I. Introduction
Microwave interferometry is an ideal means for displacement measurement in various engineering applications. This is due to its ability to provide fast noncontact measurements, applicability to dusty or smoky environments (as distinct from laser Doppler sensors or vision-based systems using digital image processing techniques), and simple hardware implementation. In microwave interferometry, the displacement of the object under measurement (target) is extracted from the phase shift between the signal reflected from the target and the reference signal. At present, this phase shift is usually determined using special hardware incorporating a power divider and a phase-detecting processor, which is an analog [1] or a digital [2] quadrature mixer. In doing so, measures have to be taken to minimize the nonlinear phase response of the quadrature mixer, which is caused by its phase and amplitude unbalances.