Symmetrical Quasi-Reflectionless SAW-Based Bandpass Filters With Tunable Bandwidth

Surface acoustic wave (SAW)-based bandpass filters (BPFs) with symmetrical quasi-reflectionless characteristics and continuously tunable bandwidth (BW) are reported. They are based on in-series-cascaded symmetrical quasi-reflectionless acoustic-wave (AW)-lumped-element resonator (AWLR)-based networks whose input and output ports are connected to resistively terminated AWLR-based bandstop filter (BSF) sections. The proposed concept is presented through the coupling routing diagram (CRD) formalism that allows their design using coupled-resonator-based synthesis. It is shown that by incorporating variable reactance elements in their BPF sections, continuous-type BW tuning can be realized. In addition, the fractional BW (FBW) of the filter can be designed to be wider than the electromechanical coupling coefficient k_t² of its SAW resonators. This allows enhanced-FBW passbands to be obtained. For proof-of-concept validation, two prototypes were designed and measured. They include a single-stage prototype with 1.9:1 continuously tunable BW and a static two-stage with effective quality factor > 6500, FBW of 1.2k_t², and return loss >15 dB throughout its passband and stopband ranges.