Temperature stable bulk acoustic wave filters enabling integration of a mobile television function in UMTS system

Multi-mode mobile equipment requires efficient radio-frequency (RF) filtering solutions to solve interference problems in an UMTS system integrated with a mobile television function. A RF filtering solution comprising a band-pass filter and a band-stop filter based on temperature-stable bulk acoustic wave resonator (BAW) technology is proposed in this letter. Both filters measurably demonstrate low insertion loss (∼3 dB), high rejections (35-40 dB), and fast roll-off (∼10 MHz} from pass band to stop band). A temperature coefficient of frequency (TCF) as low as - 4 ppm/°C} in the filters is achieved, which is five times lower than that of a conventional BAW filter. The low TCF is of critical importance, particularly in the sharp skirts, because 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.