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

Abstract

A modified magnetic-LC (MLC)-based microwave sensing system is proposed in this manuscript. The proposed microwave sensing system consists of the modified MLC resonator and the active RF circuits. The modified MLC resonator is evolved from a traditional MLC resonator, and the density of electrical field for the modified MLC resonator is increased significantly than 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 bandpass filter (BPF), and an envelope detector. The operating principle of proposed sensing system can be illustrated as follows: Assuming that the signal generator produces a RF signal with a fixed frequency of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , then the RF signal passes through the passive resonator, 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 known, the DC output voltage will be altered by loading liquid under tests (LUTs) with different permittivity, 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 ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ′) 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.