Quasi-elliptic SAW Filters Using Multi-Resonant Acoustic-Wave Lumped-Element Resonator Stages

Surface acoustic wave (SAW) resonator-based bandpass filters (BPFs) with enhanced fractional bandwidth (FBW)-i.e, that is wider than the electromechanical coupling coefficient $kt^{2}$ - quasi-elliptic transfer functions are reported. They are based on cascaded acoustic-wave-lumped element (AWLR) multi-resonant stages that are shaped by one SAW resonator and one or more lumped-element (LE) resonators. Each of the multi-resonant stages contributes to the overall transfer function one pole and two transmission zeros (TZs). Thus, the proposed three-stage BPF configuration exhibits a transfer function shaped by three poles and six TZs. A detailed RF design methodology using analytical equations is provided for each multi-resonant AWLR stage. Various design examples for each stage and the overall three-pole/6-TZ BPF are presented. For proof-of-concept demonstration purposes a three-pole/six-TZs filter prototype was designed, manufactured and measured at 916.6 MHz. It exhibits the following characteristics: Center frequency: 916.6 MHz, FBW: $1.26kt^{2}$ and minimum insertion loss 1.88 dB that corresponds to an effective quality factor of 6000.