Balanced BPFs with wideband CM suppression and high selectivity based on double-sided parallel-strip line

In this paper, two novel balanced bandpass filters (BPFs) with wideband common-mode (CM) suppression and high selectivity are proposed based on the out-of-phase double-sided parallel strip-line (DSPSL) structure. The out-of-phase DSPSL structure is composed of a pair of out-of-phase transmission lines as the balanced ports and a lambda/2 resonator placed in the center. Based on the DSPSL structure with out-of-phase characteristics, a wideband CM suppression without affecting the differential-mode (DM) performance is achieved in two BPFs. By using a pair of shorted-coupled lines in BPF I and introducing a ring resonator in BPF II, two transmission zeros are attained, achieving high selectivity in both BPF designs. For experimental demonstration purposes, two balanced BPFs are designed and fabricated. The measured results are in good agreement with the simulated ones.