A tunable SAW duplexer

The success of SAW technology for RF front end filtering in high performance hand held mobile phones and radios is well demonstrated. The small size, high rejection and relatively low insertion loss which can be achieved using SAW impedance resonators gives this technology a significant advantage over competing approaches. One of the remaining issues for SAW impedance resonator filters has been to further improve the performance margins, particularly in the case of pass band edge insertion loss, in order to reduce average insertion loss and improve manufacturing yield. This paper describes the development of a SAW duplexer component that tunes the response of the Tx filter over a frequency range in order to improve its effective performance. The duplexer is fabricated using multi-layer LTCC technology that permits the easy interconnection of numerous diverse components, including SAW filter die, inductors, transmission lines, capacitors and voltage variable capacitors. The duplexer is designed for the dual-mode AMPS/CDMA cellular system, i.e., for transmission in the band from 824-849 MHz and for reception in the band from 869-894 MHz. The duplexer is packaged in a 9mm X 10mm X 2.5mm form factor, attains 0.8 dB minimum insertion loss in the Tx band and 1.0 dB average insertion loss and provides good return loss in a 50 ohm system. The duplexer can be tuned under software control through the use of external DC voltages which are readily available at the RF board.