A Parallel Multistepped-Impedance Transmission Line (PMSITL)-Based Microwave Measurement System for Characterizing Binary Aqueous Mixtures

A parallel multistepped-impedance transmission line (PMSITL)-based microwave measurement system for retrieving complex permittivity of binary aqueous mixtures is proposed in this article. The proposed measurement system is composed of a standard characteristic impedance transmission line with multistepped-impedance resonators connected in parallel, a reflective radio frequency (RF) oscillator based on pseudomorphic high electron mobility transistor (PHEMT) ATF34143, and a frequency demodulation circuit. The transmission coefficient phase of standard characteristic impedance transmission line with parallel multistepped-impedance resonators (PMSIRs) loaded is utilized to detect the permittivity of liquid samples, the optimal parameter values of PMSITL are calculated by analyzing the mathematical model of PMSIR-based transmission line, and the optimal parameter values of PMSITL imply the highest change rate of transmission coefficient phase with regard to permittivity. Next, the reflective RF oscillator is constituted by a PMSIR-based transmission line and PHEMT ATF34143 is designed by negative impedance theory, which adopts the oscillation frequency to measure the permittivity of liquid samples. To improve the system integration level, the frequency demodulation circuit is added to the RF oscillator, which can transform the change of oscillation frequency into the variation of direct current (dc) voltage. In measurement, the proposed microwave measurement system exhibits an average sensitivity of approximately 0.2287% and 1.01% for the two channels, which are increased by dozens of times compared to state-of-the-art microwave sensors.