Abstract: Apparatus for measuring vibrations in turbines and like objects in which electromagnetic signals are directed at the object by a transmitter and reflections of the signals from the object are received by a signal receiver. The transmitted and received signals are continuously compared to provide a signal representative of the phase relationship therebetween. The latter signal is used in calibrating apparatus which further includes means for intermittently frequency modulating the transmitter oscillator and provides automatic control of the oscillator frequency to bring the mean frequency of the transmitted signal to a value such that the phase difference between the transmitted and received signals is equal to n.sup.. 180.degree., wherein n is an interger. Between the intermittent calibration periods, the signal representative of the phase relationship is applied to indicating means as indicative of the vibrations of the turbine.