Hi Ed,
On 4/29/2012 12:28 PM, ed breya wrote:
Yes, very nice pictures. That thing is really built and looks like it
should be easy to work on and experiment with.
I still have to say that I doubt the cavity is off-tune unless
something serious happened to it mechanically. Is it even adjustable?
Yes, it's quite adjustable. The end screws in and out over a range of
~0.9 to 1.07 inches for the inside length of the cavity. I suspect that
the reason the cavity was mistuned wasn't because of the cavity changing
mechanically but because the cell drifted over the 40+ years since it
was manufactured. After all, isn't that what happens to Rb standards?
If so, maybe someone previously tried to adjust it and messed up. If
not, or it appears original, then I think any mistuning will be in the
multiplier instead.
Unfortunately I won't be able to test/adjust the multiplier until I can
build an extender card. But even if the multiplier was slightly off (
can an analog multiplier by _slightly_ off?), wouldn't that just change
the required frequency for the OCXO? The SRD would still be an exact
multiplier and if the physics package was working properly, I should see
the fundamental and 2nd harmonic as expected.
You can sweep the cavity by placing coupling loops in there, and then
see what happens as you go say +/- 100 MHz around the desired center,
and then at narrower sweeps. You should get an observable peak at or
near the right frequency, and it should be broad enough to include the
ideal Rb frequency. When the excitation lamp is on and the cell has
some light going through, there should be some absorption, and the
cavity Q may decrease a bit, but I doubt it will have much effect. You
can try this by sweeping with everything off, and then with the lamp
on to see if it's noticeable.
My test equipment isn't good enough for that. I was able to look for
frequencies in the cavity and saw the 92nd, 93rd, and 95th harmonics of
the driver frequency. But the Rb frequency is at the 98th harmonic. I
have retuned the cavity so that the strongest signals are the 97th and
98th harmonic. I hope to get things reassembled today to see if it
changed anything.
Ed
Also, as someone else mentioned, it's good to see fully-utilized bench
space in the background - plenty of stuff everywhere, at your
fingertips. I especially liked the open-sided desktop PC. All of my
garage PCs are just like that (I don't even know if I can find the
covers) - it gives better cooling, is easy to modify and experiment
with, and provides some handy storage space that otherwise would be
wasted.
Ed
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