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 [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
