Cryogenic front-end circuit for capacitive sensing in superconducting gravimeters

Capacitive sensors are commonly used in superconducting gravimeters due to their high resolution and low drift. This study developed a cryogenic front-end circuit for superconducting gravimeters to reduce the negative effects of parasitic capacitance on capacitive sensors. The front-end circuit comprises a noiseless superconducting transformer and a low-noise cryogenic preamplifier, both of which are positioned adjacent to the capacitive sensor probe. Compared with the front-end circuit operating at 300 K, the transfer coefficient of the front-end circuit increases from 131 to 1070 V/m, and the equivalent displacement noise reduces from 1.4 × 10-10 to 5.0 × 10-11 m/Hz1/2 within a frequency band from 10-3 to 1 Hz. The temperature coefficient of the cryogenic preamplifier is 0.3%/K, and the superconducting transformer's matching factor has adjusted as low as (2.4 ± 0.2) × 10-4. The cryogenic front-end circuit was finally applied to a superconducting gravimeter. The observed gravity data satisfactorily fit the theoretical tidal model, implying good long-term stability for the developed front-end circuit.