Reducing the effect of interrogation phase noise on the frequency stability in passive frequency standards: experimental results and new investigation means

It is known that the local oscillator phase noise limits the frequency stability in passive frequency standards. When the probe signal is permanently applied to the atomic resonator, this limitation is due to an intermodulation effect. We have established that, with a square wave frequency modulation, a proper signal processing using a notch filter in the synthesis chain and a peculiar demodulation waveform significantly reduces the limiting effect. Experimental tests confirm the optimization of the performance by the theoretical demodulation waveforms. The previous State of the Art is improved by a factor of 4.5. At last, in order to determine the optimum signal processing for different modulation conditions, we have realized an automatic system. It compares the frequency stabilities obtained for various signal processing conditions, it increases the number of harmonics involved in the demodulation and changes their rates. This system makes it possible to determine the minimum limiting effect on frequency stability, in passive frequency standards where atoms have a permanent interaction with the probe signal.