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 (Qeff) 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 (kt²) 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 kt²), and minimum IS levels between 25-46 dB.