A Dual-Band Balun BPF Using Codirectional Split Ring Resonators

In this letter, a novel dual-band microstrip balun bandpass filter (BPF) is designed by using interdigital capacitor-loaded codirectional split ring resonators (CDSRRs). The utilized resonator is analyzed by coupled line theory and exhibits independently controllable dual resonance behavior. The proposed balun BPF is constructed by coupling two identical CDSRRs to rotationally symmetrical balanced output ports. Two transmission zeros (TZs) between the passbands can be observed by means of the CDSRRs. The designed balun BPF centered at 1.59 and 1.92 GHz was also fabricated and measured. The measured results show a good agreement with the predicted results. Phase and magnitude imbalances between the balanced output ports have been measured as better than 180 ± 5° and 0.4 dB, respectively. The proposed dual-band balun BPF exhibits high filtering performances in terms of closely spaced passbands, selectivity, and clean upper stopband.