A Microwave Sensing System Based on Reflective RF Oscillator and High-Sensitivity Coupled-Line Sensor for Extracting Permittivity of Liquid Samples

A microwave sensing system based on a reflective RF oscillator and high-sensitivity coupled-line sensor is proposed in this article. The coupled microstrip lines, with the pass-through and isolation ports open-circuited and the coupling port short-circuited, are adopted to analyze the conditions for achieving the maximum phase variation of the reflection coefficient using the Z-parameter matrix. The maximum value of phase variation of the reflection coefficient is obtained by acquiring the optimal values of odd- and even-mode characteristic impedances, and the electrical length of the coupled line. The RF oscillator designed by optimal coupled-line sensor and pseudomorphic high electron mobility transistor (PHEMT) ATF34143 can transform the relationship between reflection coefficient phase and permittivity into the relation between oscillation frequency and permittivity. Further, the microwave components, including the power divider, phase shifter, mixer, and low-pass filter (LPF), are cascaded with the RF oscillator to form the microwave sensing system. In measurement, the measured sensitivities of two circuit channels are about 0.44% and 0.31%, respectively, which is separately improved by hundreds of times and dozens of times compared to the other recently reported detection systems. The proposed microwave sensing system has the merits of high sensitivity, nonuse of vector network analyzer (VNA), and high portability, which determines it to play an excellent role in the field of detecting liquid samples.