[time-nuts] Re: time-nuts Digest, Vol 14, Issue 28
Tom Clark, W3IWI
w3iwi at toad.net
Sat Sep 17 19:22:11 EDT 2005
Tom van Baak, responding to Dave Brown noted:
> 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
> 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
> 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.
The hyperfine transition that defines the frequencies of all the
atomic standards are subject to the magnetic field in the atomic
"cloud"/RF interaction region. Cesium is a good reproducible reference
standard because the magnetic interaction is small.
In the case of Rubidium, the "C-field" magnetic field is used to tune
the Rb cell slightly off frequency to make it possible to lock a clean
crystal signal to the atomic signal. In most Rb standards I know of,
the electronics are configured so that the tail-end digits of the
frequency are equal to 5/16 MHz (xxx.3125 MHz) so that the RF
frequency can be easily generated. The frequency knob that you tweak
to correct the Rb frequency passes some tens of ma thru a coil
surrounding the RF interaction region. If you try to phase lock a Rb
to GPS, you need to develop a current source error signal.
Another Rb FYI item. In earlier Rb designs, the xtal oscillator was
given a slight frequency modulation, so that the frequency wobbles
over a few Hz around the "proper" frequency. The optical detector
would the look for the phase of optical level modulation and center
the xtal on the Rb transition. This causes many of the older Rb
standards (including 5061s) to have a lot of RF phase noise when the
5/10 MHz signal is multiplied up into the microwaves. Amateur
microwave enthusiasts have learned that they need to phase-lock their
radios with time constants ~1 second in order to improve the ratty
In some later standards (including even the inexpensive SRS unit), the
tail-end frequency and FM are generated by a DDS driven from a "clean"
unmodulated xtal and are not subject to as much noise.
Regards, Tom Clark
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