Rubidium frequency standard with a high resolution digital synthesizer

The development and updated results for a rubidium frequency standard with a high-resolution digital synthesizer are described. A fraction frequency synthesizer is being incorporated in the frequency-lock loop (FLL) of the standard. This allows frequency settability in steps of 1.5×10^-12 over a range of 6×10^-9 without changing the C-field. A square-wave frequency modulation/demodulation is applied to the FLL. A C-field modulation which is alternated coherently with the frequency modulation is also applied. The AC C-field method is being used to reduce the magnetic field sensitivity. Both modulations give rise to transient signals which are related to adiabatic and nonadiabatic phenomena that occur in the Rb spin system. These transients and the means to eliminate them are described