This letter presents a dumbbell-shaped defected ground resonator and its application in the design of differential filters. The operation principle of the dumbbell-shaped resonator (DSR) coupled to differential microstrip lines is studied through a circuit model analysis. The proposed circuit model is validated through the comparison with the electromagnetic simulation results. It is shown that the bandpass configuration of microstripline- coupled DSR can be used to design higher order bandpass filters. The design procedure is explained by developing a thirdorder filter prototype. The designed filter shows more than 57 dB common mode rejection within the differential passband.
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