Wideband Bandpass Filter Cell using a Hybrid BAW-Assisted Topology

Bulk acoustic wave (BAW) filters are widely used to meet the stringent specifications of modern radio-frequency (RF) front-end modules. However, the increasing demand for wider frequency bands in recent mobile standards continues to place significant stress on conventional ladder topology. In this work, a complete design procedure for a BAW-assisted topology is presented to realize wideband filters using a Bridge-T configuration that combines BAW resonators with reactive elements. The proposed methodology uses the coupling matrix (CM) concept to extract the circuit elements before proceeding with the electromagnetic simulation. For validation purposes, a proof of concept BAW-assisted filter employing 20% Sc-doped AlN BAW resonators is designed and fabricated from 2.9 GHz to 4.1 GHz. The resulting wideband filter, with a 33% FBW, exhibits 1.5 dB of insertion loss at the band edges and 1.0 dB at the center frequency.