An integrated rat-race coupler for balun applications based on high quality factor Slow-wave CoPlanar Waveguides transmission lines (S-CPW Tlines) at millimeter wave frequencies was realized in the 65-nm CMOS technology from STMicroelectronics. Optimized criteria for a CPW phase-inverter and Tlines' characteristic impedance enable to minimize insertion loss and surface on the die. The various building blocs were designed thanks to 3D full wave EM software coupled to a circuit simulator. Thanks to the S-CPW Tlines, the device occupies a small area of 0.085mm~2. From 51 GHz till a minimum of 67GHz, i.e., a 16 GHz bandwidth, the measurements show the return loss is lower than -15dB. The magnitude imbalance of the measured couplings are included between to -4.2 and -4.1dB and -6.5 and -6dB, respectively, and are really flat over a wide bandwidth. Finally, in the bandwidth the phase shift between the output ports is kept very flat, equal to 175.9° ± 0.4°, while the isolation is better than -26dB. The rings are smaller than 290 μm on a side. It shows the potential of this approach for further increasing working frequencies.
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