Monitoring and Control of MEMS Tunable Filters Using Inductive Proximity Sensing

This paper presents a monitoring and control system for MEMS tunable evanescent-mode cavity filters. Inductive proximity sensing is utilized to detect the deflection of the tuning MEMS membrane. The sensed deflection is used as a feedback for a closed-loop control of the MEMS bias voltage to deliver the desired deflection and, hence, desired RF response based on an input from the user. The sensing spiral inductor also functions as an electrode for the electrostatic actuation of the membrane. The presented system is analyzed, optimized, and experimentally implemented on a bandstop filter tunable in the 30–42-GHz range. The measured results show that the system is capable of tuning the filter to the desired response with good repeatability of notch frequency (less than 1% error) and with stable long-term response (better than 1.6% drift), which falls within the 20-dB bandwidth of the filter.