[time-nuts] Homebrew H Maser

J. Forster jfor at quik.com
Mon Aug 30 00:05:10 UTC 2010

Yes and no.

I watched while a cryogenic MASER experiment was done at Harvard.

First off, a H MASER built at Smithsonian Astrophysical Observatory, was
set up and a GPS set up as a comparison. This was late 1980s.

That MASER was used as a reference for a synthesizer which was swept a
Hertz two around the 1421 MHz frequency to excite the cavity. The cavity
was peaked with a reflectometer using a narrow sweep and then H was added.
Parameters were adjusted while the cavity microwave field monitored.

The RF field was pulsed, as I remember, and they watched as the RF decayed
in a damped exponential. You could see what was happening to the gain of
the medium, the decay took longer, and parameters were slowly adjusted to
the point where there was enough gain to oscillate and thing "took off".

Fun stuff, if you have lots of time.




> The impression I got from my reading on the subjecct is that tuning an
> H-maser isn't something you can do incrementally.  It's not a conventional
> RF tank circuit -- there's a list of factors as long as your arm that have
> to be just right, or you will get nothing at all for your trouble.
> Getting
> those factors right seems to require a graduate-level understanding of
> both
> the materials and the math.  Ars longa, vita brevis.
>> -this maser has two vacuum sections, an outer that enclosed the cavity
>> and thermally insulates it. This allows the cavity temperature to be
>> regulated to within .001 degree C. The inner chamber keeps the cavity
>> evacuated so all you have in it is the H atoms.
> I imagine the cavity has to be pretty stout not to either collapse from
> barometric pressure or flex excessively.
>> I'll dig them up and see if anyone could host them on a website. (Files
>> are quite large!)
> It would be great if you could upload these to the Manuals page at
> www.ko4bb.com.  I've looked over the Symmetricom maser's manual but it's
> pretty terse.
>>> "Building Scientific Apparatus" by Moore, Davies, and Coplan
>> It's Davis, as in C.C. Davis of the University of Maryland, my former
>> MS thesis adviser.
>> Sorry about that... yes..
>> Its a great book, but very expensive.
>> worth every penny if you're doing this sort of thing..
> The current edition is $70 at Amazon, not too bad by textbook standards.
> It's a good book, but often more valuable as a pointer for further reading
> than as a practical, up-to-date handbook.  Seems like a good overview of
> vacuum technology.
> The book you really want to start with is Major's "The Quantum Beat,"
>>>In thinking about it, it would be a terrific project to run with
>>> LabView!
>>Rubbish, LabView would _never_ be able to do that.
> (Shrug) PLLs are PLLs.  I don't see a role for a PC in the cavity tuning
> or
> oscillator disciplining loops, but that's a very small part of the overall
> control picture.  Most of the actual software work would involve UI design
> for monitoring (and ideally graphing) the dozens of operating parameters
> over time.  It would probably make the most sense to use either analog or
> microcontroller-based controls for the realtime (RF) loops, and use
> Labview
> or another instrumentation package to monitor everything.
> There are also various high-latency thermal loops that could be controlled
> as well as monitored by Labview-like software on the PC.
> -- john, KE5FX
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