Reflective and quasi-reflectionless multiband bandpass filters using multi-resonant acoustic-wave lumped-element resonators

Surface acoustic wave (SAW) resonator-based compact multi-band bandpass filters (BPFs) with quasi-elliptic transfer functions (TF) and reconfigurable reflective or quasi-reflectionless characteristics are reported. They utilize bandpass-type multi-resonant acoustic wave lumped resonator (AWLR) stages, shaped by multiple in-parallel cascaded distinct SAW resonators and one lumped-element (LE) inductor. By incorporating resistively-terminated bandstop-type AWLR stages at the Radio Frequency (RF) input/output, their power reflection response can be tailored to be quasi-reflectionless. The multi-band BPF can be expanded to TF with: i) a high number of passbands by increasing the number of the acoustic wave resonators (AWRs) in the AWLR stages, and ii) higher selectivity by cascading bandpass-type AWLRs stages using impedance inverters, and iii) symmetric quasi-reflectionless characteristics. For N in-series cascaded stages, each comprising K distinct AWRs, K passbands with enhanced fractional bandwidth (FBW) can be created, with overall TF having N•(K) poles and N•(K+1) transmission zeros (TZs). The operating principles of the multi-band BPF concept are provided through detailed design examples. The concept is demonstrated through detailed design examples, with three multi-band BPF prototypes manufactured and characterized, exhibiting dual-/triple-band centered between 1029.8 and 1039.9 MHz, FBWs> 0.4kt2, and effective Q-factors> 5,000. The quasi-reflectionless prototype showed a maximum reflection of -11.5 dB.