LIMITATION OF THE CLOCK FREQUENCY STABILITY BY THE INTERROGATION FREQUENCY NOISE - EXPERIMENTAL RESULTS

Recent advances in atomic physics will make it possible to soon reach frequency stabilities in the range 10(-14)-10(15) tau-1/2 for passive atomic frequency standards. However, it is now recognized that the frequency noise in the interrogating microwave signal limits the frequency stability, approximately to the range 10(-12)-10(-13) tau-1/2, depending on the spectral purity of the VCXO and on the modulation frequency f(M). In order to experimentally investigate this effect, we have realized two identical microwave generators at 9192 MHz. These two microwave generators enable us to measure the PSD of their frequency fluctuations with or without additive frequency noise, and to separately characterize 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 has been 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.