A Novel Microwave Sensor System Based on Feedback-Type Oscillator and Modified Coplanar Waveguide Resonator for Retrieving Complex Permittivity of Liquid Samples

A microwave sensor system based on feedback-type oscillator and modified coplanar waveguide resonator for retrieving complex permittivity of liquid samples is proposed in this article. The proposed microwave sensing system consists of modified split-ring resonator (MSRR)-based coplanar waveguide passive sensor, RF amplifier, and phase detector. The MSRR-based coplanar waveguide passive sensor is evolved from a traditional coplanar waveguide structure, the coplanar waveguide transmission line is located on the top surface of dielectric substrate, and two split rings with different diameters are located at the bottom layer of substrate. Then, the closed-loop interdigital structure (CLIDS) is added between the two split rings, and the electrical field density between two split rings is enhanced, which is beneficial to improve the sensitivity of passive sensor. The MSRR-based passive sensor and the RF amplifier constitute the feedback-type oscillator, and the passive sensor is utilized as a feedback loop. In the simulation, the oscillation frequency would shift toward low-frequency band when a liquid sample with increased permittivity is loaded at the sensing area. In addition, three power dividers, two phase shifters, and two phase detectors are added to a feedback-type oscillator, and the proposed microwave sensing system is formed, which can get rid of the use of vector network analyzer (VNA). In measurement, the phases of two phase shifters are adjusted to be different values, and thus, the output dc voltages of two phase detectors are different at the initial stage. When liquid samples with different complex permittivity are loaded, the phase shifts for the two branches are changed. Different phase shifts denote different output dc voltages for phase detector, and the two output dc voltages are adopted to extract the complex permittivity of liquid samples. In measurement, the designed passive sensor has the dimensions of
$30\times {40}\times {1}.{6}~ \text {mm}$
, the unloaded frequency is about 1338.47 MHz, the minimum detectable volume is about
${0}.{707}~\text {uL}$
, and average sensitivities for the two branch circuits of the proposed microwave sensing system are about
${1}.{81}$
and
${{4}.{45} ~{\text {mV}}} \mathord {\left /{{\vphantom {{{4}.{45} ~{\text {mV}}} \varepsilon _{\text {r}}^{\prime }}}}\right. \hspace {-1.2pt} } \varepsilon _{\text {r}}^{\prime }$
, respectively, which are improved by about 411.11%–139.24% than other reported ones. The maximum errors for the two channels are 5.02% and 2.54%. In comparison to other microstrip sensors, the proposed microwave sensing system can get rid of the use of VNA and lower the cost. In the future, the components of detection system can be integrated to form a portable device and achieve the purpose of real-time detection. All in all, the proposed microwave sensing system is a good candidate for characterizing liquid samples.