Novel Low-Cost Clapp Oscillator Circuit for Active RFID Materials Sensing

There are numerous applications for low-cost materials sensing; however most RFID sensing solutions are limited to measurement of permittivity. To address this need, we present a novel materials sensing circuit for active RFID tags that enables the ability to measure relative changes in electrical permittivity and magnetic permeability as well as conductivity and AC loss. This design is based on a JFET Clapp oscillator topology that uses minimal parts, and where the resonator coil is also used as the sensor antenna. We show how the loss and reactive components of the material impedance can be independently derived from the amplitude and frequency of the oscillator and measured using an integrated frequency counter and diode detector. The completed sensing circuit, containing few parts and a cost of approximately US$1, was demonstrated using an integrated active RFID tag that is compliant with the IEEE 802.15.4 protocol. Our oscillator design has high stability and quick start-up, which enables very low duty cycle operation and achieves an average power consumption of less than 1 mW.