A limit to the frequency stability of passive frequency standards due to an intermodulation effect

It is shown that in passive frequency standards, the fre- quency noise of the probing signal, at Fourier frequencies equal to even multiples of the modulation frequency, is translated into the frequency band of the selectively amplified resonator response. Then an addi- tional perturbation of the slaved frequency source arises, which sets a limit to its achievable frequency stability. A quantitative estimate of this limit is given for the first and the third harmonic locking tech- niques. As an example, numerical values are given assuming a good crystal oscillator' as a frequency source. It is concluded that the limi- tation considered is very serious and may impinge on the expected fre- quency stability of newly developed frequency standards in which the resonance line can be observed with an enhanced signal to noise ratio. Such is the case, for instance, in devices using laser optically pumped cesium beam, rubidium cell, or stored ion(+