A radio frequency, RF, receiver circuit is configured to simultaneously monitor a two or more different RF frequencies. The RF receiver circuit uses a sub-sampler to sub-sample an RF signal that is at any of the monitored RF frequencies, and the sub-sampled signal is then demodulated and a digital code that was encoded in the RF signal is recovered. The RF receiver circuit may be particularly low power, in part owing to using the same sub-sampler for each of the two or more monitored RF frequencies, and not relying on superheterodyning. Furthermore, monitoring two or more different RF frequencies simultaneously means that signals received on the monitored RF frequencies may be acted on very quickly. These characteristics make the RF receiver circuit particularly suitable for use in low-power wake-up receivers, such as Bluetooth Low Energy (BLE) wake-up receivers.