Bridged micromechanical filters

Abstract

High-order micromechanical filters comprised of 3 to 4 mechanically coupled resonators, and employing for the first time "bridging" between nonadjacent resonators to affect loss poles for better filter shape factors, have been demonstrated using a polysilicon surface micromachining technology with insertion loss less than 3 dB, sharper passband-to-stopband roll-offs than achievable by equivalent non-bridged filters, and around 50 dB of stopband rejection - 15 dB better than a previous 2-resonator filter design. The filters of this work consist of several clamped-clamped beam resonators with a more general coupling scheme than previously used, where non-adjacent, as well as adjacent, resonators are coupled to generate loss poles, leading to frequency characteristics with faster roll-offs from passband to stopband and greater stopband rejections. Via this bridging design approach, 20 dB shape factors down to 1.95 have been achieved for filters centered at 9 MHz using only 3 resonators.