[time-nuts] Re: time-nuts Digest, Vol 14, Issue 28
tractorb at ihug.co.nz
Sun Sep 18 05:06:35 EDT 2005
Thanks for the informative replies, all.
Interested to see your comments (TomC) ref the methods of fine tuning
the Rb cell-that in fact was where this train of thought all started
as I was wondering about applying an EFT arrangement to an existing Rb
unit for GPS locking. Got sidetracked on the 'integer Hz' issue
Dave Brown, NZ
----- Original Message -----
From: "Tom Clark, W3IWI" <w3iwi at toad.net>
To: <time-nuts at febo.com>
Sent: Sunday, September 18, 2005 11:22 AM
Subject: [time-nuts] Re: time-nuts Digest, Vol 14, Issue 28
> 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
>> some aspect of the physics of these transition frequencies that
>> the Rb resonance has to be an integer number of Hz, based on the
>> Or is the Rb resonance frequency, which is generally quoted as
>> 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
> standard because the magnetic interaction is small.
> In the case of Rubidium, the "C-field" magnetic field is used to
> the Rb cell slightly off frequency to make it possible to lock a
> crystal signal to the atomic signal. In most Rb standards I know
> 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
> 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
> to GPS, you need to develop a current source error signal.
> Another Rb FYI item. In earlier Rb designs, the xtal oscillator
> 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
> 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
> 5/10 MHz signal is multiplied up into the microwaves. Amateur
> microwave enthusiasts have learned that they need to phase-lock
> radios with time constants ~1 second in order to improve the ratty
> In some later standards (including even the inexpensive SRS unit),
> tail-end frequency and FM are generated by a DDS driven from a
> unmodulated xtal and are not subject to as much noise.
> Regards, Tom Clark
> time-nuts mailing list
> time-nuts at febo.com
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