Low-loss W-band CPW bandpass filter using annular CPW branch-line coupler with open load stubs

A W-band coplanar waveguide (CPW) low-loss annular branch-line bandpass filter for planar integrated millimeter-wave circuits is presented. It is designed by using the annular branch-line coupler as a transversal filtering section by loading the coupled ports of the coupler with suitable open load stubs and taking the isolated port as the output node. The proposed W-band bandpass filter shows very good performance, insertion-loss of 0.7 dB with a 14.9% of 3 dB relative bandwidth at center frequency of 94 GHz and return loss is better than -27 dB at the center frequency. The designed and fabricated bandpass filter exhibits the good performance for monolithic integrated millimeter-wave circuit applications.