Capacitance transducer with a point-like probe for receiving acoustic emissions

Abstract

A high-sensitivity capacitance transducer for the detection of ultrasonic signals has been developed. The transducer incorporates capacitance pickup circuitry designed by Radio Corporation of America (RCA) for their VideoDisc player. We use a 0.1-mm-diam platinum wire tip as the capacitance probe. This diameter is small compared to ultrasonic wavelengths. The wire forms part of a transmission line resonant circuit whose center frequency (910 MHz) is modulated by the signal capacitance. The signal capacitance is generated by the distance fluctuations between the probe and the vibrating sample surface. The probe is maintained in position at ∼0.1 μm from the sample surface by feedback control (with a 0.3-s time constant) of current supplied to a coil and magnet positioner. The frequency response has been measured between 100 kHz and 6 MHz and is flat in this range. Calibration tests using a step-function point source on an elastic half-space indicate a sensitivity of 7 mV/nm and a minimum displacement resolution of ∼10−11 m, limited by pickup circuit noise.