High Selectivity and CM Suppression Frequency-Dependent Coupling Balanced BPF

In this letter, a compact second-order balanced microstrip bandpass filter (BPF) with high selectivity and common-mode (CM) suppression performances is presented based on the magnetically coupled resonators. Under differential-mode (DM) operation, a transmission zero (TZ) near the passband can be created by the frequency-dependent coupling between two resonators. Meanwhile, the intrinsic high CM suppression performance can be maintained due to the magnetically coupling between two resonators under CM operation. Another two DM TZs can be generated by the frequency-dependent source-load (S-L) coupling. Finally, a compact high CM suppression balanced BPF with three TZs for WLAN application has been designed and fabricated, and the measured results show good agreement with simulated ones.