FBAR Duplexers with Minimal Shunt Inductance for Better Isolation and Packing Density

Relentless pressure to reduce the size of filters, duplexers and multiplexers in mobile phone RF front end modules requires close packing of the filters and any associated PCB or SMT inductors. In these multiplexers, however, every Tx filter has to reject every Rx frequency and vice versa, while TDD bands/filters must be rejected by and reject nearly every other band -60 dB typical with -55 dB guaranteed. These seemingly contradictory requirements can both be satisfied if we reduce the magnitude of the grounding inductors of the filters to a point that their mutual inductance is not significantly degrading isolation. The Band 25 duplexer described is 1.5 mm x 1.1 mm x 0.6 mm.