Acoustic Wave Resonator-Based Absorptive Bandstop Filters With Ultra-Narrow Bandwidth

Ultra-narrowband absorptive bandstop filters (ABSFs) are presented in this letter. They are based on a hybrid RF-design approach in which high-unloaded quality-factor (Q) acoustic wave (AW) resonators and low-Q lumped-element components are effectively combined to create narrow $({<} 0.1\hbox{\%})$ fractional-bandwidth (FBW) stopbands that exhibit theoretically infinite attenuation and effective quality factor $(Q_{eff}$) of the order of a thousand. The proposed ABSF design approach is experimentally validated with two manufactured prototypes at 418 MHz that consist of commercially-available surface acoustic wave (SAW) resonators and surface mounted devices (SMDs). Measured FBWs as narrow as 0.02% that correspond to a $Q_{eff}$ of 10,000 and maximum stopband isolation of 68 dB validate the conceived ABSF design concept.