7th order sharp-roll-off bridged micromechanical filter

Abstract

A 7th-order capacitive-gap transduced 8-MHz microme-chanical filter has been demonstrated with a channel-selecting bandwidth of 24 kHz (0.3%) and a shape factor of 1.45, which bests the previous mark of 1.86 for similar frequency MEMS-based filters. This shape factor arises from not only the sheer order of the filter, governed by seven coupled clamped-clamped beam resonators, but also from strategic bridging of its non-adjacent 1st, 4th, and 7th resonators to generate loss poles that further steepen the roll-off from passband to stopband. Interestingly, it is the extremely strong electromechanical coupling on the order of keff2~ (Cx/C0) ~17% offered by capacitive-gap transduction at HF that allows the use of so many resonators, thereby enabling this filter. The steepness of this filter's roll-off greatly increases the density of available channels in HF radios for military and Ham applications, as well as future sensor network applications enabled by these results.