A Novel Differential Microwave Sensor Based on Reflective-Mode Phase Variation of Stepped-Impedance Transmission Lines for Extracting Permittivity of Dielectric Materials

A differential microwave sensor based on stepped-impedance transmission lines for retrieving the permittivity of dielectric materials is proposed in this article. The proposed differential microwave sensor consists of a modified passive sensor and active circuits. The modified passive sensor is composed of two identical short-ended stepped-impedance transmission lines, and the high- and low-impedance microstrip lines constitute the stepped-impedance transmission line. Through theoretical analysis and experimental validation, the optimum geometrical dimensions for the passive sensor are given. The optimum structure can realize the maximum phase variation of the reflection coefficient for the modified passive sensor. The modified stepped-impedance transmission line has the maximum phase variation of reflection coefficient with permittivity changing by one unit, and the stepped-impedance transmission line itself is a good passive sensor in characterizing dielectric materials. In the experiment, one high-impedance line is regarded as a sensing area, another is regarded as a reference, and this differential configuration can inhibit the interferences of the external environment and improve the detection sensitivity. The active circuits consist of an RF oscillator, phase shifter, mixer, low-pass filter (LPF), and in-phase proportional amplifier. In the RF oscillator, the modified stepped-impedance transmission line is used as a load network, and the pseudomorphic high-electron mobility transistor (PHEMT) ATF34143 is adopted to design the RF oscillator. The design of an RF oscillator is a preparation for fabricating a microwave sensing system. In the microwave sensing system, the oscillation frequency will be changed when dielectric materials with different permittivity are loaded, which further leads to the variation of output dc voltage. The mathematical relationship between permittivity and output dc voltage can be established to characterize unknown materials. In measurement, the average sensitivity is about
$169.6 {\text {mV}}/ {\Delta {\varepsilon }_{r}^{\prime }}$
, which is several hundred times higher than other sensors. The maximum error is about 1.745%, which is reduced by about 34.39% than another one. The proposed differential microwave sensing system can not only decline some interferences of the external environment, but also get rid of the use of a vector network analyzer (VNA), and lower the cost. The proposed differential sensing system is a good candidate in the field of characterizing dielectric materials. In the future, the proposed microwave sensing system can be designed as a portable electronic device, and it can be applied to other detection scenarios.