A Differential Coupled-Line-Based Active Microwave Sensor System for Retrieving Real Permittivity of Binary Aqueous Solution

Abstract

An active microwave sensor system, which is based on a differential coupled-line, is proposed. The sensor comprises a differential coupled-line, as well as RF circuits. The coupled-line with through and isolation ports short-circuited, and coupling port open-circuited, is used to design the coupled-line sensor. The phase of reflection coefficient of coupled-line is utilized to extract real permittivity of liquid sample, and in order to realize the maximum sensitivity of coupled-line sensor, it is necessary to obtain the optimal condition for achieving the maximum phase-variation rate with respect to real permittivity. The mathematical analysis is conducted to obtain the optimal geometrical parameters of coupled-line, and it is inferred that the values for odd- and even-mode impedances of coupled-line are smaller, the sensitivity would be higher with the electrical length of 90°. After determining the optimal parameters, a pair of coupled-line is mirrored to produce a differential sensor, with one line used as a reference and the other for testing. The employed differential structure can mitigate external interference and boost sensitivity. To eliminate the need for a vector network analyzer (VNA) and reduce costs, active microwave circuits are incorporated. With the coupled lines functioning as load networks and the PHEMT ATF34143, a pseudomorphic high electron mobility transistor, providing the negative impedance, the proposed unloaded differential reflective RF oscillator can attain a stable oscillation frequency of 1962.66 MHz. Some microwave circuits are added into reflective RF oscillator to form microwave sensor system. Notably, the measured average sensitivity is about 5.7%. Characterized by high sensitivity and low cost, the proposed differential active microwave sensor system stands out as a promising candidate for liquid sample characterization.

Keywords

• Differential
• coupled-line
• active microwave sensor
• sensitivity
• real permittivity