A 240-GHz Sub-THz Direct-Conversion Transmitter With <i>I</i>/<i>Q</i> Phase Calibration in 40-nm CMOS

A 240-GHz direct-conversion transmitter (TX), consisting of an LO chain and fundamental I/Q mixers, is proposed for sub-THz communication applications. The LO chain integrates phase-shifter-embedded impedance matching networks (IMN) and frequency triplers with an optimized harmonic IMN, delivering I/Q LO signals at 240 GHz with high output power, 360∘ phase shifting range, and I/Q phase calibration capability. The I/Q mixer incorporates two transformer baluns for I/Q signal combining and groundshielding structures, ensuring layout symmetry and reducing coupling. This can significantly enhance the image rejection ratio (IMRR) and suppress LO feedthrough. Fabricated in a 40-nm CMOS process, the proposed TX provides an output power of -11.7 dBm at 240 GHz with a 3-dB bandwidth (BW) from 224 to 244 GHz. It achieves LO feedthrough suppression and IMRR better than -17.7 and -16.3 dBc, respectively, within the 3-dB BW.