Design of Balanced RF Filter for Wireless Applications Using FBAR Technology

We present a balanced RF filter based on thin film bulk acoustic resonator (FBAR) technology for wireless applications, and demonstrate its usage in the Wide-Band Code Division Multiple Access (WCDMA) front-end as post-LNA filter. The filter, which consists of two lattice sections and one symmetric ladder section, provides deep rejection at stop-band, particularly at transmitter band (1920-1980 MHz), steep roll-off at pass-band edges, and excellent common mode rejection ratio (CMRR). The typical broadband rejection is better than 60 dB. The small size, low in-band insertion loss, sharp roll-off, high stop-band rejection, good temperature stability, and compatibility with semiconductor processing make this FBAR filter an ideal choice for GHz wireless applications.