Dual-Band Balun Bandpass Filter With Hybrid Structure

Dual-band balanced-to-unbalanced (balun) bandpass filter (BPF) is proposed, using the hybrid structure with resonators implemented on the upper microstrip plane and bottom coplanar-waveguide (CPW) plane, respectively. By providing two coupling paths through microstrip and CPW planes, one may separately control the two passbands and also increase the freedom in filter design. The implemented dual-band balun BPF centered at 2.4 and 5.4 GHz shows the desired filtering performance with good amplitude and phase imbalances, which are less than 0.13/0.08 dB and 0.82°/1.65° in the two passbands, respectively.