Study Of The On-line Detection Method For Cable Capacitance In Water Based On High Modulation Frequency Digital Lock-in Amplifier

The real-time detection of weak capacitance signal in strong noise condition is always required in modern scientific research and industrial production line,as the real-time measurement of water capacitance is needed in the production process of the cable.The lock-in amplifier is one of the most widely used instrument in the weak signal detection,but the traditional digital lock-in amplifier has a high requirement on hardware and consumes a large amount of resources.Thus,it is suitable for measuring the DC or slowly varying AC signals,but not suitable for real-time measurement systems.Therefore,the research of weak capacitance on-line measurement method based on digital lock-in amplifier has a very important theoretical and practical value.To resolve the problems of the traditional digital lock-in amplifier in the measurement of rapidly changing signals,a new digital lock-in amplifier based on high modulation frequency algorithm is designed to on-line measure the cable capacitance in water,in this paper.The main contents are as follows:(1)Analyze the capacitance to voltage signal convert circuit based on the AC excitation method and use the T-type resistor network to replace the large value resistor,to reduce the error caused by the thermal noise of the resistance.(2)Analyze the autocorrelation and cross-correlation detection theory,then study the basic principles of analog and digital lock-in amplifier and compare their performance.Finally,a high modulation frequency digital lock-in amplifier algorithm is proposed according to the basic theory of traditional lock-in amplifier,and the algorithm is the analyzed in this paper.(3)According to the characteristics of the FPGA,a kind of lock-in amplifier is designed to measure weak capacitance.(4)Design the external circuit of measurement system,including capacitance to voltage signal convert unit,analog to digital and digital to analog conversion unit,and gives the design schematics.(5)Analyze and design the various functional modules for digital lock-in amplifier in FPGA,including the control module of analog to digital and digital to analog conversion,signal generation module,phase-sensitive detection module,the amplitude calculation module and the serial port module.(6)Use the modelsim to run functional simulation of FPGA logic and analyze the simulation results,the results show that the FPGA logic modules implement the required functionality.Finally,the system is tested by experiment and the results show that the design can achieve the on-line measurement of the cable capacitance in water.