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This paper presents, for the first time, the design of planar resonators and admittance inverters that can operate at two widely separated frequency bands. Design equations of the proposed structure and its application to pass-band filter design are given. These building blocks feature dual-band operation, compact planar design and low-insertion loss. For demonstration, the measured results of a dual-band microstrip filter operating at 900 & 2000 MHz based on the proposed configuration are shown.