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) <[EMAIL PROTECTED]> 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 > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts