Research On Dielectric Testing Technology Based On Interference Planar Transmission Line

Dielectric testing technology is widely used in the fields of chemistry,biology and medicine,including chemical material testing,biological sample research and medical diagnosis.It is very important to accurately measure the change of dielectric parameters for judging material performance.However,when background signal of transmission line is strong,the weak change of dielectric parameters will be difficult to measure.According to the principle of on-chip interference cancellation,design and material measurement of the dielectric testing device based on interference planar transmission line are completed in this paper.The device uses coplanar waveguide as signal transmission line and is composed of two Wilkinson power dividers and two transmission branches that differ in path by half a wavelength.The device structure is very symmetrical and smaller than the discrete measuring device.When the device is working,input signal is divided into two signals by a Wilkinson power divider.After passing through two transmission branches,the two signals have opposite phase and are synthesized at output to eliminate background signal.The material under test and the reference are respectively placed on two transmission branches.The difference of dielectric parameters between the two materials can be reflected by measuring cancellation effect of the two signals.The center frequency is set to 5GHz.Simulation results show that the S₂₁ reaches-109.96dB at frequency of 5.004GHz.Simulated materials with different permittivity and conductivity are added to the model for simulation.When the difference between parameters of reference and material under test is larger,variations of frequency and S₂₁of the interference point are larger.The changing trend accords with theoretical prediction.Variations of S₂₁ of the interference point are more than ten times that of single coplanar waveguide transmission line with same material.It proves that the device can achieve measurement target,and shows variation of material parameters more clearly.A tuning module is also designed in this paper.The module not only prevents device from being deformed by external force,but also adjusts output results and ensures that device is in a horizontal state when liquid is added.When device is measured,the S₂₁ of the interference point of device without tuning module is-69.23dB.After adding tuning module,the S₂₁ is optimized to-81.03dB.Deionized water is selected as reference in the experiment,and deionized water,absolute ethanol and n-propanol are used as materials to be tested.The measurement results are very close to simulation results,and verify that the device is effective.Correlation analysis and regression analysis are carried out on measurement results to obtain correlation equations and regression equations between frequency and permittivity,S₂₁ and permittivity.Then,a regression model is established,in which frequency and S₂₁ as independent variables and permittivity as dependent variables,and functional relationship between permittivity and measured results is obtained.Ethanol solution with a concentration of 50%is used as test sample.The test results are substituted into regression equation to obtain permittivity of solution,which is only 0.56%different from the permittivity recorded in literature.It proves that the dielectric testing device based on interference planar transmission line designed and manufactured in this paper has practical application value.