Radio-frequency bulk acoustic wave duplexer with a differential receiver port

This paper presents a novel bulk acoustic wave (BAW) duplexer with a single-ended to differential receiver (Rx) filter. The single-ended to differential Rx filter uses a single-ended ladder section on the antenna side of the Rx path to provide the proper impedance in the transmitter (Tx) band as to not load the Tx hurting its performance in the duplexer. Then a balun follows the initial ladder section to convert single-ended to differential. Finally a lattice filter follows with its small size and beneficial rejection. Taking full advantage of the characteristics of ladder and lattice structures respectively, the filter features low insertion loss, steep near-band skirts and deep out-of-band attenuations. As an optimal implementation the ladder, the balun and the lattice sections have been fabricated on a single die. This novel “balun in the middle of the filter” solution provides the best combination of loss, size and functionality in a duplexer.