A 40-nm CMOS IEEE 802.11ba low-power wake-up receiver (LP-WUR) is presented. It receives 802.11ba messages generated by an 802.11 orthogonal frequency-division multiplexing (OFDM) transmitter operating at 5.8 GHz. The power consumption of the RF front-end is minimized by removing active RF gain stages and using a third harmonic passive mixer with a ring-based local oscillator (LO) operating at one-third of the RF frequency. The noise figure is 14 dB, taking advantage of a 1:3 transformer that provides passive voltage gain in front of the high switch loss mixer-first RF front-end, and matching across 5.5–5.8 GHz with $S_{11}$ . The receiver achieves a sensitivity of −83-dBm and −20-dB adjacent channel signal-to-interference ratio (SIR) while consuming $220~\mu \text{W}$ at a bit error rate (BER) of $10^{-3}$ and data rate of 62.5 kb/s, which shows the best sensitivity-power tradeoff among >3-GHz receivers.