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

• Published in: IEEE microwave and wireless components letters Vol. 29; no. 7; pp. 447 - 449
• Main Authors: Psychogiou, Dimitra, Gomez-Garcia, Roberto
• Format: Journal Article
• Language: English
• Published: IEEE 01-07-2019
• Subjects: Acoustic wave (AW) filter; Band-pass filters; Couplings; k2t enhancement; microwave filter; Passband; Prototypes; reflectionless filter; Resonant frequency; Scattering parameters; surface acoustic wave (SAW) filter; Surface acoustic waves
• ISSN: 1531-1309; 1558-1764
• DOI: 10.1109/LMWC.2019.2918413

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 <inline-formula> <tex-math notation="LaTeX">k_{t}^{2} </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">1.2k_{t}^{2} </tex-math></inline-formula>, and return loss >15 dB throughout its passband and stopband ranges.