Nowadays, the efficiency of the Radio-Frequency (RF) energy harvesting circuits is continuously increasing and, at the same time, the energy consumption of connected devices is drastically decreasing. Despite that, collecting, storing and delivering such kind of harvested energy to the device in an appropriate manner is still a challenge. This paper focuses on a strategy of harvesting energy in an efficient way from both low and high levels of the RF field. The objective is to power an ultralow power consumption wake-up radio requiring a regulated voltage. The starting point is the characterization of a commercial power management integrated circuit which operates with ultralow power levels. Then, some design guidelines of the RF power rectifiers, specifically designed for our envisaged application are given.