Single-to-balanced BAW filter combining CRF with lattice filter
Although FBAR filters have successfully penetrated the CDMA duplexer market, they still cannot displace the SAW filters in traditional RF front-end filter applications. Most of present cell phone systems like GSM or WCDMA require the filter device to have both single- to-balanced conversion and impedance transformation functions. Current BAW filter with ladder or lattice topology cannot satisfy any of these requirements. Therefore the coupled resonator filter (CRF) has gained a lot of attention since it posses the potential to meet all the requirements of state-of-the-art wireless systems. In the past, CRF was fabricated in a SMR structure with proper selection of reflector layer materials. The reason of using the SMR structure is to suppress the redundant resonant response of the CRF. However, comparing with a membrane-type structure, the SMR has slightly lower Q and effective K 2 . In this paper, a new design structure combining CRF and lattice filter for single-to-balanced conversion applications is proposed. It can help to eliminate the need of SMR structure and thus the performance could be improved. This concept is verified by simulation and the comparison to conventional ones is discussed.