CMOS Closed-Loop Control of MEMS Varactors for Robust Ka-Band Tunable Filters

A closed-loop capacitance sensing and control mixed-signal CMOS circuit with a dedicated sensor electrode and a proportional-integral controller was designed to have a sensitivity of 1 fF, a settling time of 200 μs, and a voltage capacity of 150 V. In a hybrid integration with a MEMS varactor, the control circuit could switch the varactor capacitance from 270 to 365 fF in 15 ms and hold it within ±2% between 25 °C and 80 °C. Furthermore, using the varactor as the tuning element of a substrate-integrated-waveguide (SIW) resonator, the control circuit could tune its resonance frequency from 33.2 to 33.9 GHz while keeping it within ±0.08% over a temperature range of 25 °C-90 °C. The control circuit occupied an area of 1.7 mm 2 and consumed 6 mW. The present closed-loop-controlled MEMS-tunable resonators could be used in a quartz SIW multi-pole filter to realize tunable, small, light, low-cost, robust, and reliable millimeter-wave filters that are critical to 5G and other future-generation communication and radar systems.