Hybrid Resonators Based Balanced Diplexer with Wide Stopband and High/Wideband Common-Mode Suppression

A balanced-to-balanced diplexer with wide stopband and high/wideband common-mode suppression is proposed. To increase filter order while achieving a high degree of common-mode suppression, the microstrip feedlines were appropriately designed to feed each I/O slotline resonator of each channel. To extend the spurious-free bandwidth for enabling two flexible operating frequencies, an intrinsic transmission zero from the dual-mode slotline resonator was utilized for stopband suppression. A fourth-order diplexer was prototyped with center frequencies at 2.31/4.85GHz and 3-dB bandwidths of 8.7/9.9%. Measured in-band insertion loss, common-mode suppression, and isolation are 3.4/2.5 dB, greater than 52.5/46.4 dB, and greater than 44.3/43.5 dB, respectively. Common-mode rejection is greater than 31.5 dB over 1-10 GHz and for greater than 18-dB suppression, the differential-mode stopband extension is larger than $4.33 f_{L}$.