A Novel Single-Ended to Balanced Bulk Acoustic Wave Filter for Wireless Communications

This letter presents a new architecture of BAW filter with single-ended to balanced mode conversion. The single-ended to balanced BAW filter comprises a single-ended ladder section at the filter input, a fully balanced lattice section at the output, and a balun in the middle to couple the two sections electrically. By taking full advantage of the characteristics of ladder and lattice structures respectively, a single to balanced filter with low insertion loss, steep near-band skirts and deep out-of-band attenuations can be achieved. As an optimal implementation the ladder, the capacitors in the balun and the lattice sections have been fabricated on a single die. The measurement results demonstrate that the proposed new architecture can deliver a single to balanced mode conversion with an ideal combination of performance, size, and functionality in a filter or a duplexer for wireless communications.