Two-frequency microwave interferometer with an error-free phase indication

A microwave interferometer is described in which the microwave generator operates successively at two nearby frequencies. The measured plasma density is determined from the phase difference at these frequencies. Homodyne frequency conversion is used in the interferometer and an electronic switch serves to distinguish intermediate frequencies. The frequency difference is chosen so that the measured phase is less than 360{degrees}. Hence there are no errors to affect subsequent values of the measured phase. 3 refs., 3 figs.