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State-of-the-art wireless sensor nodes are mostly supplied by batteries. Such systems have the disadvantage that they are not maintenance free because of the limited lifetime of batteries. Instead, wireless sensor nodes or related devices can be remotely powered. To increase the operating range and applicability of these remotely powered devices an electro-magnetic energy harvester iPs developed in a 0.13 μ m low cost CMOS technology. This paper presents an energy harvesting system that converts RF power to DC power to supply wireless sensor nodes, active transmitters or related systems with a power consumption up to the mW range. This energy harvesting system is used to power a wireless sensor node from the 900 MHz RF field. The wireless sensor node includes an on-chip temperature sensor and a bulk acoustic wave (BAW) based transmitter. The BAW resonator reduces the startup time of the transmitter to about 2 μs which reduces the amount of energy needed in one transmission cycle. The maximum output power of the transmitter is 5.4 dBm. The chip contains an ultra-low-power control unit and consumes only 190 nW in idle mode. The required input power is -19.7 dBm.