Single-to-balanced filters for mobile phones using coupled resonator BAW technology

The coupled resonator filter (CRF) is a new type of bulk-acoustic-wave (BAW) device in which two piezoresonators are stacked on top of each other in a way that a certain degree of acoustic interaction occurs. Experimental results of single-ended and mode-converting CRF for GSM applications also featuring impedance conversion, operating at 1.8 GHz and manufactured at Infineon Technologies are presented. For the balanced port, those results demonstrate excellent amplitude- and phase-imbalance, obtained by a very symmetrical design. Also, a method for suppression of spurious resonances is demonstrated. Moreover, the latest results confirm that unwanted passbands can be overcome by modification of the acoustic mirror. Furthermore, the temperature coefficient of frequency (TCF) has been measured. Due to the compensating effect of SiO/sub 2/ in the CRF stack the values obtained are exceptionally small.