[time-nuts] Cs tube pics

Attila Kinali attila at kinali.ch
Wed Nov 2 05:41:03 EDT 2016

On Tue, 1 Nov 2016 09:10:25 -0700
"Tom Van Baak" <tvb at LeapSecond.com> wrote:

> > I really would like to do that. But they are a tad bit expensive.
> > Especially on this side of the big pond. If anyone is willing
> > to part with a Cs standard and want to have it a good home, feel
> > free to contact me :-)
> It's true that a cheap GPS receiver is more accurate long-term than
> a surplus cesium standard, but you're right about the "just because" part.
> To me at least, this time nut hobby is not so much about the pursuit of
> accuracy as it is an appreciation for the variety, ingenuity and complexity
> of timekeeping. In some cases "how it works" is far more interesting that
> "does it work".

I think, most of us are in it for the "how it works" and
"how far can I push it". :-)

> A used but known working cesium standard can be expensive, but like most of 
> you almost all my gear comes from eBay via automated daily keyword searches. 
> Many of my mil- or telecom-surplus FTS 4050 and HP 5061 were obtained for 
> just a couple hundred dollars. You may search for months or even years, but 
> amazing bargains show up. Not all of them work, of course, but the op/svc 
> manuals are superb, the design / construction is very repair-friendly, and 
> there's a weird group called time nuts with helpful advice.

It still is prohibitevly expensive in Europe. There are much fewer
Cs standards going around than in the US in the first place, and there
is also less a tinkering mentality. Ie a lot of companies just say
"it's broken, it's no use for anyone anymore, let's just scrap it" and
thus a lot of stuff ends up in recycling instead of on ebay. Heck, a couple
of years ago i got an 3458 because the company wanted to throw it away.
Mind you, it was in full working condition, only the NVRAM batteries were low.

I still would like to try to build my own atomic clock at some point,
even if it would be a quite costly, and a many years project. 

IMHO the easiest to build would be an Rb or Cs vapor cell using either
dual resonance or coherent population trapping. Cells can be bought
for 300-500€. For a bit more you can get them made to spec. Machining
a resonant cavity from aluminium is pretty easy and cheap these days,
if one wants to go for the dual resonance. The biggest issue for both
types would be the laser system. Either getting the laser diodes selected
(makes them expensive) or build an external cavity for them (creates
the need of a complex control system and not so simple mechanically).
Putting all toghether would probably cost something in the order of
1000€ to 5000€.

One up in difficulty would be a passive hydrogen maser. This requires a
vacuum system and things like platinum leaks to generate atomar hydrogen
and state selection magnets. If one knows glass blowing, part of it can
be made using pyrex tubes, which simplifies some stuff (like keeping the
state selection magnets outside the vacuum system). Also, the cavity
needed would be quite big. A normaly used TE011 cavity is huge. One can
load it with aluminia and get it down to 15-20cm diameter, but this requires
crystaline aluminia to maintain low loss. Maybe one could use other resonating
structures that are smaller, TE111 or loop-gap resonators have been proposed.
The biggest cost here is definitely the needed vacuum system. Although
the rough pumps are rather cheap (around 500-1000€ if one does not need
fast pumping) and some of these actually end up on ebay without being
destryoed by "testing", the high vacuum pumps (ion pumps, turbo pumps etc)
are not cheap and need to bought new (the stuff you see on ebay are either
systems that were removed from labs because they don't work anymore, or
were destroyed by "testing" them in free air).
An active hydrogen maser should not be that much more difficult. It mostly
involves a low loss, correctly tuned cavity and low noise detection electronics.
The input stream of hydrogen atoms needs to be more precisely controlled as

Another step up in difficulty would be a system using a magneto optical trap.
Glass cavities with flanges for vacuum system are readily available and
also cheap. Again, the vacuum system is one difficulty, though probably
simpler than for the hydrogen maser (less parts that need to be custom made).
But the requirements for the vacuum are a bit higher. The laser system poses
a similar difficult as with the CPT system above, but now there are more
lasers and all need to be locked to eachother. The traping laser also need
to be directed at the cavity from 6 sides, all meeting in the center of the
cavity, which makes alignment problematic. This is also the first system
that offers to be a primary standard, although it probably does not get to
the stability of a 5071, as the trapping lasers will induce a light shift
that is not so easy to control unless one goes for the expensive laboratory
grade lasers. 

Next on the list would be an Hg ion trap. But there are so many parts
in such a system that are not easily bought and require a very good
understanding of the physics involved to design custom parts, that
it becomes almost unrealistic that an amateur could be build one at home.
Same goes for ion and neutral atom optical clocks. Yes, they can be build,
yes the principle is simple, but getting it actually to work needs a lot
of understanding and tinkering.

Ah... I'm dreaming again :-)

				Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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