A Low-Loss Hybrid Acoustic Bandpass Filter with Continuously Tunable Fractional Bandwidth

This research proposes a low-loss hybrid acoustic bandpass filter with continuously tunable fractional bandwidth. A reconfigurable acoustic-wave-lumped-element resonator (AWLR) module with a varactor diode is studied. Based on the reconfigurable AWLR modules, two ladder-type tunable filters, including a single-series/single-shunt configuration (Type I) and a single-series/two-shunt configuration (Type II), are designed. Their analytical expressions are derived to directly predict their electrical characteristics. Their calculated and simulated insertion loss and 3 dB fractional bandwidth show good agreement, which validates the correctness of the theoretical analysis. The proposed two configurations of the reconfigurable AWLR-based filters are fabricated and measured. The Type I filter achieves a 3 dB fractional bandwidth tuning range from 1.16k_t² to 1.76k_t² with a minimum insertion loss of 1.2 dB and a maximum of 2.2 dB. The Type II filter provides a wider tuning range from 0.96k_t² to 1.85k_t² with a minimum insertion loss of 1.7 dB and a maximum of 2.7 dB. The results indicate that the proposed filter offers a favorable balance between bandwidth tunability and insertion loss.