Research On Novel Portable Icrowave Sensors Applied To Dielectric Constant Detection

As an important property of matter,dielectric constant contains much valuable information.Accurate material property detection system are highly required for numerous applications in medicine,geological exploration and concentration measurement of mixed solution.Among so many detection technologies,the microwave approach,thanks to its non-invasive and nondestructive feature,is becoming a popular tendency.The traditional microwave measurement method generally rely on measuring the change of frequency response of the sensitive element to obtain the dielectric constant of the material,which usually requires the vector network analyzer with large volume and expensive price to complete.This does not work in many scenarios that require real-time and low-cost detection.Therefore,this paper will carry out the research on portable microwave sensors applied on dielectric constant detection,which focuses on avoiding vector network analyzer and reducing the test cost.Firstly,this work proposed two highly sensitive planar resonators,the substrate integrated waveguide(SIW)and the compact microstrip resonator(CMRC),which can be applied to cavity perturbation method.And study the frequency variation characteristics before and after materials filling into the cavity with the help Electromagnetic simulation software in the following stage.Secondly,according to the planar resonant element,a feedback voltage-controlled oscillator based on SIW structure and a negative resistance voltage-controlled oscillator based on CMRC are designed respectively.Then the change of resonant frequency caused by loading material is converted into the change of oscillator output frequency.This paper also discussed the change with principle of oscillator.Finally,the PLL frequency synthesis system applied to permittivity detection is studied.As a feedback system,PLL can automatically track the output signal frequency to the input signal frequency.Therefore,the change of oscillation frequency can be converted into the change of voltage by embedding the designed VCO into the PLL system,and the dielectric permittivity can be measured simply by voltmeter.According to the test results of several standard solutions,the system achieves a small measurement error within 4%.