Device for indicating changes in the position of an object

A device for indicating vibrations of an object comprising

• a signal transmitter including an oscillator for emitting electromagnetic signals toward the object,
• a receiver for receiving reflections of the transmitted signals from the object,
• a phase comparing circuit energized in accordance from the transmitted and received signals respectively to provide an output signal indicative of the phase relationship therebetween,
• indicating means for connection to said phase comparing circuit,
• means for intermittently frequency modulating the oscillator of said signal transmitter by a symmetric sweep signal over a frequency range small compared to the frequency of the transmitted signal,
• the frequency range of said means for intermittently frequency modulating the oscillator being such that the phase difference between the transmitted signal and the reflected signal at said receiver caused by said means somewhat exceeds 180.degree., and
• means responsive to the output of said phase comparing circuit and the means for intermittently frequency modulating the oscillator to set the oscillator to such a frequency that during the periods between the intermittent frequency the phase difference between the transmitted and received signals is n .sup.. 180.degree. where n is an integer, and the output of said phase comparing circuit during said periods is representative of the vibrations of the object.