A Dielectric Constant Measurement System for Liquid Based on SIW Resonator

A self-sustained dielectric constant (i.e., permittivity) measurement system for liquid is proposed. A tunable substrate integrated waveguide (SIW) resonator is exploited to form a feedback-type voltage controlled oscillator (VCO) that is further embedded into a phase locked loop (PLL)-based frequency synthesizer. The SIW resonator acts as the feedback element of the VCO. When the SIW resonator is exposed to liquid, its frequency response and the corresponding VCO oscillation frequency change accordingly because of electromagnetic field perturbation. Due to the presence of the PLL, the eventual frequency of synthesizer is kept constant, while this potential frequency shift is translated into the output voltage of the PLL loop filter. This voltage is readily measured by a digital multi-meter. Principles and design considerations of the SIW resonator as well as implementations of the VCO and the PLL synthesizer are discussed in detail. In addition, a prototype sensing platform is devised. Correlations between the dielectric constant of liquid MUT and the physical parameters, such as the SIW resonant frequency, the VCO oscillation frequency, and the PLL output voltage are analyzed comprehensively. According to the experimental results of several organic liquids, the proposed system can achieve an accuracy better than 4% for permittivity characterization.