A generic 2.0 × 2.5 mm2 UMTS FBAR duplexer based on 8-pole near-elliptic filters

UMTS antenna duplexers for handsets require high levels of wide-band rejection while still achieving high isolation, low insertion loss, and good return loss in-band. Several of the UMTS duplexers also require very steep filter skirts due to a very small guardband between transmit (Tx) and receive (Rx) frequency bands (Bands 2 and 8 have ~1% guardband). Without sizeable inductors to stretch the bandwidth, this demands a higher effective coupling coefficient and a near-ideal placement of a large number of poles and zeroes to achieve steep filter skirts while maintaining acceptable insertion loss with so many resonators. Signifcantly higher quality factor (Q) than previously available is essential to maintain good insertion loss in these designs. Cross-coupling elements shift the frequency of existing ladder filter zeroes to place them near ideal alignment to meet in-band rejection requirements without the need for tight tolerance on components.