Novel Microwave Frequency-Locked-Loop-Based Sensor for Complex Permittivity Measurement of Liquid Solutions

In this article, a new microwave frequency-locked-loop (FLL)-based sensor is proposed for complex permittivity measurement of liquid solutions. An interdigitated electrode split-ring resonator (IDESRR) is embedded in the FLL and used as a sensing device to distribute electric field in the near-field region and sense the permittivities of the liquid solutions with different concentrations. When the test liquid is placed on the IDESRR, a deviation will be introduced to the resonance frequency based on the perturbation theory. This frequency deviation accompanies a corresponding phase shift in the FLL operating frequency. The phase-shifted signal then leads the FLL to output a frequency-modulated (FM) signal, whose frequency deviation depends on the concentration level of the test liquid. The quadrature frequency discriminator is used to demodulate this FM signal and transfer it into dc voltages, VI and VQ. Reference liquids with known permittivities are used to calibrate the proposed sensor and relate the measured voltages to the dielectric constant and loss tangent. In this article, the water-ethanol mixtures and water-glucose solutions are used as the test liquids to evaluate the measurement performance of the proposed FLL-based permittivity sensor. The measured results obtained using the proposed sensor agree very well with those obtained using the commercial dielectric probe. Moreover, since the proposed permittivity sensor has the benefits of a simple system architecture, high sensitivity, and not requiring a vector network analyzer (VNA), it has great potential to be developed as a new type of biomedical sample sensor.