Liquid Permittivity Measurement Method Based On Microwave Planar Sensor

In the field of liquid permittivity measurement,the planar microwave resonator measurement method is widely used because of its high reliability and high performance.At present,the traditional resonator measurement method often needs a broadband instrument to measure the offset of resonant frequency and retrieve the dielectric constant of liquid,which is difficult to avoid the measurement error caused by ignoring the frequency dependence of permittivity.In addition,the planar sensor also has problems such as the complicated structure of the stage and the need to accurately control the liquid volume for measurement.Focusing on the above-mentioned problems,this article first studies the dropping-based sensor with a simple structure and no need to control the liquid volume.Subsequently,the dropping-based fixed frequency sensor that meets the requirements of narrowband measurement is studied,and the fixed-frequency measurement method is studied based on the equivalent circuit model of the dropping-based fixed frequency sensor.The main research contents are as follows:(1)Based on the structure of Open Complementary Split Ring Resonator(OCSRR),a dropping-based permittivity sensor is designed.To improve the sensitivity of the sensor,the sensing area with electric field distribution concentration is designed on the sensor,and the matching sample container is designed.Based on the above research,the electromagnetic full-wave simulation of the dropping-based sensor is carried out to analyze its electromagnetic distribution characteristics.The influence of different volume samples on the measurement results was analyzed by the dripping experiment.Through repeated measurement of different liquid reflection trap frequencies and their reflection coefficients,the measurement formula of the liquid complex permittivity is fitted,and the accuracy of the dropping-based measurement is analyzed based on the literature data.The working frequency band of the sensor is 100 to330 MHz,and there is no need to accurately control the volume during measurement.The complex permittivity of liquid can be measured by dropping liquid over 0.08 ml.(2)Focusing on the goal of dropping-based fixed-frequency measurement,the dropping-based sensor which meets the requirements of the fixed frequency measurement,and the fixed frequency measurement method based on sensor circuit model parameters are studied.Based on the dropping-based sensor,the sensor with the functions of dropping-based measurement and fixed frequency measurement is designed,and the sensor sample is made for measurement verification.Then,the equivalent circuit model of the fixed frequency measurement sensor is established,and the reflection and transmission characteristics are analyzed,the equivalent circuit parameters of different samples were extracted by ADS software.At a single sampling point,the complex permittivity of the sample is retrieved by extracting and analyzing the equivalent parameters of the liquid to be measured in the circuit model.The variation rule of the capacitance and conductivity of the resonator with the complex permittivity is obtained.The data curve is consistent with the dielectric theoretical analysis.To obtain the permittivity of liquid materials at different frequencies,the fixed frequency measurement of multiple sampling points is studied according to the technology of loading lumped element technology and the principle of differential measurement.The experiment of liquid samples verifies the feasibility of the proposed multi-point fixed frequency measurement to measure liquid permittivity at different frequencies.In this paper,from two aspects of dropping-based measurement and fixed frequency measurement,through the design of the dropping-based sensor,the improvement of dropping-based fixed frequency sensor and the experimental verification of the fixed frequency measurement,a new method for measuring the permittivity of liquid is proposed,which provides a reference for related research.