Temperature Compensated Piezoelectric Bulk Acoustic Wave Filters

Small sized, high performance bulk acoustic wave (BAW) band pass filters areof great interest for mobile and wireless systems. These filters are based onelectro-acoustic high Q resonators, which exploit the thickness extensional mode ofa thin piezoelectric AlN film. Modelling of the filter and resonator response isperformed by means of a1D electro-acoustic Mason model.Multi-mode mobile equipments require efficient radio-frequency (RF) filteringsolutions to solve interference problems in a UMTS system integrated with a mobiletelevision function. TD-CDMA mobile television (TDtv) is one of the mobiletelevision technologies developed in Europe; it seeks the1900-1910MHz frequencyspectrum that is10MHz below the UMTS3G transmitter (Tx) band (1920-1980MHz) to transmit television data to mobile handsets. To allow TDtv and UMTS tocoexist in a wireless phone requires a highly selective band-pass filter to preventdisturbances from interfering frequencies out of the TDtv band. Meanwhile, toreduce the UMTS Tx output from increasing noise figure of the TDtv receiver (Rx),another filter is adopted in the path of UMTS Tx that features a low insertion loss inthe UMTS band and a high rejection in the TDtv band.We designed and fabricated a pair of RF filters consisting of a band-pass and aband-stop filter, based on temperature-stable bulk acoustic wave resonator (BAW)technology. Both filters measurably demonstrate low insertion loss (~3dB), highrejections (35~40dB), and fast roll-off (~10MHz from pass band to stop band).Furthermore, they achieve a low temperature coefficient of frequency (TCF) of-4ppm/oC, which is five times lower than a conventional BAW filter. The low TCF isof critical importance, particularly in the sharp skirt region, since trivial frequencychanges associated with temperature variation could lead to failure against filterspecifications at the skirt corners. The proposed temperature compensated BAWfiltering solution is very representive, and it plays a key role of minimizinginterference present in similar systems, such as mobile phones integrated withsatellite radio or GPS navigation.