Single and Multiband Acoustic-Wave-Lumped- Element-Resonator (AWLR) Bandpass Filters With Reconfigurable Transfer Function

Abstract

New types of single- and multiband acoustic-wave resonator-based bandpass filters (BPFs) with continuously variable transfer function in terms of bandwidth (BW), selectivity, and out-of-band isolation (IS) are presented. They are based on single-/multiband resonant branches that are shaped by high-quality-factor ( $Q$ ) acoustic-wave-lumped-element resonators (AWLRs) and are connected in parallel to an all-pass network through variable impedance inverters. With the proposed configuration, quasi-elliptic-type passbands with wide fractional BWs (FBWs)—i.e., larger than the electromechanical coupling coefficient $k_{t}^{\mathrm {2}}$ of its constituent acoustic-wave resonator—can be realized and continuously tuned while preserving the high- $Q$ characteristics of its acoustic-wave resonators. Furthermore, by reconfiguring the location of the transmission zeros (TZs) of the AWLRs, tunable out-of-band IS profiles and asymmetrical passband BWs can be created. The operating principles of the devised spectrally agile filter concept are demonstrated through coupling-matrix-based synthesis and linear-circuit simulations. They are experimentally validated at the UHF band through the realization of: 1) a three-pole/six-TZ single-band prototype at 418 MHz with tunable BW between 0.16 and 0.49 MHz (FBW: 0.5– $1.5k_{t}^{\mathrm {2}})$ for an insertion loss (IL) between 3.3 and 1.2 dB (i.e., effective $Q >10$ 000), reconfigurable order (first-to-third-order states), out-of-band IS controllability in terms of BW (18–26.5 MHz at the upper stopband) and magnitude (23–46 dB at 426 MHz), and an intrinsically selectable all-pass mode and 2) a six-pole/nine-TZ dual-band BPF with two transmission bands located at 418 and 433.9 MHz. They, respectively, feature symmetrical and asymmetrical BW tuning between 0.3–0.54 and 0.6–1.23 MHz with in-band IL <1.7 dB for all tunable states.