A CPW hybrid coupler with an enhanced coupling microstructure

A new micromachined coplanar waveguide (CPW) 3-dB hybrid coupler has been demonstrated. A micromachined enhanced coupling structure at the middle of two coupled transmission lines is employed to obtain high directivity. The coupling structure is composed of alternately overlapping plated conductors employing micromachined air-gap structures. This structure relaxes the tolerance of the gap between the coupled transmission lines, and mitigates production error. Moreover, this coupling structure enhances the coupler directivity by equalizing phase velocities of both even- and odd modes. We fabricate the CPW hybrid coupler with the enhanced coupling structure on a silicon substrate to verify the performance. The results showed the return loss of better than 28 dB, the isolation of more than 25 dB over 11 to 15 GHz.