Improving the short-term stability of laser pumped Rb clocks by reducing the effects of the interrogation oscillator

This paper studies the limitations due to the local oscillator on the short-term stability of a laser pumped rubidium frequency standard. Two effects have been considered: the increase of current noise on the resonance signal and the “aliasing” effect originated by the microwave phase noise. After reducing these two effects, the standard has demonstrated a short-term stability of σ_y(τ)=3.10^-13 τ^-1/2. We have measured the clock stability as a function of local oscillator phase modulation (PM) noise at harmonics of the sine wave modulation frequency. At present we are not aware of a complete theory for PM noise at Fourier frequencies that are very large compared to the line width. In some cases the PM noise at 100 times the nominal line width dominated the short term frequency stability. The details presented here should prove useful for testing a complete theory of this effect and give some insight into the design requirements of local oscillators for passive standards