Slotline-Based Balanced Filter With Ultra-Wide Stopband and High Selectivity

Slotline-based balanced filter with ultra-wide stopband is proposed in this brief. The proposed balanced filter integrates split-ring slotline resonators (SRSRs) and π-type slotline structures, which not only increase the selectivity, but also enhance suppression in a stopband range of 7.1 times as large as center frequency. The SRSRs are implemented by two symmetrically folded stepped-impedance resonators and used as a second-order resonant circuit to obtain high-selectivity bandpass response. Moreover, the π-type slotline structure enhances harmonic rejection significantly. The fabricated balanced filter demonstrates an insertion loss of 1.07 dB @ f₀ (3.5 GHz), a differential-mode (DM) stopband suppression of 20 dB @ up to 7.1 f₀, a common-mode (CM) suppression of 40.5 dB over the passband. To the best of the authors' knowledge, with maintaining competitive in-band performances, the proposed balanced filter achieves the widest stopband suppression.