Precision differential capacitor

A 4-electrode multi-port precision differential capacitor for testing the gravitational reference sensor (GRS) electronics of the ST7 spacecraft has been built and tested. The differential capacitor has a tunable and intentional mismatch of 2 aF out of ∼2 pF total nominal capacitance, and holds stable for time-periods on the order of hours. pacitor has two electrodes representing a pair of nominally identical drive plates, one electrode representing the metallic proof-mass, and one electrode acting as a non-contact pickup. The required adjustability has been achieved by introducing two pairs of grounded micrometers located between the drive and the ‘proof-mass’ electrodes for making coarse and fine adjustments. The capacitance is changed by advancing or retracting the micrometer spindles in the electric flux path to alter the capacitive coupling of each arm. The change in capacitance by coarse adjustment is 30 fF for each arm of the capacitor. The fine adjustment has a range of 1 fF with a setting resolution of 2 aF per arm. This mechanism of changing the capacitance achieves the fine control, repeatability and stability required to test the ST7 electronics.