A Microwave Sensor System Based on M-SRRs for Assessing the Complex Permittivity of Liquid Samples

In this article, a differential modified split-ring resonator (M-SRR)-based microwave sensor system is proposed to extract the complex permittivity of liquid samples. The proposed sensor system is constituted by the differential circuit part and demodulator; meanwhile, the differential circuit part consists of an RF generator, two RF switches, and two M-SRR-based passive sensors. The confinement of electrical field can be enhanced for the SRR-based passive sensor with interdigital capacitor added, and two RF switches are added to realize the capability of differential detection. For the differential circuit part, the upper path would be activated when the RF switches are turned to the upper branch, and this output signal is used as a reference signal. Besides, the lower path would be activated when the RF switches are turned to the lower branch, which is regarded as a test channel. The demodulator contains a power divider, hybrid coupler, transmission line (TL), mixer, and low-pass filter (LPF), and it can convert the power level of RF signal into an output dc voltage. As known, the injected liquid samples with various complex permittivity would induce the different output dc voltages, and then, the mathematical model describing the association between dc voltage and complex permittivity is built. In the test, the RF generator outputs an RF signal at about 1790 MHz with the power level of 10 dBm, and the average sensitivities of about 0.807 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.12}~\text {mV} /{\varepsilon }_{r}^{\prime }$ </tex-math></inline-formula> are obtained, which are enhanced by about 7.6%–49.3% than recently reported ones. The sensitivity of per unit volume is calculated as 0.615/0.229 mV/(<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}^{\prime }\cdot \text { VL}$ </tex-math></inline-formula>) as the volume of liquid sample is about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.9~\mu \text { L}$ </tex-math></inline-formula>. The proposed sensor system is a strong contender in the field of detecting liquid samples.