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 a modified high-sensitivity substrate-integrated waveguide (SIW) for detecting glucose concentration in an aqueous solution is proposed in this article. An RF signal generator, a broadband 3-dB directional coupler, two modified SIWs, and a gain/phase detector constitute the proposed microwave sensor system, wherein, the broadband 3-dB directional coupler is a second-order 3-dB directional coupler, whose relative bandwidth is 53.42% higher than the single-order 3-dB 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 the coupler are each connected to an SIW. Meanwhile, one is regarded as a reference, and the other for testing, both being SIWs. The two output ports of the SIWs are connected to the two input ports of the gain/phase detector. In the system, the signal output port of the RF generator is connected to the input port of the coupler. When the RF generator outputs 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 average sensitivities of about 0.698 and 1.014 mV/(mg/dL) for the two output dc voltages. Furthermore, the dc voltage amplifiers are added to the proposed sensor system, and the average sensitivities become 3.04 and 4.21 mV/(mg/dL) for the two channels. The proposed microwave sensor incorporates differential detection to diminish the impact of external environment factors, and the added active RF gain/phase detector can abandon the utilization of a vector network analyzer (VNA) and lower the cost. With these advantages, the proposed microwave sensor is an excellent candidate in the region of characterizing liquid samples.