  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.
