| Closed-loop control of RF MEMS capacitive switches was demonstrated by using an intelligent CMOS circuit. The control was based on fine-tuning the bias magnitude of the switches according to the difference between sensed and targeted capacitances. Innovative designs were used to allow the CMOS circuit to sense low capacitance of tens of femto-farats, and to handle high voltage up to ±40 V.;Simulations were carried out to further study and optimize the CMOS circuit performance in terms of control sensitivity, speed, and high-voltage handling capability. A fast actuation/deactuation speed of 15 μs was achieved in simulation, with a capacitance sensing resolution of about 13 fF, which corresponds to a ±1% control accuracy of a switch with 550-fF down-state capacitance.;The control circuit was implemented in 0.5-μm CMOS silicon-on-sapphire technology. The CMOS die occupied an area of 3×1.5 mm2, which was dominated by input/output and voltage regulation/protection circuits; the actual capacitance sense/control circuit was smaller than 0.1 mm2. The entire circuit consumed 0.7 mW of power during active sense/control, which could be significantly reduced with less frequent sense/control and more advanced CMOS technology.;For expeditious proof of such closed-loop control concept, hybrid integration approach was adopted, which significantly increased the parasitics and degraded the circuit sensitivity. Nevertheless, a control accuracy of ±2.5% was demonstrated on MEMS switches with 550-fF down-state capacitance. Under accelerated dielectric charging test, the circuit could intelligently adjust the bias voltage to compensate the switch dielectric charging effect and hold MEMS switch capacitance at the target value. Intelligence was also programmed to alternate the bias sign when its magnitude required to maintain the targeted capacitance drifted significantly due to dielectric charging. As a result, indefinitely operation of RF MEMS capacitive switch despite dielectric charging could be realized.;Such intelligent control could also be used to compensate for process variation, material creep, ambient temperature change, and RF power loading, which would make MEMS capacitive switches not only more reliable, but also more robust. |
