Temperature compensated band-pass and band-stop bulk-acoustic-wave filters solution reducing interference in wireless systems

Multi-mode mobile equipments requires efficient radio-frequency (RF) filtering solutions to solve interference problems in a UMTS system integrated with a mobile television function. We designed and fabricated a pair of RF filters consisting of a band-pass and a band-stop filter, based on temperature-stable bulk acoustic wave resonator (BAW) technology. Both filters measurably demonstrate low insertion loss (~3dB), high rejections (35~40 dB), and fast roll-off (~10 MHz from pass band to stop band). Furthermore, they achieve a low temperature coefficient of frequency (TCF) of -4 ppm/°C, which is five times lower than a conventional BAW filter. The low TCF is of critical importance, particularly in the sharp skirt region, since trivial frequency changes associated with temperature variation could lead to failure against filter specifications at the skirt corners. The proposed filtering solution can be applied to minimize interference present in similar systems, such as mobile phones integrated with satellite radio or GPS navigation.