A 2.4 GHz to 27 MHz non-linear RFID topology in flexible electronics

A non-linear commercial topology of passive RFID tag is transported onto a low-cost flexible substrate by means of antenna embedded distributed decoupling capacitors, which allow to embed both a far-field antenna, operating at 2.4 GHz, and a near-field antenna, operating at 27 MHz, into a compact low-cost layout. The sensor is based on two co-designed planar antennas and a demodulating diode, which receives a two-tone gigahertz signal and radiates back their frequency difference. Filtering elements are embedded directly into the antennas to decouple the near-field and far-field section. The layout transport is realized using silver ink printing on a flexible plastic substrate and can be used for instance as soil moisture sensor when a simple planar capacitance is added to the design. Two-tone measurements of the sensor show the amplitude of the 2^nd order intermodulation product at 27 MHz to be only 9 dB lower than the 3^rd intermodulation product at 2.400 GHz – 27 MHz, thanks to the high non-linearity of the sensor and the decoupling effect of the distributed capacitors in the metal traces of the folded-slot antenna topology.