Acoustic Wave Resonator-Based Bandpass Filters With Continuously Tunable Fractional Bandwidth

A new class of acoustic-wave resonator-based bandpass filters (BPFs) with continuously tunable fractional bandwidth (FBW) are presented. They are based on cascaded multi-resonant stages of hybridly-integrated surface acoustic wave (SAW) resonators with lumped element (LE) components. Each stage comprises one SAW resonator and one or more LE resonators that contribute to the overall transfer function to one pole and two transmission zeros (TZs). As such highly-selective quasi-elliptic transfer functions with ${N}$ poles, $2{N}$ TZs and enhanced FBW can be created for two series and N-2 parallel stages. It is shown that by reconfiguring the resonant frequency of the LE resonators, continuously-tunable FBWs can be obtained. The operating principles of the multi-stage concept are demonstrated through design examples of multi-stage prototypes. The concept has been validated at 916.6 MHz through a four-pole/eight-TZ BPF with tunable BW between 0.63-1.32 MHz (i.e., FBW $=\,\,0.57- 1.32k_{t}^{2}$ ), minimum in-band insertion loss (IL) 3.9-2.3 dB $(Q_{eff}\,\,=$ 7700-6000) and out-of-band isolation > 27 dB.