A planar balanced branch-line coupler with filtering function

A planar balanced branch-line coupler with filtering function is proposed, which is realized by four quarter-wavelength microstrip lines and eight quarter-wavelength coupled lines. The proposed design not only exhibits the functions of power dividing and filtering for differential-mode signal, but also can suppress the common-mode signal. Furthermore, the proposed planar design is easy to process when compared with the reported double-layer balanced filtering branch-line coupler. The measured results show the insertion loss of 2.1 dB at 1.7 GHz, the 3-dB bandwidth of 5.6%, the isolation better than 15 dB from 1.65 to 2.03 GHz, and the common-mode suppression higher than 20 dB from 1.64 to 2.01 GHz.