Room and Low-Temperature Performance for a Sub-Attofarad Capacitance Sensor for Precision Measurements, Including Applications in Gravitational Wave Detectors

Abstract/Background

We describe the design principles, fabrication, and characterization of a precision AC capacitance sensor with a measured 0.33 aF/rtHz short term sensitivity, at 118 kHz bridge resonance frequency and at 1 Hz Fourier measurement frequency. The sensor is based on a high-Q resonant differential planar printed circuit board transformer that provides an excellent improvement in signal-to-noise of over two orders of magnitude, compared with a standard transformer-based capacitance bridge. Cryogenic operation of the transformer at 120 K resulted in a measured sensitivity of 0.21 aF/rtHz at 1 Hz, which, extrapolated to 3 K, would yield a capacitance sensitivity ~0.03 aF/rtHz. Repeating the experiment with a second transformer core yielded sensitivities of 0.42 aF/rtHz and 0.34 aF/rtHz at 293 K and 125 K respectively, providing a first order approximation of the variability of the sensor performance. 28-hours room temperature runs with the two cores demonstrated long-term sensitivities of 0.36 +/- 0.01 aF/rtHz and 0.46 +/-0.01 aF/rtHz, respectively.

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