A low power radio is provided with automatic interference rejection. The radio is comprised generally of: an antenna, a rectifier, a comparator, and a correlator. The comparator receives an input signal from the rectifier, compares the input signal to a reference signal and outputs a digital signal. The correlator in turn receives the digital signal from the comparator, correlates the digital signal to a wake-up code and outputs a wake-up signal having a high value when the digital signal is highly correlated with the wake-up code. The radio further includes an automatic threshold controller which adjusts sensitivity of the comparator. Of note, the rectifier, the comparator, the correlator and the automatic threshold controller are comprised in part by circuits having transistors operating only in subthreshold region.