Design of differential mode wideband bandpass filter with Common mode suppression using modified branch line coupler

Abstract

In this paper, application of branch line coupler to design a differential mode wideband bandpass filter and to suppress common mode noise is proposed. The schematic contains one modified branch line coupler with centrally loaded open circuited stubs in the symmetrical plane to achieve high attenuation under common mode excitation. First differential mode and common mode circuits are analyzed separately for the proposed schematic and then quantitative approach, based on distributed transmission line method is proposed to explain the simulated frequency response of differential filter. No via hole is required to design differential filter, is other advantage for fabrication of the filter. A 65 % fractional bandwidth (FBW) is calculated at 5.7 GHz Centre frequency and Common mode attenuation (CMRR), greater than 13 dB is calculated under the differential passband bandwidth.