Self-Calibrating Resonant Sensor for Dielectric Material Characterization

This article presents the design, fabrication, and characterization of a self-calibrating resonant sensor for the noninvasive dielectric characterization of materials. It is based on a ring resonator operating in the subgigahertz range, and it exploits the mode-splitting phenomenon to perform differential measurements, wherein one resonance is sensible to the dielectric properties of a sample under test, while the orthogonal resonance serves as an internal reference. This strategy effectively mitigates common-mode perturbations, such as temperature drift. The prototype fabrication was carried out by inkjet-printing a conductive ink into a Rogers RO4003C substrate. The resulting prototype exhibits a nominal resonant frequency near 750 MHz. To characterize the sensor, several samples with known dielectric properties were placed on its surface, and the scattering parameters were measured over the 550-950-MHz frequency range. An analytical fitting procedure was employed to extract the central frequency, amplitude, and quality factor of each resonant peak in the magnitude of the forward transmission coefficient, and these parameters were observed to vary as a function of the samples' dielectric properties. Additional tests demonstrated the sensor ability to reject common-mode disturbances. The promising results of these investigations are discussed in detail within this article.