Electrically coupled MEMS bandpass filters

This paper, the second of two parts, introduces electrostatic coupling of micromechanical resonators for implementation of high-order narrow-bandwidth bandpass filters. The concept of electrostatic coupling of closely spaced microresonators and its electromechanical modeling is presented. Electrostatic coupling of resonators does not require any distinct physical coupling elements. The electrostatic coupling strength between the resonators is sharply dependant on the applied polarization voltages to the resonators. Hence the coupling strength and consequently filter bandwidth can be tuned over a wide range after fabrication. It is shown that electrostatic coupling provides the highest degree of tunability and design flexibility among the available microresonator coupling approaches. Low frequency single crystal silicon prototypes of second order electrostatically coupled filters are fabricated and characterized. Filter quality factor as high as 6800 (0.015% BW) with more than one decade of bandwidth tunability is demonstrated for a second order electrostatically coupled beam filter at center frequency of 170 kHz.