Continuously-tunable-bandwidth acoustic-wave resonator-based bandstop filters and their multi-mode modeling

This paper reports on high-quality-factor (Q) acoustic-wave resonator (AWR)-based bandstop filters (BSFs) with continuously-variable stopband bandwidth (BW). They are based on mixed-technology resonant branches that are shaped by one acoustic-wave-lumped-element resonator (AWLR) and two lumped-element (LE) impedance inverters that are in-parallel cascaded to an all-pass-type network. Despite the use of AWRs, transfer functions with fractional bandwidths (FBWs) that are wider than the electromechanical coupling coefficient (k
_t
²
) of the AWRs can be created and controlled through the integration of variable LE capacitors that preserve the AWR's high-Q characteristics. A coupling-matrix-based design approach that facilitates the prediction of high- and low-frequency spurious modes - typically present in a realistic AWR filter response - already at the synthesis level is presented for the first time for AWR-based BSFs. An experimental BSF prototype constructed with commercially-available surface acoustic wave (SAW) resonators was built and tested at 418 MHz for proof-of-concept purposes. It exhibits tunable BW between 0.18-0.71 MHz (0.55-2.2k
_t
²
), passband insertion loss (IL) between 0.55-2 dB and maximum stopband attenuation between 25-40 dB (i.e., Q>8,000).