Design of Balanced Dual-Band Bandpass Filters Using Asymmetrical Coupled Lines

In this paper, we present balanced dual-band bandpass filters (BPFs) using asymmetrical coupled lines. The proposed dual-band BPF has an enhanced common-mode suppression level because the proposed asymmetrical coupled line inherently has the bandstop characteristic on common-mode signals without either additional stubs or lumped elements. The proposed asymmetrical coupled line has dual-band performance on differential-mode signals. The common- and differential-mode equivalent circuits of the proposed filter are given and analyzed to demonstrate the common- and differential-mode responses. Two passbands of the differential-mode response are designed based on a lumped equivalent circuit of the proposed filter. Therefore, two bandwidths can be easily controlled. Two second-order balanced dual-band BPFs are designed and tested at center frequencies of 2.4 and 3.57 GHz with fractional bandwidths (FBWs) of 7.5% and 5% and with FBWs of 7.5% and 6.16%, respectively.