Holst Centre, in collaboration with imec, the Delft University of Technology and the Eindhoven University of Technology, have designed and fabricated a self-calibrating RF energy harvester. The device is capable of harvesting RF energy at lower input power levels than state-of-the-art solutions.
When used in combination with a dedicated or even ambient RF source, the new RF energy harvester has the potential to power small sensor systems. The harvester shows excellent wireless range performance, leading to an increased area that can be covered by the RF source.
A self-calibrating and highly sensitive RF energy harvester is presented that is capable of generating 1V output at -26.3dBm sensitivity. Measurements in an anechoic chamber in the European 868MHz ISM band show a -26.3dBm sensitivity for 1V output and 25 meter range for a 1.78W RF source in an office corridor. The maximum end-to-end power conversion efficiency of the harvester is 31.5%.
Key building blocks of the RF energy harvester are a 5-stage cross-connected bridge rectifier, a high-Q antenna and a 7-bit capacitor bank. The rectifier is brought at resonance with the antenna by means of the capacitor bank. A control loop is added to compensate for any variation in the antenna-rectifier interface and passively boosts the antenna voltage to enhance the sensitivity. The capacitor bank and the rectifier have been implemented in standard 90nm CMOS technology, and are ESD protected. The active die area occupies only 0.029mm2.
With this design, several limitations of existing RF energy harvesters have been overcome. Today's RF harvesters either have poor sensitivity, or require calibration, a special technology process or a large chip/antenna area. Compared to existing solutions, the new device consumes a smaller antenna area - favoring applications demanding a small form factor - while operating at a lower frequency. And thanks to its superior sensitivity and wireless range performance, an increased area can now be covered by the RF source. This makes the device suitable for powering small sensor systems in applications where other energy sources such as light, vibrations or thermal gradients are not available. It also paves the way towards harvesting ambient RF energy, as supplied by e.g. ambient WiFi or GSM signals.
These results have been presented at the 2013 Symposia on VLSI Technology and Circuits.