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A Michelson-type unequal path interferometer for measuring the effective thickness of dielectric slabs is presented. Due to unequal paths, this interferometer is frequency dependent so that frequency tuning of the source controls the phase difference between the two paths and serves as a phase shifter. FM modulation of the source enables determination of the null point with an electronic sensitivity of 0.02°. High spatial resolution (≃λ/2) is achieved by spacing a suitable horn antenna close to the dielectric surface. Detailed design considerations and experimental results concerning an X-band interferometer are presented.