CMOS Control Circuit for Robust MEMS Varactors

A CMOS closed-loop control circuit was designed, fabricated, and tested for order-of-magnitude improvement of the robustness of a MEMS varactor against ambient temperature change. The control is based on real-time sensing of the capacitance of the MEMS varactor and fine-tuning its bias voltage according to the difference between the sensed and targeted capacitances. Simulation shows that this difference can be sensed with femtofarad sensitivity and a linear output over a piccofarad dynamic range. Measurement shows that, in a hybrid integration of the CMOS control circuit and the MEMS varactor, the varactor capacitance can be kept within ±2% of the targeted value despite a temperature change from 25°C to 80 °C. When the MEMS varactor is used as the tuning element of a substrate-integrated-waveguide (SIW) resonator, the control circuit can maintain the resonance frequency within ±0.08% over the temperature variation.