A wireless receiver includes a receiver mixer having at least two sets of switches connected to selectively pass a portion of a received RF signal to the mixer's output. Different sets of the switches are activated by different phases of a local oscillator to effect passing of different phases of the RF signal to the output. These outputs are combined in various ways to obtain operation of the main receiver and the wake up receiver modes at harmonically related carrier frequencies with as low as possible subharmonic multi-phase clock generation network. This can be used to detect a wake up signal with a very fast response time, minimal area overhead, minimal power usage, and equal loading to the local oscillator network providing the phases to the mixer to maintain excellent phase balance and precise harmonic selectivity.