Compact Differential Wideband Bandpass Filters With Wide Common-Mode Suppression

A compact differential wideband bandpass filter (WBPF) composed of a coupled full-wavelength loop is presented. The differential-mode circuit of the proposed filter is analyzed, showing that the proposed filter exhibits a three-pole differential-mode (DM) wideband bandpass filtering response. Moreover, both the fractional bandwidth (FBW) and the inband return loss can be adjusted by changing the parameter of the characteristics of coupled-line. Furthermore, the proposed filter provides a wider common-mode (CM) rejection characteristics owing to its multiple periodical transmission zeros. Additionally, the filter has a very compact size occupation. To verify the design concept, one filter example has been designed, fabricated and measured. The fabricated filter demonstrates good differential WBPF performance including a broad 3 dB fractional bandwidth of 66.6% centered at f₀ = 1.62 GHz, a wider 14.5 dB CM rejection from dc to 2.96f₀, and a very compact size of 0.163 λ_g².