Capacitive-Based, Closed-Loop Frequency Control of Substrate-Integrated Cavity Tunable Filters

In this paper, we present an integrated capacitive sensing technique that can be used to achieve closed-loop feedback control of variable capacitors. The technique is well suited for diaphragm-based actuators that can be used to tune the center frequency of microwave reconfigurable filters. A commercially available capaci-tance-to-digital converter is used to measure the capacitance of a custom-made monitoring capacitor. This capacitor is completely external to the filter structure and does not contribute to any added losses. A tunable, second-order, bandpass filter –using substrate-integrated, evanescent-mode cavity resonators– is created to demonstrate the concept. The frequency tuning is achieved using piezoelectric actuators to displace a flexible copper diaphragm that forms the top part of the loading capacitor. The monitoring capacitor consists of a second cavity that is mounted above the actuator forming a second air-filled, metal-insulator-metal capacitor. There exists a direct relationship between the monitoring capacitance and the loading capacitance in the evanescent-mode cavity. Therefore, by tracking the monitoring capacitance the center frequency of the filter can be monitored, which allows for direct in-situ closed-loop control of the filter. An algorithm for the tuning operation is presented, which includes an automatic calibration technique to initialize the controller. The effectiveness and repeatability of the technique is evaluated as the filter is tuned from 3.3 GHz to 3.7 GHz. Having stable feedback control integrated with this type of evanescent-mode cavity filters, brings the technology one step closer to actual fielding.