Input-Reflectionless Acoustic-Wave-Lumped-Element Resonator-Based Bandpass Filters

This paper reports on an RF design methodology for acoustic-wave-resonator-(AWR)-based bandpass filters (BPFs) with input-reflectionless behavior in both their passband and stopband regions. The proposed concept is based on acoustic-wave-lumped-element resonators (AWLRs) that are incorporated in series-cascaded reflectionless stages (RLSs). Each RLS comprises a first-order bandpass section — shaped by three impedance inverters and one AWLR — and a first-order resistively-terminated bandstop section — shaped by two impedance inverters and one AWLR — that are designed to exhibit complementary transfer functions. In this manner, an input-reflectionless behavior can be obtained both at the passband and stopband regions of the filter. In addition, the use of AWLRs in the RLSs facilitates the realization of high-quality-factor (Q > 10,000) quasi-elliptic-type transfer functions with fractional bandwidths (FBWs) that are wider than the electromechanical coupling coefficient (kt2) of its constituent AWRs. For proof-of-concept validation purposes, one- and two-state prototypes were manufactured, and measured at 418 MHz using commercially-available surface-acoustic-wave resonators.