Quasi-Elliptic Multi-band BPFs using Multi-Resonant Acoustic-Wave Lumped-Element Resonators

Compact surface acoustic wave (SAW) resonator based multi-band bandpass filters (BPFs) with quasi-elliptic transfer functions are reported. They are based on multi-resonant acoustic wave lumped resonator (AWLR) stages that are shaped by multiple one-port type acoustic wave resonators (AWRs) and one lumped-element inductor. The concept can be scaled to transfer functions with: i) a high number of passbands by readily increasing the number of the AWRs in the multi-resonant stage and ii) a higher order by cascading AWLRs using impedance inverters. As such, for N in-series cascaded multi-resonant stages, each comprising K AWRs, K passbands with enhanced fractional bandwidth (FBW) can be created, and the overall transfer function will have $K\bullet(N)$-poles and $N\bullet(K+1)$-transmission zeros (TZs). The operating principles of the multi-band AWLR concept are provided through detailed design examples. For proof-of-concept demonstration purposes, two-stage dual-band (i.e. $N=K=2$) and triple-band ((i.e. $N=2$ and $K=3$) BPF prototypes were designed, manufactured and characterized. They exhibit i) two passbands with $f_{cen1,2}=1033.4$ and 1039.9MHz, FBWs $\gt0.78k_{t}{}^{2}$ and $Q_{\text{effS}}\gt6200$ ii) three passbands with $f_{cen1,2,3}=1029.8,1033.4$ and $1039.9\mathrm{MHz},\mathrm{FBWs}\gt0.48k_{t}^{2}$ and $Q_{effS}\gt5400.\ (k_{t}{}^{2}$ is the AWR’s electromechanical coupling coefficient)