meter
readings, and measurements indicate frequency within parts in e12. I
don't have any reason to believe that it's not tuned or working
properly, except for the Zeeman-setting results.
The problem is that when I tune the audio source around 53.53 kHz, per
both the manual and the sticker on the door, I don't see any change in
Beam I at all. Nor do I see anything at the alternate frequency of 42.82
kHz.
Instead, I see the expected three peaks -- primary with a smaller
secondary on either side -- at about 48.21 kHz, which doesn't show up
anywhere in the literature I've found.
There are in total 7 peaks, you want the center peak of those.
Tom has made measurements:
http://leapsecond.com/images/cfield.gif
I have done the same to one of my tubes, but I don't recall where I have
that flimsy picture, so the above is a good start.
Below, I've cut and pasted a years-old message from TVB and Corby that
explains the Zeeman frequencies. I've measured the synthesizer output
and it's nominally 12.6317725 MHz, which per that message should
correspond to a 53.53 Zeeman. Where 48.21 kHz comes from, I have no idea.
The separation of the peaks depends on the C-field value you have.
For low C-field strength, the side-peaks separate my the square of the
C-field (B), and the cluster shifts gently linear with the C-field.
You want to spread the side-peaks out, such that they do not confuse
your measures. Also, looking at the above you have the 7 Rabi
distributions, and on top of those the Ramsay fringes, at which you want
to lock onto the center one. It may be interesting to learn that certain
systematics skews the shape of these, and thus causes a systematic
miss-tuning, so great care is taken to reduce that effect when
manufacturing the tube.
Look at the above C-field plot again and you see how the side features
move with different C-field settings.
Modern "digital" caesium clocks measure the side-features in order to
Servo the C-field into a stable value, and hence also stabilize C-field
drift out of the equation first degree.
I'm using a Rigol arbitrary function generator locked to an external
reference as the audio source, in sine wave mode. I know that's not the
cleanest device in the world, but the wave doesn't look too bad on my
scope and a counter indicates the frequency is what the dial says. As I
adjust the audio amplitude, the beam current responds, and I see a peak
at around 500mV, which the 5061B manual says is correct.
Any ideas why I might be seeing this very off-the-wall result? Could
distortion in the audio source cause something like this? I'm more
inclined to blame technique or gremlins than the 5061B -- again,
external measurements indicate that the thing is tuned correctly and
operating properly, just having this goofy Zeeman response.
You want a clean source, as spurs or distortion would cause you to look
at multiple points in the spectrum at the same time and you would get
the combined result of those features. Still, the distortion values
doesn't have to be stellar to get decent readings.
Cheers,
Magnus
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