How to identify common mode interference
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How to identify common mode interference
1) judging from the interference source: the interference generated by lightning, nearby arc, nearby radio station or other high-power radiation device on the cable is common mode interference.
2) judging from frequency: common mode interference is mainly concentrated above 1MHz.
This is because the common mode interference is induced to the cable through space, which is easy to occur only at a higher frequency. But there is an exception, when the cable passes by a strong magnetic radiation source (for example, switching power supply), the common mode interference with lower frequency will also be induced.
3) measurement with instrument: as long as there is a spectrum analyzer and a current caliper, it can be measured and judged. The steps of judgment are as follows:
Clamp the caliper on the signal line or ground line (live line or zero line), and record the interference intensity of a frequency of interest (F1);
/Clamp the signal wire and ground wire at the same time. If the interference at (F1) can be observed, then (F1) interference contains the common mode interference component. To judge whether it only contains the common mode component, carry out step 3;
Clamp the caliper on the signal line and the ground line respectively. If the interference amplitude measured on the two lines is the same, the interference only contains common mode component; if not, the interference also contains differential mode component.