A Differential Microwave Sensor Based on Modified High-Sensitivity Substrate-Integrated Waveguide (SIW) for Detecting Glucose Concentration in Aqueous Solution

A differential microwave sensor system based on modified high-sensitivity substrate integrated waveguide (SIW) for detecting glucose concentration in aqueous solution is proposed in this manuscript. A RF signal generator, a broadband 3dB directional coupler, two modified substrate integrated waveguides (SIWs), and a gain/phase detector constitute the proposed microwave sensor system. Wherein, the broadband 3dB directional coupler is a second-order 3dB directional coupler, whose relative bandwidth is 53.42% higher than the single-order 3dB directional coupler. Two complementary split-ring resonators (CSRRs) are carved onto the upper surface of SIW, and the meander slots are embedded into CSRRs to confine more electrical field. Besides, the etched cross shaped grooves are added to the area between the two meander slots to concentrate electrical field density. In the proposed microwave sensor, the through and coupled ports of coupler are each connected to a SIW. Meanwhile, one is regarded as reference, another for testing, both being substrate integrated waveguides. The two output ports of the SIWs are connected to the two input ports of gain/phase detector, respectively. In the system, the signal output port of RF generator is connected to the input port of the coupler. When the RF generator output an oscillation signal, the detector can separately convert the differences in amplitude and phase of these two input signals into a respective output DC voltage. The output DC voltages can be used to detect glucose concentration, as changes in glucose concentration would be reflected in various of the DC voltages. In measurement, the proposed microwave sensor has the average sensitivities of about 0.698 mV/(mg/dL) and 1.014 mV/(mg/dL) for the two output DC voltages. Further, the DC voltage amplifiers are added into the proposed sensor system, and the average sensitivities become 3.04 mV/(mg/dL) and 4.21 mV/(mg/dL) for the two channels, respectively. The proposed microwave sensor incorporates differential detection to diminish the impact of external environment factors, the added active RF gain/phase detector can abandon the utilization of VNA and lower the cost. With these advantages, the proposed microwave sensor is an excellent candidate in the region of characterizing liquid samples.