[time-nuts] Antique Rb Standard - Thanks, Pictures, Parts Request, Question

[Ed Palmer]

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