This paper presents an ultra-low power 802.11ba receiver for Internet of things (IoT) devices. A low-power third-harmonic mixer based on a 6-phase non-overlap local oscillator (LO) operating at 1/3 RF frequency is proposed to improve the out-of-band rejection. The analog finite-impulse-response (AFIR) filter in the baseband is designed for better adjacent channel rejection (ACR) while maintaining the low power consumption. A tunable tapped-capacitor resonator is designed to provide the passive voltage gain and input matching with zero power consumption.