Complex Permittivity Detection of Organic Chemicals and Mixtures Using a 0.5–3-GHz Miniaturized Spectroscopy System

In this paper, a miniaturized broadband dielectric spectroscopy system is presented for complex permittivity detection of organic chemicals in the 0.5-3-GHz frequency range. A sensing capacitor exposed to a material under test exhibits changes in its capacitance and resistance according to the complex permittivity. The sensing capacitor is embedded inside a wideband impedance divider circuit excited by an external microwave signal at the sensing frequency. The amplitude and phase variation of the microwave signal across the impedance divider is a function of the complex permittivity. Wideband in-phase and quadrautre mixers are used to measure the resulting amplitude and phase variations. A unique calibration algorithm using reference liquids is applied to the fabricated sensor and sensor characteristics are extracted using 2-D surface fitting. Complex permittivity detection of pure organic chemicals is performed with an error less than 1.5% in the 0.5-3-GHz frequency range. The measured points of the permittivity versus frequency are used to estimate the static permittivity using extrapolation with an error less than 1.5% compared to theoretical values. The sensor is also applied to measure the permittivities of binary mixtures with a mixing ratio accuracy of 1%.