Design of unbalanced and balanced radio frequency bulk acoustic wave filters for TD-SCDMA

This paper presents the design methods and simulation results of an unbalanced filter and a balanced filter for RF signal filtering in the receiver path of a TD-SCDMA handset. Film bulk acoustic resonator (FBAR) technology is employed in the filters. Transmission line model has been built to emulate the behaviors of an FBAR, which is then used in the design and optimization for the filters. The unbalanced filter is formed by connecting the FBAR resonators in a ladder topology, and low insertion loss, sharp transition from pass band to near stop band, high out-of-band attenuations are achieved. The filter die features a small size of 0.7 mm × 0.5 mm. The balanced filter involves a balun which performs the single ended to differential conversion. A novel approach to integrate the capacitors needed in the balun circuit on the filter die is proposed for small form factor package of the balanced filter.