20.8–51 GHz highly balanced CMOS balun
An ultra-wideband asymmetric spiral-stacked Marchand balun fabricated in the standard 0.18 μm CMOS process is demonstrated. An innovative technique named in-phase current return path is incorporated into the design to enhance the balun's balance metrics. The proposed balun has a measured insertion loss <5 dB (3 dB for an ideal balun) and a return loss better than 10 dB from 20.8 to 60 GHz. Significantly, it provides a measured amplitude imbalance <0.3 dB and a phase imbalance <2° within 10–51 GHz range. The balun occupies an area of only 0.043 mm2 without pads. The resultant highly balanced CMOS balun is shown as a high-performing millimetre-wave component suitable for the full Ka and/or Q band applications.