A Modified MLC-Based Microwave Sensing System for Retrieving Permittivity of Liquid Samples

A modified magnetic- LC (M-MLC)-based microwave sensing system is proposed in this article. The proposed microwave sensing system consists of the M-MLC resonator and the active RF circuits. The M-MLC resonator is evolved from a traditional MLC resonator, and the density of electrical field for the M-MLC resonator is increased significantly than the traditional one, and the more concentrated electrical field can improve the sensitivity of retrieving real permittivity. The RF detection circuits is composed of a low noise amplifier (LNA), a f bandpass filter (BPF), and an envelope detector. The operating principle of the proposed sensing system can be illustrated as follows: assuming that the signal generator produces an RF signal with a fixed frequency of ${f}_{{{0}}}$ , then the RF signal passes through the passive resonator, and the magnitude and phase of original RF signal will be changed. Then, the BPF can suppress the out-of-band spurious signals and make the desired RF signal cleaner. Next, the modulated RF signal is demodulated by envelope detector, and after demodulation, the direct current (dc) output voltage can be measured by multimeter. As is known, the dcf output voltage will be altered by loading liquid under tests (LUTs) with different permittivities, and a mathematical model for dc output voltage and real permittivity can be established. The permittivity of LUTs with different volume fractions of water can be predicted by the established mathematical model. In measurement, the proposed sensing system shows an average sensitivity of about 55.35 mV/(per unit ${\varepsilon }_{r}^{'}$ ) in detecting ethanol-water mixed solution with different volume fractions of water, which is several times higher than the other designed ones. The proposed M-MLC-based microwave sensing system is a good candidate in the application of characterizing liquid samples.