A Millimeter-Wave Direct Digital Transmitter Using Multiphase Subharmonic Switching PA

This article presents a direct digital transmitter (TX) employing a multiphase subharmonic switching (MP-SHS) power amplifier (PA) to achieve both high peak and power back-off (PBO) efficiency with wide signal bandwidth. A phase-shifted subharmonic (SH) local oscillator (LO) divider is proposed to suppress SH spurs with minimal implementation overhead by introducing an additional phase shift during LO frequency division. Furthermore, a dual-rate hybrid digital-to-analog converter (HDAC) is integrated with the MP-SHS TX, which enables noise shaping and lowers in-band error vector magnitude (EVM). A proof-of-concept prototype, fabricated in 65-nm CMOS, operates at 24 GHz, achieving 19.2-dBm peak output power with 38.3% peak power-added efficiency (PAE). This TX demonstrates 22.3%/20.3% average PAE with EVM of −34.3 dB/−30.8 dB at 1.2/3.2 Gb/s for 64-QAM and 256-QAM single-carrier (SC) modulated signals, respectively.