A 24-GHz Frequency-Locked Loop-Based Microwave Microfluidic Sensor for Concentration Detection

This article presents a 24-GHz microfluidic sensor using frequency-locked loop (FLL) technology for detecting liquid concentrations. The sensor, based on FLL, features a microfluidic channel placed over an asymmetrical coplanar waveguide resonator (ACPWR) that functions as a sensing device. For testing purposes, we use ethanol–water mixtures and glucose–water solutions as the liquid under test. Due to the electric field distribution in media with varying dielectric constants, the phase of the signal undergoes different phase deviations. The FLL-based sensor is capable of detecting these phase deviations and, in response, produces a frequency-modulated signal. This signal is subsequently demodulated into a corresponding voltage with the aid of a frequency demodulator, realized through a phase detector. Consequently, the sensor demonstrates the capability to differentiate between tested liquids of varying concentrations and offers a linear response that correlates the output voltage with the liquid concentration. The proposed 24-GHz FLL microfluidic sensor offers advantages, such as cost effective, high sensitivity, and compact size. It has a great possibility to implement this sensor using the system-on-chip technology. As it combined with Internet of Things technologies, it may have a capability of real-time biomedical specimen sensing for daily life.