Influence of termination impedance to radiated emission from AC cable with ferrite cores array below 300 MHz

The common-mode current on AC cable is one of radiated disturbance source below 300 MHz. The method of using common-mode absorbing device (CMAD) is discussed to improve reproducibility of test result by stabilizing the termination impedance of AC cable. However, CMAD can´t stabilize the differential-mode impedance because this acts as a kind of common-mode choke coil. In this paper, the influence of the differential-mode termination impedance was studied. The measurement results of disturbance current on AC cable showed that the differential-mode level between AC power line and ground line is larger than the common-mode level. The study using a simple circuit model suggested that the differential-mode current could be converted to the common-mode current by changing the termination impedance between AC power line and ground line. The experiment was carried out by using the artificial AC network which has a function to change both common-mode and differential-mode termination impedance. The results showed that a common-mode level fluctuated by changing the termination impedance. The results also showed that the influence to the common-mode current and the radiated electric field strength in the case of the comb generator was smaller than that in the case of the PC.