Limitation of the frequency stability by local oscillator phase noise: new investigations

In passive frequency standards where atoms have a permanent interaction with the probe signal, the local oscillator phase noise limits the short and medium term frequency stability. It has been shown that this spurious effect cannot be suppressed whenever there is any truncation in the spectrum of the resonator response. However, a simultaneous processing of the probe signal, similar to that of the NIST, and of the resonator response-by means of an appropriate demodulation waveform-makes it possible to significantly reduce the limiting effect. First achieved with a square wave frequency modulation, this result is now extended to various modulation waveforms. It is shown that, for various modulation waveforms, the use of an optimized demodulation provides a moderate additional improvement to a moderate 2f _M rejection: this result confirms that, for a cesium clock using a slow frequency modulation, a high 2f_M rejection and a proper demodulation are necessary to strongly reduce the limiting effect. It is also shown that a non controlled distortion in the demodulation waveform may degrade the performance. At last, for a square wave frequency modulation and in the case of a flicker phase noise, a convenient demodulation reduces the spurious effect by a few dBs without any rejection in the microwave spectrum