Balanced quad-band diplexer with wide common-mode suppression and high differential-mode isolation

A balanced quad-band diplexer is proposed with wide common-mode (CM) suppression and high differential-mode (DM) isolation in this study. The six-port balanced diplexer was formed by combining two dual-band bandpass filters operating at 1.92/5.25 and 2.45/5.8 GHz through two stepped-impedance resonator (SIR) inserted T-junctions. By carefully designing the section length of the T-junction and the SIR length from its open-end to the joint-point with the T-junction, high isolation between diplexer channels can be attained. A prototype of the proposed balanced quad-band diplexer was implemented and measured with a DM isolation of higher than 51.8 dB and a CM suppression of larger than 47.9 dB in the four DM passbands. Good agreement was observed between the simulated and the measured results.