Differential broadband filter based on microstrip coupled line structures

A coupled line schematic is proposed to build the differential filter featuring broadband, low insertion loss and good common-mode suppression. The proposed filter is composed of four quarter-wavelength coupled lines and four quarter-wavelength microstrip lines. The calculated, simulated and measured results are in good agreement with each other. The differential-mode signal can propagate with low insertion loss, whereas the common-mode signal will be suppressed below -10 dB. The fractional bandwidth of the proposed filter is 82% centred at 3 GHz.