[time-nuts] Frequency processing scheme of HP5065 vapour rubidium standard

[Magnus Danielson]

From: Normand Martel <martelno at yahoo.com>
Subject: Re: [time-nuts] Frequency processing scheme of HP5065 vapour rubidium standard
Date: Mon, 6 Nov 2006 16:28:47 -0800 (PST)
Message-ID: <20061107002847.49574.qmail at web50707.mail.yahoo.com>

> 
> 
> --- Ulrich Bangert <df6jb at ulrich-bangert.de> wrote:
> 
> > John,
> > 
> > > offset that changed annually.  IIRC, it was
> > typically something like 
> > > 300x10e-10.
> > 
> > Agreed! And that is what the manual says its good
> > for! 
> > 
> > However, the question remains why different physics
> > packages need
> > DIFFERENT thumbwheel settings in order to achieve
> > the SAME time scale.
> > 
> > 73 de Ulrich, DF6JB
> > 
> I've read the answer last week on the forum... (sorry
> i don't remember who gave the answer...)
> 
> It's simple: The technology used on cesiums CAN NOT be
> applioed on Rubidiums. With Cesium, high end
> techniologies like fountains or beans can be used, and
> if i understand well, with these technologies, PURE
> CESIUM is used, so there is NOTHING that can affect or
> "pull" the resonance of cesium from it's natural
> resonance. (9 192 631 770 Hz).

Wrong.

Rubidum beams and fountains is possible. Infact, recent research have shown
that Rubidium is better suited for fountains than Cesium is, so it outperforms
Cesium.

There is frequency pulling even in Cesium beams. Only a few beams handles the
phase error pulling by reverting the beam direction. Fountains have this
together with doppler reduction.

> With rubidium, buffer gases must be used, and these
> gasses "pull away" the resonance away from Rubidium's
> natural resonance. And that is the source of all our
> problems with rubidium: Buffer gas/rubidium
> evaporation/absorption will affect the frequency
> "pulling", thus the drift in Rubidiums. This also
> explins why two rubidiums clocks wiill have different
> settings (Rb/buffer mixture tolerances). With Cesium,
> no pulling, with rubidium, some pulling. That's why
> Rubidiums aren't primary standards. If we could build
> a Rb (or ANY OTHER element) atomic clock with the same
> technology that we use with Cs (total element purity),
> there wouldn't be any pulling over time and Rb's would
> be considered primary standards.

The Rubidium was found especially suited for the gas cell atomic standard.
The gas cell atomic standard is cheap, but the inherent wall shift is so large
that it prohibits repeatability of the same degree as the beam standards.
Later buffert gas mixtures and pressure was found to balance up the wall shift,
but it still prohibits the same repeatability in frequency.

Currently work on Cesium gas cells is being performed to reach chip scale
atomic clocks, but to the best of my knowledge they also suffer from wall
shift.

We could be measuring the SI second in Thallium, which according to those days
investigations proved even more suitable, but it was judged harder to acheive
the repeatability necessary in the technology of late 50s and early 60s so
Cesium was the winner. All to the best of my knowledge.

So, again, do not confuse the atomic standard measuring/realisation methods
with that of the various atoms, and the usual mappings. Certain combinations of
methods and atoms have proven especially suited for various purposes.

Toss in stuff like ion traps with all the fancy cooling stuff. Maybe we will go
for Mercury 199 ions and the laser combs on top of that.

Cheers,
Magnus