Limitation of the frequency stability by local oscillator phasenoise: new investigations and natural improvements

摘要

In frequency standards in which the atoms have a continuousninteraction with the probe signal, local oscillator phase noise maynlimit medium term frequency stability. This spurious effect cannot bensuppressed whenever there Is any truncation in the spectrum of thenresonator response. Nevertheless, a simultaneous processing of the probensignal, similar to that of the NIST, and of the resonator response (bynmeans of an appropriate demodulation) makes it possible to reduce thisnlimiting effect. Previously achieved with a square wave frequencynmodulation, this result is now extended to various frequencynmodulations. An uncontrolled distortion in the demodulation waveform maynsignificantly degrade the performance. For the case of a square wavenphase modulation, the limiting effect also exists, but it is smallernthan for a frequency modulation. When the phase noise of the localnoscillator is naturally “not flat”, it is possible to easilynreduce the spurious effect: using the quasi-static approximation, onencan calculate various optimized demodulation waveforms and thencorresponding improvements. For the simplest optimized demodulation (fnM, 3fM), theoretical predictions arenexperimentally confirmed for flicker phase noise and flicker frequencynnoise