Differential readout for a magnetic gradiometer

A magnetic gradiometer system being developed by the authors uses a vibrating string as its sensing element. The next generation of sensor will use a wide ribbon instead of a string. In order to precisely measure the displacement of the ribbon we have designed a new differential capacitive readout with displacement noise below 10−1310−13 m/Hz. Two capacitive plates are placed at the one quarter and three quarter positions adjacent to the ribbon. An inductor is added in series to make an LC series circuit. Displacement is determined by measuring the phase. From a scan of phase versus frequency, the linear dynamic range is measured to be ∼360∼360 nm. The LC series readout is highly immune to stray capacitance and is designed for use with long cables in a magnetic gradiometer system.