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. Instead of increasing scandium (Sc) doping in aluminum nitride (AlN) BAW resonators at the cost of reducing the quality factor, alternative configurations to the conventional ladder topology enable higher fractional bandwidth (FBW) while preserving low insertion loss. In this work, a complete design procedure for a BAW-assisted topology is presented to realize wideband filters using a Bridge-T (BT) 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 Sc-doped AlN BAW resonators is designed and fabricated from 2.9 GHz to 4.1 GHz. The resulting broadband filter exhibits a FBW of 33.07% with 1 dB of insertion loss at the band-edges and 0.6 dB at the center frequency.