A Micro-Displacement Meter
R.F. Post, R. L. Howard · The Journal of the Acoustical Society of America · 1947
This paper deals with an equipment capable of detecting and measuring the amplitude of vibration of small surfaces (such as the vibrating faces of ultrasonic piezoelectric crystal assemblies) when the displacements are less than one micro-inch, with the introduction of negligible acoustic loading. The usable frequency range is 100 c.p.s. to 100 kc. The operation of the micro-displacement meter is based on the frequency-modulation technique of measurement, previously described. The method consists of determining the amount of frequency modulation produced in a vacuum-tube oscillator circuit which is coupled electro-mechanically to the vibrating system under measurement. The equipment employs a frequency-modulated oscillator or simple construction, so designed that the vibrating system is electromagnetically at the open end of a quarter-wave line—a point of very high impedance. Thus the vibrations produce a change in the terminating impedance of the resonent line, which in turn results in a large change in the oscillator frequency. The sensitivity of the system is also enhanced by the relatively high oscillator frequency (500 megacycles) which is used. At this frequency small percentage frequency variations correspond to sizeable absolute variations in frequency. The design is applicable for use in many related problems, such as the measurement of shock waves, the “probing” of acoustic fields, and employment in an extremely lightweight phonograph pick-up.