An Active and Portable Alcohol Solution Concentrations Sensing System Based on Microwave Rectifier Circuit

Traditional microwave solution sensors suffer from limited sensitivity and require expensive equipment. In this paper, an innovative alcohol solution concentrations sensing systems based on RF active circuit is proposed. The system consists of a passive modified split ring resonator (MSRR) and RF active circuits. The MSRR is improved from the conventional split ring resonator (SRR), and the electric field is enhanced through the mutual coupling of multiple split rings to improve sensitivity. The RF active circuit is mainly composed of a voltage-controlled oscillator (VCO), a power amplifier (PA) and a microwave rectifier circuit (RC). The VCO is used to provide a stable and tunable microwave signal, and the PA serves as a compensatory element to offset power attenuation within the system. The detecting system demonstrates the capability to convert the varying concentration of liquids under test (LUTs) into a readable DC voltage value. Based on the correlation between alcohol solution concentrations and voltage values, a mathematical model has been developed to predict the concentration of unknown samples. According to the results, the maximum error in extracting concentration is 3.67% and the maximum sensitivity is 3.82mV/%. The proposed system eliminates the need for expensive experimental equipment such as a vector network analyzer (VNA), RF signal generator, and spectrum analyzer, thus reducing costs. Additionally, the proposed design has the feasibility detecting scenarios in solid and gas samples characterization with high precision, low cost, and portable features.