[time-nuts] Rb resonance
cfmd at bredband.net
Sat Sep 17 11:39:34 EDT 2005
From: "Tom Van Baak" <tvb at leapsecond.com>
Subject: Re: [time-nuts] Rb resonance
Date: Sat, 17 Sep 2005 01:44:55 -0700
Message-ID: <000a01c5bb64$151d69a0$7d0ff204 at computer>
> > The second is defined in terms of the Cs133 resonance- hence there is
> > an integer number of Hz in the resonance frequency viz. 9192 631 770
> > Hz.
> A definition doesn't imply it has to be an integer. An
> inch, for example, is defined to be 25.4 mm.
Fortunatly, there is two definitions, the C.E. Johansson definition of 25.4 mm
which also became the internationally accepted one and then the US Metric law
one which derives backwards from a meter causing an inch to be 100/3937 of a
meter or in the neighborhood of 2,54000508 mm. Mmm, standards are good,
everybody has one up their sleve they are not aware of!
> > But what about the Rb resonance frequency? It is always quoted as
> > being an integer number of Hz as well- viz 6834 682 608 Hz. Is there
> > some aspect of the physics of these transition frequencies that means
> > the Rb resonance has to be an integer number of Hz, based on the Cs133
> > definition?
> > Or is the Rb resonance frequency, which is generally quoted as above,
> > actually rounded to be an integer number of Hz for convenience?
> > Perhaps a set of environmental conditions need to be specified as
> > well?
> > Regards,
> > Dave Brown, NZ
> Yes, depending on where you see the value it will be
> rounded. The most accurate measurement that I've
> seen for the Rb resonance is 6 834 682 612.904324 Hz.
> But it is common to see 6.8 GHz, 6 834 MHz, and
> 6 834 682 612 Hz as well. Not sure about the ...608
> vs. ...612 thing.
> For Hydrogen and Mercury frequencies see the last
> paragraph of:
> Note that due to a variety of factors atomic frequency
> standards don't actually operate right at their magic
> frequencies. Most Cs, for example, run a few Hz higher
> and the correction is transparently applied internally in
> a combination of hardware or firmware.
The same is done to the Rubidium cells to, basically derived back to the
C-field being non-zero just as for Cesium beams. By keeping the buffer gas
influences to a minimum and also compensating for the wall shift, the C-field
shift is used to both get a suitable frequency synthesis chain and a correction
range for errors which is both positive and negative. Frequency corrections is
done by varying the C-field. Naturally, any such shift needs to be compensated
for in primary standards where as for secondary standards the need is really
not the same.
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