[time-nuts] How Rubidiums make their frequency

Matt Ettus boyscout at gmail.com
Wed Apr 19 19:55:01 UTC 2006


Since we can now make DDS's with arbitrary frequency resolution, could
you make an Rb oscillator without the magnetic field adjustment? 
Wouldn't that reduce a source of error in frequency?  Then we'd be
left with the ideal resonance frequency, right?

Are there any other influences on the resonance frequency?  I assume
temperature and density don't matter.

Matt

On 4/19/06, Tom Clark, K3IO (ex W3IWI) <K3IO at verizon.net> wrote:
>
>
>
>    Christopher Hoover asked:
>
> one issue remains:   i have to crank the magnetic field setting almost to
> its high limit (9.91/10.00) to get 5 MHz out; lower settings give a
> frequency that is too low.   i presume this is unusual.
>
> i have a rudimentary understanding of the rubidium oscillator physics, but i
> do not understand what would cause this.  can i buy a clue?
>
>
>    I don't know the Tracor, but I imagine it is like most of the other
>    Rubidiums in it's innards.
>    Inside the physics package of a Rb, a cell with some Rubidium is
>    heated to (that's why Rb's run not!) enough so that it is turned into
>    a gas. Both light and microwaves illuminate the cell. If no magnetic
>    field is present on the cell, the Rb gas has a hyperfine resonance
>    (the difference in frequency between two infrared transitions of the
>    Rb gas) at 6.8346826128 Mhz. When a magnetic field is imposed, the
>    energy difference between the two hyperfine states changes.
>    In the RF part of the signal path (here, the block digram of a typical
>    Rb standard helps. See Page 3 of [1]this Symmetricom White Paper .)
>    Let's start with some convenient oscillator at, let's say 10 MHz.
>    Multiply it up to 60 MHz and then hit a Step Recovery Diode to get the
>    114th harmonic at 6840 MHz.
>    Then difference between the 6834.. and 6840 MHz is 5.31738+ MHz. In
>    the standard Rb configuration, we apply a magnetic "C-field" to bring
>    the difference frequency upwards by 4.89 kHz to 5.31250000 MHz which
>    happens to be  5MHz + 5/16MHz. Back in the early days, we didn't have
>    nice programmable DDS chips, but simple dividers/multipliers could
>    make the 5/16 MHz "adder".
>    So what you are doing by tweaking the magnetic field to shift the RF
>    resonance of the Rb cell so that it matches the arithmetic "quirk"
>    that the 6834 MHz is almost contains the neat 5/16 MHz in the tail-end
>    digits.
>    Hope that helped -- 73, Tom
>
> References
>
>    1. http://www.symmttm.com/pdf/Precision_Frequency_References/wp_mmrfs.pdf
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