Acoustic-Wave-Lumped-Element-Resonator Filters with Equi-Ripple Absorptive Stopbands

Acoustic-wave-lumped-element-resonator (AWLR)-based absorptive bandstop filters (ABSFs) with enlarged-bandwidth (BW) equi-ripple stopbands are presented. They are based on reflection-type architectures whose reflective loads are shaped by N identical series-cascaded AWLRs and N static impedance inverters each. In this manner, equi-ripple stopbands whose BW is no longer limited by the electromechanical coupling coefficient (k_t²) of the acoustic-wave (AW) resonator are obtained through the distribution of 2N transmission zeros (TZs). In contrast to conventional ABSF cascades, fewer inverters and identical AW resonators are employed. This results in benefits in terms of lower insertion loss (IL), reduced size, and higher fabrication robustness. A continuous-type tunable-isolation (IS) and BW-control mechanism by means of variable resonator resistance is reported as a novel contribution. Two-, four-, and six-TZ 418 MHz AWLR-based ABSF prototypes with low passband IL (0.96-1.14 dB), large levels of IS (up to 70 dB), and stopband BWs in the range of 0.03-0.11 MHz were manufactured and characterized as proof-of-concept demonstrators. Reconfigurable prototypes with variable levels of IS between 17-65 dB and BW tuning ratio of 1.33:1 were also measured.