Expanding The Low-frequency Bandwidth For Near-field EMI Scanning System

In order to reduce the electromagnetic radiation of power electronic equipment,near-field scanning technology is widely used,and it has become a common means for electromagnetic compatibility pre-testing of electronic products.In recent years,with the rapid development of highly integrated low-frequency electronic devices such as wireless power supply,switching power supply,and class D power amplifier,the measurement requirements for low-frequency electromagnetic interference sources have been proposed.For this demand,near-field scanning technology still has some shortcomings.In order to accurately locate the interference source on the highly dense circuit surface of the device,the current near-field scanning system needs to miniaturize the near-field probe components in the system to ensure the system has high spatial resolution.However,the high spatial resolution is conflict with the low-frequency bandwidth and sensitivity of the near-field probe.Therefore,the existing near-field scanning system has problems of low-frequency bandwidth shortage and low sensitivity when measuring and locating low-frequency interference sources that beyond the lower limit of the traditional electromagnetic compatibility test frequency.These problems have brought great difficulties to the measurement of low-frequency radiation sources by near-field scanning systems.In this paper,in order to meet the requirements of near-field scanning measurement of electromagnetic interference in low frequency applications,and to solve the problems of low frequency bandwidth and low sensitivity of traditional electric/magnetic near-field probes.Based on the analysis of the traditional near-field probe equivalent circuit model and the main factors affecting the low-frequency bandwidth of the probe,a new method is proposed to extend the traditional near-field probe's low frequency bandwidth,and a new low-frequency near-field probe is designed by this method.Finally,a low-frequency electromagnetic interference two-dimensional near-field scanning measurement system is developed by combining the newly designed low-frequency near-field probe.The main work of this paper mainly includes:1.The structure and working principle of the traditional near-field probe are analyzed in detail,and the equivalent circuit model of the traditional electric field probe and the magnetic field probe is proposed.According to the model,the formula of the near-field probe lower cut-off frequency is derived and analyzed,and then the load impedance of the near-field probe and its own distribution parameters are obtained by the formula,which are the main factors affecting the probe's low-frequency bandwidth.2.A new method for expanding the low-frequency bandwidth of the near-field probe is proposed.Based on the determination of the main factors affecting the low-frequency bandwidth of the probe,the lower limit cut-off frequency of the traditional near-field probe is analyzed,and a new method is proposed to extend the probe's low frequency bandwidth and increase its sensitivity by matching the electric field probe with the high input impedance preamplifier and matching the magnetic field probe with the low input impedance preamplifier without sacrificing the spatial resolution of the near field probe.3.According to the new method,a new low-frequency near-field probe is designed,and the calibration and frequency response experiments of the low-frequency near-field probe are carried out,and the experimental results were compared with the traditional near-field probe of the same size.The results are obtained:(1)The original lower cut-off frequency of the 1cm conventional loop magnetic field probe is 300MHz,and the extended lower cut-off frequency is reduced to 200kHz,so the bandwidth is expanded by 1500 times;(2)the original lower cut-off frequency of the 6mm conventional short pointed electric field probe is 300MHz,the lower cut-off frequency after extension is 30Hz,so the bandwidth is expanded downward by 10~7 times.Therefore,from the experimental results,the newly designed low frequency near field probe satisfies both the wide low frequency bandwidth and the high spatial resolution.4.A two-dimensional near-field scanning measurement system for low-frequency electromagnetic interference is designed,and the low-frequency applications such as DC motor PWM driver board operating at 13 kHz,Class D digital audio amplifier board at 303kHz,and wireless charging transmitter board at 205 kHz were measured by this system.Finally,the results show that the system can accurately obtain the field strength distribution information of the low-frequency radiation source and quickly locate the interference source.Through the research in this paper,a new low-frequency near-field probe with wide and low frequency bandwidth is obtained,and the sensitivity at low frequency is significantly improved.A two-dimensional near-field scanning measurement system with low frequency electromagnetic interference is designed.The low-frequency electromagnetic interference measurement system can meet the requirements of field strength distribution characteristics analysis and positioning of many low-frequency radiation sources such as DC motor PWM driver,wireless power transmission system and class D power amplifier.