Experimental Analysis of a 60 GHz Compact EC-CPW Branch-Line Coupler for mm-Wave CMOS Radios

A 60 GHz reduced-size branch-line coupler has been developed using elevated-center coplanar waveguide (EC-CPW) transmission lines. The coupler is fabricated in a 90 nm CMOS technology and has a chip area of 0.102 mm². The measurements indicated an amplitude-imbalance of 0.7 dB and a phase difference of 88 degrees at 60 GHz, a return loss of greater than 15 dB at every port over the 60 GHz band (57-64 GHz), and an isolation better than 17 dB. A 73% size-reduction compared to a conventional CPW coupler design has been achieved using capacitively loaded high-impedance EC-CPW lines. The proposed coupler is well suited for the design of millimeter-wave (mmW) CMOS subsystems including: balanced amplifiers, vector modulators, and balanced mixers.