A limit to the frequency stability of passive frequency standards

It is shown that in passive frequency standards (e.g. cesium, rubidium, stored ion(s)), the noise of the interrogation signal at even multiples of the modulation frequency entails a limitation to the achievable frequency stability. The effect will be significant in laser-pumped future frequency standards, in which an improved signal-to-noise ratio is obtained. The frequency stability capability of future semiconductor-laser-pumped cesium beam tube or rubidium cell frequency standards being on the order of 10/sup -14/ tau /sup -1/2/ to 10/sup -13/ tau /sup -1/2/, the effect constitutes a very serious limitation. The same conclusion applies to microwave and optical frequency standards based on stored ions. It will therefore be necessary to reduce as much as possible the level of frequency or phase noise that the interrogation signal exhibits at Fourier frequencies equal to even multiples of the modulation frequency, unless a different processing of the resonator response can relax the requirement on the spectral purity.<>