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This article presents a 0.9/1.23/2.4/3.5 GHz quad-channel diplexer by using a C-section impedance matching circuit to cascade two dual-band bandpass filters (DB-BPFs). Due to the virtual grounds, the canceling effect of mixed electric and magnetic coupling, and the cross coupling, there are six transmission zeros around two passbands, which result in sharp skirts property of DB-BPF. Synthesized step-to-step design process is given to guide the quad-channel diplexer design. Compact circuit area of 0.03396λ g1 2 , low insertion loss of 1.2/0.9/1.2/0.9 dB, sharp skirts and better than 32 dB high port-to-port isolation are also achieved in the proposed quad-channel diplexer design.dual-band bandpass filters, quad-channel diplexer, transmission zeros