NSRR Microwave Sensor Based on PLL Technology for Glucose Detection

Microwave biosensor shows great potential in mediator-free glucose detection. However, most microwave biosensors are passive devices; therefore, the high-stability and SNR frequency spectrum are necessary for ensuring the detection precision. In this article, a nested split-ring resonator (NSRR) microwave sensor integrated with phase-locked loop (PLL) technology is proposed for building a portable glucose detection system. PLL drives the NSRR operating at the specific frequency, in which the insertion loss is utilized to characterize the glucose level. By establishing the noise contribution model of PLL to NSRR, the loop parameters of PLL are optimized for higher linearity and sensitivity. The results show that when the loop bandwidth K is set to 20 kHz for a Type-II fourth-order PLL, -76.3 dBc of spur suppression is achieved in the noise-sensitive area (660-760 MHz). In this case, the operation frequency is set at 743 MHz to realize a sensitivity of 17.2 dB/mg mL-1 and linearity of 0.97 for glucose detection.