An investigation of dual-band balanced-to-unbalanced (blaun) bandpass filters (BPFs) is presented in this letter. Two types of balun BPFs named Type-A and Type-B filters based on coupled ring resonators are discussed and fabricated. Both the simulated and measured results show that these balun-BPFs have not only good amplitude performances but also excellent phase difference performances. The center frequencies of these balun BPFs are set at 2.4 GHz/5.6 GHz for Type-A and 1.57/4.65 GHz for Type-B balun filters. The differences are 180° ± 5° in phase and within 0.6 dB in magnitude of type A and 0.73 dB of type B, respectively. Thus, these balun BPFs can be used in many wireless communication systems.
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