Low-power mm-wave sensors using an FMCW radar technology are opening up unprecedented opportunities in high-resolution object detection (e.q., gesture and breathing). Building a low-power mm-wave LO generator (LOG) fulfilling the stringent phase-noise (PN) requirement of the FMCW transceiver is still challenging [1]. A number of indirect mm-wave LOGs have been explored. As depicted in Fig. 17.9.1, a 20GHz VCO followed by a frequency tripler (x3) effectively lowers the PLL operating frequency by 3 times, benefiting not only the power budget but also the frequency-tuning range. Injection locking [2], harmonic-mixing [3], and voltage-mode implicit frequency tripling [4] are the mainstream x3 techniques, where the first technique faces the trade-off between locking range and output power, whereas the second and third techniques call for a power-hungry mm-wave mixer or amplifier to recover a large output swing. Finally, all of them suffer from severe subharmonic spurs.