Dual-Mode Diplexer using Stepped-impedance Resonator with Controllable Transmission-zero

The proposed dual-mode diplexer uses a stepped-resonator based on a closed-loop rectangular dual-mode resonator with internal stubs. The stub sizes are adjustable to achieve center frequencies of 1.95 GHz and 2.14 GHz, utilizing the IE3D design software. The circuit integrates two filters in a T-junction configuration, forming a compact 3-port dual-mode diplexer, that operates independently, providing easy control over the desired frequency and transmission-zero (TZ) position. This design allows for improved frequency response in both low-frequency and high-frequency applications. The feeding structure is optimized to achieve insertion losses better than 0.92 dB and 0.98 dB, and isolation frequencies with performance better than 20 dB. This makes the circuit suitable for applications requiring precise frequency control and efficient signal separation.