Limitation of the clock frequency stability by the interrogation frequency noise: experimental results

Recent advances in atomic physics will make it possible to reach frequency stabilities in the range of 10^-14-10¹⁵ τ^-1/2 for passive atomic frequency standards. However, it is recognized that the frequency noise in the interrogating microwave signal limits the frequency stability, approximately to the range 10^-12-10^-13 τ^-1/2, depending on the spectral purity of the VCXO and on the modulation frequency f _M. In order to investigate this effect experimentally, two identical microwave generators are realized at 9192 MHz. These two microwave generators enable one to measure the power spectral density (PSD) of their frequency fluctuations with or without additive frequency noise, and to characterize separately the limit of the frequency stability related to the interrogation frequency noise. The relation between the microwave frequency noise at 2nf_M and the frequency stability limit is verified: experimental results very precisely confirm the predictions of the model for slow modulation frequencies. In any case, the spurious effect is related only to the frequency noise present in the microwave interrogation at Fourier frequencies which are even harmonics of the modulation frequency