High-Q Bandstop Filters Exploiting Acoustic-Wave-Lumped-Element Resonators (AWLRs)

This brief focuses on the radio frequency (RF) design of novel high-quality-factor (Q) reflective-type bandstop filters (BSFs) with narrow bandwidth (BW), small physical size, and large stopband attenuation levels. They are realized by merging acoustic-wave-lumped-element resonators (AWLRs) and lumped-element impedance inverters in a single filter architecture. In this manner, deep stopband notches that correspond to resonators with effective quality factors (Qs(eff)) on the order of 1000 are created. Furthermore, the obtained rejection bands exhibit fractional BWs that are no longer restricted by the electromechanical coupling coefficient k(t)(2) of the constituent acoustic wave (AW) resonators as opposed to the traditional all-AW filters. Experimental prototypes of first-and third-order frequency-static BSFs were built and tested at 418 MHz to validate the proposed concept. Measured stopbands with 3-dB BWs from 0.2 to 0.5 MHz, maximum stopband attenuation in the range of 15-43 dB, and passband insertion loss between 0.37 and 1.6 dB are reported. Moreover, an electronically switchable BSF with dynamic notch allocation is presented as a practical demonstrator device for switchable RF-interference suppression. A design methodology that relates to coupled-resonator filter formalism is also addressed to facilitate the systematic theoretical synthesis of the engineered AWLR-based reflective-type BSFs.