A miniature BAW duplexer using flip-chip on LTCC

The challenge for developing a duplexer for US-PCS mobile phones comes from the close proximity of TX and RX pass bands as well as from very stringent rejection and isolation requirements. Until recently, only rather bulky ceramic filters were able to provide the necessary performance. However, the need for smaller footprint and device height has brought up competing technologies based on SAW according to T. Matsuda et al. (2002) and FBAR by J. D. Larson et al. (1999) and P. D. Bradley et al. (2002) filters. We present experimental results on a PCS duplexer derived from combining a high-Q solidly mounted BAW technology by R. Aigner et al. (2002) with a flip-chip-on-LTCC package (CSSPlus). The ground inductances of the filters were carefully tuned to maintain excellent attenuation up to 3.4 GHz. Due to high resonator Q>1200 the filters have roll-offs <1500 MHz. A low TCF of only -19 ppm/K has been achieved. The resulting duplexer has small footprint (5 × 5 mm²) and height (<1.3 mm). An insertion loss <3 dB, TX-to-RX isolation >40 dB in the RX band and >50 dB in the TX band have been demonstrated.