[time-nuts] Re: time-nuts Digest, Vol 14, Issue 28

Dave Brown tractorb at ihug.co.nz
Sun Sep 18 05:06:35 EDT 2005

Thanks for the informative replies, all.
Much appreciated.
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 
>> 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.
>   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
>   signal.
>   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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