Abstract: Frequency stabilization of a beam type device is disclosed that is particry well suited for avoiding cavity phase shift problems. A beam generator provides a molecular or atomic beam that is directed through a pair of spaced and separate interaction regions to a detector. The two interaction regions, such as cavities, provide independent regions into which different signals are injected with the injected signals being derived from an oscillator such as a crystal oscillator. The signal coupled to the interaction regions may be varied according to a general equation. The first signal may also be coupled into one of the interaction regions at a frequency substantially the same as the resonance frequency of the particles in the beam directed through the cavity, while the signal coupled to the other cavity may be caused to vary above and below the resonance frequency.