Model-Based Microwave Dielectroscopy of Fluids With Impedance Sensors

A broadband permittivity extraction method for fluids by means of electrically small impedance sensors is reported. It relies on an equivalent circuit model of the sensors, which takes into account the most significant parasitic effects. It assumes the sensor's intrinsic susceptance and conductance to depend linearly on the real and the imaginary part of the permittivity, respectively. The simplicity of the approach eases calibration. Besides conventional thru-reflect-line standards, which define the reference plane, only air and a single reference liquid are needed. The method is applied to three different devices, namely, a common open-ended coaxial line, a compact printed circuit board design, and an integrated microchip sensor for (milli-)liter, microliter, and picoliter sample volumes, respectively. It is shown how the frequency limits can be determined, and how the sensitivity can be maximized in terms of the model parameters. The method is shown to be robust against production tolerances.