[time-nuts] PTTI 2012, part 2

Bill Dailey docdailey at gmail.com
Tue Dec 4 14:14:11 UTC 2012


I used to work on oxygen clean systems... that cleaning is done primarily
with liquid freon (R-113 and ultrasound - may be hard to get now).  The
Vacuum stuff is expensive but off the shelf.  The complicated part to me is
the lasers and microwave... in addition to the super-stable oscillator you
need to take maximum advantage of this stuff.   Seems like a lot more than
plumbing to me.  If you look at the papers on portable rubidium fountains
they are significantly bigger than a shoebox (65 cm).

Doc
KX0O


On Tue, Dec 4, 2012 at 8:06 AM, paul swed <paulswedb at gmail.com> wrote:

> OK now that I am setting out to build one. Would the challenge for a
> amateur be that the components you build with are basically dirty. Some how
> on the copper pipe it would need to be clean and then brazed I might guess.
> All of that makes for a dirty element.
> To the vacuum. I used to make vaccuum pumps out of old refrigerator motors.
> That would be the first stage of pump down. But how do you take it down
> below that? Then I speculate you use a ion pump to get rid of the stuff
> that remains.
> Regards
> Paul.
>
> On Tue, Dec 4, 2012 at 7:37 AM, Bob Camp <lists at rtty.us> wrote:
>
> > Hi
> >
> > I went through a similar process quite a while ago. The dimensions of the
> > actual fountain can be quite small. One could make one the size of a shoe
> > box and still have it perform quite well.
> >
> > Bob
> >
> > On Dec 3, 2012, at 2:07 PM, Poul-Henning Kamp <phk at phk.freebsd.dk>
> wrote:
> >
> > > --------
> > > In message <AB5B0278225B4BD483382A39E6834203 at pc52>, "Tom Van Baak"
> > writes:
> > >
> > >> - USNO rubidium fountains
> > >>
> > >> While many national labs have developed cesium fountains (for
> > >> accuracy), USNO has been gradually building rubidium fountain clocks
> > >> (for stability) and 4 of them are now fully operational. The ADEV
> > >> of these clocks gets well under 1e-16. The paper will have all the
> > >> details but the note I made was that with 20 months of data, the
> > >> stability was near 5e-17 at tau 4 months. That's 100x better than
> > >> a commercial cesium standard; better than all of USNO's other 70
> > >> cesium clocks and 15 H-masers combined. Yes, I've added "rubidium
> > >> fountain" to my automated eBay searches.
> > >
> > > I happened to miss a turn (or something...) and stumbled into the
> > > building where they keep those fountains when I visited USNO some
> > > months ago.  WhatI found most remarkable about them were how compact
> > > they were, I still expected fountains to be room size, but these
> > > were rack-sized.
> > >
> > > I asked what the material cost would be and if a competent amateur
> > > would be able to do something like that, and the clear message was
> > > that the single biggest problem was the vacuum for a vessel that
> > > size (when you can't use ferromagmnetic materials) and getting
> > > the optical bench calibrated.  "Apart from that it's just some
> > > plumbing"
> > >
> > >> - Loran/UrsaNav
> > >>
> > >> CW instead of very low duty cycle Loran pulses would improve S/N [...]
> > >
> > > Actually, it probably will not.
> > >
> > > The one smart thing about the LORAN signal is S/N, which means that
> > > LORAN for timing purposes is incredible insensitive to noise and
> > > at the same time, incredible transmitter power economy.
> > >
> > > The one caveat is that the GRI has to be a good number, preferably
> > > a four (or more!) digit prime number.
> > >
> > > (You need to grok moduls-arithmetic to really appreciate this, but
> > > its the magnitude of the prime factors of the product of the GRI
> > > and the disturbing CW which counts:  The smaller the are, the harder
> > > it is to filter the CW-RFI out.)
> > >
> > > This is why Europe switched to 4-digit GRIs and almost totally solved
> > > CW-RFI by doing so, and why the Russian Chayka at GRI 8000 is totally
> > > useless near anything resembling a transmitter.
> > >
> > >
> > > --
> > > Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> > > phk at FreeBSD.ORG         | TCP/IP since RFC 956
> > > FreeBSD committer       | BSD since 4.3-tahoe
> > > Never attribute to malice what can adequately be explained by
> > incompetence.
> > >
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-- 
Doc

Bill Dailey
KXØO


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