Feedback-Type RF Oscillator and Modified Magnetic-LC Resonator-Based Microwave Sensing System for Characterizing Liquid Samples

A novel microwave sensing system based on a feedback-type RF oscillator and modified magnetic-LC (M-MLC) resonator is proposed in this article. The M-MLC-based passive sensor has evolved from a traditional substrate integrated waveguide (SIW), and the SIW has a passband covering from 3.2 to 7.6 GHz. The traditional MLC resonator is etched on the top surface of the SIW, and the odd-mode resonant frequency is obtained at 2.29 GHz. In order to improve the confinement of electrical field, the meander slot and complementary split-ring resonator (CSRR) are added, so another even-mode resonant frequency is generated. As the CSRR introduces additional capacitance coupling to MLC, the equivalent capacitance value for odd-mode is enhanced. In the measurement, the odd-mode is adopted to characterize the liquid sample. Then, the RF amplifier used as a direct path and M-MLC-based passive sensor regarded as a feedback loop are implemented to construct the RF oscillator; the feedback-type RF oscillator emits different oscillation frequencies as liquid samples with different complex permittivities are loaded. In order to get rid of the utilization of the spectrum analyzer and decrease the cost, the demodulation circuit can use different output dc voltages to reflect the different liquid samples. In measurement, the average sensitivities for the proposed microwave sensing system are about 0.869 and
${0.483}~\text {mV}/{\varepsilon }_{r}^{\prime }$
, and, with comparison to other recently reported sensors, they are improved by about 35.78% and 102.9%, respectively. The proposed microwave sensing system is a good candidate in the application of characterizing liquid samples.