A Hybrid Ladder Filter with Enlarged Bandwidth by Using Acoustic-Wave-Lumped-Element Resonators

SummaryIn this paper, a new class of hybrid filters with enlarged bandwidth while keeping the steepness of the transition is proposed. The proposed hybrid filter is a ladder structure, with an acoustic wave lumped element resonator (AWLR) connected in each branch. The AWLR, which can break the limit of the electromechanical coupling coefficient $\left( {k_t^2} \right)$ of conventional acoustic wave (AW) resonators, is made up of an AW resonator and two lumped elements simultaneously connected in series and shunt. The roles of the two lumped elements are to change the resonant frequency and the anti-resonant frequency of the AW resonator, which subsequently increases bandwidth or enhances the steepness of the hybrid filter. Simulation results show that the fractional bandwidth of the proposed hybrid filter is 6.0% $\left( {1.1k_t^2} \right)$, which is about 2.1 times larger than the one obtained in traditional acoustic filters.