Frequency stability of cell-based passive frequency standards: reducing the effects of local-oscillator PM noise

Experimental measurements in our present diode-laser-pumped Rb buffer-gas-cell frequency standard show that the sensitivity to microwave PM noise is approximately a factor of 10 lower when square-wave modulation is used in place of the more traditional sine-wave modulation. This result virtually eliminates what had been a serious limitation to the frequency stability attainable in cell-type standards. It also has ramifications for low-cost, commercial standards.