This letter presents a three-state diplexer design. By employing one port as the input while the other two as the output, the proposed circuit functions as three diplexers operating at different passbands. Owing to the planar stub-loaded resonator pair, two transmission zeros are generated at passband edges, ensuring sharp frequency selectivity for each filter passband. In addition, an asymmetrical feed scheme is adopted based on the discriminating coupling method, resulting in two other controllable out-of-band transmission zeros. These zeros are essential for isolation improvement between the three diplexer operations. For demonstration, the three-state diplexer is implemented. Compact size and measured isolation of more than 27.6 dB between the three states are obtained.
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