A Balanced Dual-Band BPF with High CM Suppression and Improved Selectivity

In this paper, a balanced dual-band bandpass filter (BPF) is designed based on microstrip folded stepped impedance split ring resonators (SISRRs) and balanced microstrip/slotline transition structures. The center frequencies and the fractional bandwidths (FBWs) of the two differential-mode (DM) passbands can be tuned by changing the physical lengths of two SISRRs and the gaps between the two resonators, respectively. The balanced microstrip/slotline transition structures can achieve a wideband common-mode (CM) suppression. Moreover, the DM passbands are independent from the CM responses, which significantly simplifies the design procedure. In addition, due to 0-degree feed structure and cross coupling structure, more transmission zeros can be realized, which can improve the passbands selectivity greatly. In order to validate the design strategies, a balanced dual-band BPF centered at 2.47GHz and 5.21GHz was fabricated and a good agreement between the simulated and measured results is observed.