Limitation of the frequency stability by local oscillator phase noise: Newinvestigations and natural improvements

In frequency standards in which the atoms have a continuous interaction wit h the probe signal, local oscillator phase noise may limit medium term freq uency stability. This spurious effect cannot be suppressed whenever there i s any truncation in the spectrum of the resonator response. Nevertheless, a simultaneous processing of the probe signal, similar to that of the NIST, and of the resonator response (by means of an appropriate demodulation) mak es it possible to reduce this limiting effect. Previously achieved with a s quare wave frequency modulation, this result is now extended to various fre quency modulations. An uncontrolled distortion in the demodulation waveform may significantly degrade the performance. For the case of a square wave p hase modulation, the limiting effect also exists, but it is smaller than fo r a frequency modulation. When the phase noise of the local oscillator is naturally "not flat," it is possible to easily reduce the spurious effect: using the quasi-static appr oximation, one can calculate various optimized demodulation waveforms and t he corresponding improvements. For the simplest optimized demodulation (f(M ),3f(M)), theoretical predictions are experimentally confirmed for flicker phase noise and flicker frequency noise.