Coupling-Matrix-Based Design of High-Q Bandpass Filters Using Acoustic-Wave Lumped-Element Resonator (AWLR) Modules

This paper presents an original and simple coupling-matrix-based synthesis methodology for the design of a new class of bandpass filters (BPFs) that employ hybrid acoustic-wave-lumped-element resonator (AWLR) modules with improved out-of-band isolation (IS). The proposed BPFs feature quasi-elliptic-type frequency response - shaped by $N$ poles and $2N$ transmission zeros (TZs) for an Nth-order transfer function, compact physical size, and high effective quality factors (Q_eff) of the order of 1000. Despite the use of acoustic wave (AW) resonators, passbands exhibiting fractional bandwidths (FBWs) that are no longer limited by the electromechanical coupling coefficient (k_t²) of the constituent AW resonators are obtained. A coupling-matrix-based model of a multi-mode AW resonator is also reported. It facilitates the incorporation of high- and low-frequency spurious modes that are present in a realistic filter response so that they can be anticipated at the synthesis and simulation levels. For proof-of-concept validation purposes, two BPF prototypes at 418 MHz made up of commercially-available surface acoustic wave (SAW) resonators and surface mounted devices (SMD) were built and measured. They perform first- (one pole and two TZs) and second-order (two poles and four TZs) transfer functions with measured passband insertion losses (IL) between 2.4-5.4 dB, Q_eff between 1600-1900, 3-dB absolute bandwidths ranging from 0.52 to 1 MHz (i.e., 1.6-3.2 times k_t²), and minimum IS levels between 25-46 dB.