Synthesis of Wideband Filters Based on Acoustic Wave Transversal and Ladder Topologies

The bandwidth of an Acoustic Wave filter with ladder topology shows a limitation given by the electromechanical coupling coefficient
$k_{eff}^{2}$
of the resonators when it is designed with a Generalized Chebyshev transfer function. The general rule that
$k_{eff}^{2}$
is 2 times the fractional bandwidth (FBW) is well accepted. This work demonstrates how by placing one or more reflection zeros in the complex plane, we break this limitation by decoupling the transmission zero of the filter from the resonance of the resonator. The advantage of using a Reduced Chebyshev filtering function to design wideband filters has been validated over two different topologies: the first approach consists on the synthesis of transversal topologies, while the second takes up a ladder topology. To validate the concepts for both topologies, a UNII-5 band has been used, whose FBW is around 8.4%, obtaining a feasible solution with standard aluminum nitride (AIN) of
$k_{eff}^{2}=6.7\%$
.