Dual-band and triple-band balun bandpass filters using open-circuited stubs loaded on ring resonators

A dual-band and a triple-band balun bandpass filters (BPFs) are proposed in this paper, using open-circuited stubs loaded on ring resonators. The proposed baluns are in general composed of three parallel-coupled lines and a ring resonator loaded with open-circuited stubs. The design concept originates from the dual-band characteristics of a uniform ring resonator. In order to achieve the triple-band balun, four open-circuited stubs are loaded on the uniform ring resonator to separate the two transmission poles. Afterward, the odd- and even-mode equivalent models of the proposed circuit were analyzed, and the relationship between the odd- and even-mode resonance points and frequency was plotted to confirm the adjustable dual- and triple-band characteristics of the model. To further demonstrate the validity of the proposed balun prototype, a dual-band balun BPF and a triple-band balun BPF are designed and fabricated. The measurement exhibits the dual-band balun BPF (3.41/6.88 GHz) amplitude imbalance (AI) lower than +/- 0.6 dB and phase imbalance (PI) lower than +/- 4 degrees, the triple-band balun BPF (3.19/5.82/7.17 GHz) AI lower than +/- 0.7 dB and PI lower than +/- 7 degrees. The circuit proposed in this paper can be used to design both dual-band and triple-band balun BPFs simultaneously. The designed triple-band balun BPF has advantages such as miniaturization and low amplitude imbalance. A dual-band and a triple-band balun bandpass filters (BPFs) are proposed in this paper, using open-circuited stubs loaded on ring resonators. In order to achieve the triple-band balun, four open circuit stubs are loaded on the uniform ring resonator to separate the two transmission poles. The fabricated balun BPFs have advantages such as miniaturization and low amplitude imbalance. image