Square wave frequency modulation as a mean to reduce the effect of local oscillator instabilities in alkali vapor passive frequency standards

A quick review is given of mechanisms that can transfer local oscillator instabilities onto the output frequency of an atomic frequency standard, with the possible result of representing a limitation to its short term stability. The special case of an alkali vapor cell standard operated with a frequency control loop using square wave frequency modulation is then studied, to show that this solution has an advantage over sinewave modulation with respect to the problem at hand. The analysis is quasi-static. However it is argued that, contrary to the case of sinewave modulation, a dynamical analysis would come to the same conclusions.