An Absorber-Based Self-Powered Differential Microwave Oscillator for CO 2 /O 3 Gas Sensing

This article introduces a self-powered, differential microwave gas-sensing solution that combines a perfect metasurface absorber (PMA) for energy harvesting, a high-efficiency full-bridge (FB) rectifier, and a dual-oscillator readout. This overcomes the limitations of conventional microwave sensors that either need batteries or have poor readout resolutions. The multilayer PMA robustly captures ambient RF and, together with the rectifier, delivers a stable supply with$\eta \!\approx \!82\%$at$2.4{\,}\text {GHz}$and$P_{\mathrm {in}}=5{\,}\text {dBm}$, enabling battery-less operation. Sensing is performed by two closely spaced oscillation tones (~4.66 and ~4.70GHz), where the lower branch serves as the sensing channel and the upper branch provides an internal reference to suppress environmental relative humidity (RH) artifacts. The oscillator readout exhibits ultranarrow linewidths, long-term frequency drift$\leq 20{\,}\text {kHz}$, and phase noise below$-110{\,}\text {dBc/Hz}$at a 100-kHz offset, supporting high-resolution frequency data. Material selectivity is realized by integrating Zeolite 13X for CO2(500–1000 ppm) and ZnO for O3(up to 24 ppm) within compact analyte holders; differential calibration allows tracking CO2and O3consistent with commercial meters. The proposed low-power and compact architecture is well-suited for remote, continuous environmental monitoring, where autonomy, stability, and chemical selectivity are critical.