The challenge for developing a duplexer for US-PCS mobile phones comes from the close proximity of TX and RX passbands 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 [1] and FBAR [2,3] filters. We present experimental results on a PCS duplexer derived from combining a high-Q solidly mounted BAW technology [4] with a flip-chip-on-LTCC package (CSSPlus). The ground inductances of the filters were carefully tuned to maintain excellent attenuation up to 3.4GHz. Due to high resonator Q>1200 the filters have roll-offs <15MHz. A low TCF of only -19ppm/K has been achieved. The resulting duplexer has small footprint (5x5mm²) and height (<1.3mm). An insertion loss <3dB, TX-to-RX isolation >40dB in the RX band and >50dB in the TX band have been demonstrated.
This paper describes front-end architectures of modern multi-mode, multi-band cellular phones and will discuss the requirements on RF filtering in such applications. Special focus will be on dual mode (GSM and WCDMA) cellular phones with four GSM and three WCDMA bands. On the one hand driven by forward integration -e.g., from PA function via transmit front-end to a fully integrated radio -on the other hand influenced by new requirements of 3G systems and the integration of complementary access, filtering components such as SAW and BAW filters have to solve several increased requirements simultaneously. New technologies are required to follow the demand of increased RF performance, reduced PCB area consumption and continuously decreasing component costs.
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