Sub-Attofarad Capacitance Sensor for High-Precision Sensing in LISA (Laser Interferometer Space Antenna)

This paper presents the design and performance of various sensing transformer versions for the Front End Electronics (FEE), which are utilized to sense capacitance with high precision for the LISA (Laser Interferometer Space Antenna) mission. LISA aims to detect gravitational waves in space, with a planned launch in 2035. The sensing capacitance reflects the LISA spacecraft's position relative to the gravitational reference, the TM (Test Mass). Although the technology demonstrator of LISA, LPF (LISA Pathfinder), was successful, minor system-level changes necessitated the reassessment of certain electronic components. A notable change is the increase in parasitic capacitance at the input of the sensing circuit. Without mitigation, this would reduce the resonant frequency of the sensing circuit, defined by the transformer inductance and its total input capacitance. Therefore, within the LISA framework, the LPF transformer design has been revised to achieve resonance at the injection frequency while maintaining or enhancing capacitance noise performance. We designed and manufactured 25 transformers using three different core types (P26×16-N48, P36×22-N48, B655813-N41) and three different inductance factors, i.e., core gaps between two core halves (A<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> = 400nH, 630nH, 1000nH). The transformers were assembled and tested with varying numbers of turns for each coil (48, 55, 56, 64, 69, 70, 80, 100) to achieve different coil inductances. The capacitance noise performances (0.51 aF/√Hz to 0.65 aF/√Hz) indicate potential improvement compared to the performance demonstrated in LPF (∼0.69 aF/√Hz). The measured performances are also compared with similar results recently presented in the literature.