The Research And Design Of Radio Frequency Device For Measurement Of Minute Dielectric Property Changes

The measurements of dielectric properties of liquids of small volumes are of great interest for the development of biological and chemical fields.Applications include dielectric studies of cells,medical diagnostic testing,and biological specimens for lab-on-chip advancement.However,the sensitivity of permittivity measurement always decreases with the physical volume of samples,leading to extremely low signal levels generating from the sample always submerge backgrounds coming from the measurement device.Therefore,the measurement sensitivity of dielectric property of liquids at extremely small volume is severely limited.To solve this problem,this paper proposes a method based on on-chip interference to cancel background parasitic signals,designs and optimizes six forms of structures by using coplanar transmission lines at three different frequency.This scheme consists of two identical Wilkinson power dividers and one branch of the divider is the reference signal path while the other is the test path.The required 180-degree reverse-phase is achieved by half-wavelength path difference.The simulation results show,when the conductivity changes from 0.0002S/m to 4S/m,the largest variation of S21 magnitude of this cancellation method is 37.5dB while the single CPW method is only 0.63dB.When the permittivity changes from 1 to 78.5,the maximum change of S21 magnitude of the proposed method is 34.8dB while the CPW method is only 0.1dB.Finally,a high sensitivity sensor for measurement radio frequency dielectric permittivity of extremely small volume of liquids is designed and made,and the measured insertion loss reaches up to-95dB at cancellation frequency 4.8GHz.Interference is used and parasitic effects are cancelled,which makes the sensor can catch weak signals caused by liquids with extremely small volumes.Compared with the traditional single CPW measurement at 4.8GHz,the largest |S21| change of this cancellation method can be improved by 20dB at least while using distilled water as sample.In addition,this thesis presents the relationship between transmission coefficient and permittivity of liquids under test and both simulation and measurement results show the rationality of the proposed relationship.Using this sensor,quantitative measurements of the dielectric properties at 4.8GHz are demonstrated of liquids under test.The simulation results are verified by measured results,showing that the proposed method is valid and the technology can be used for RF spectroscopic analysis of biological samples and extremely precious liquids.