An energy-harvesting stamp-sized reader for distance-immune interrogation of passive wireless sensors

Fully passive sensors (FPS) consist of a sensing element $(\mathrm{R}_{\mathrm{s}}$ or $\mathrm{C}_{2}$ in Fig. 1) and an inductor forming an RLC tank. Compared to legacy sensors (NFC, BLE, RFID), FPSs offer a simple chipless solution with battery-free operation and extremely low cost for scenarios such as implantable, biodegradable, biocompatible, and stretchable applications where legacy sensors cannot be deployed. Typically, sensor measurement is performed through near-field inductive coupling (NFIC) of a reader coil to the sensor with the goal of measuring $\mathrm{R}_{\mathrm{s}}$ or $\mathrm{C}_{2}$ (Fig. 1). Unlike the sensor, the reader remains the bottleneck due to its large size, high power consumption and distance-dependency of the results due to NFIC and may require extensive calibration. As such, existing readers [1]–[4] are not well-suited for handheld low-power operation with non-fixed readout distance. We utilize the properties of coupled resonators and a dual-mode LC-VCO as the reader to address the challenges discussed above for resistive FPS measurement.