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Coupling: Why it has a powerful effect on EMC Filter performance

Tom Downing speaks out about handling coupling issues.

As product manager at Roxburgh EMC I spend my days talking to customers about EMC filters. Many conversations are centred on the correct way to install a filter and, its natural companion: the reasons why poorly installed filters so often do not work very well. It is possible to install an EMC filter paying rigorous attention to safety regulations and the design of the finished item but end up with a filter that is not working effectively. In this article I wanted to mention one of the most important effects to consider.


In my experience a very important phenomenon is coupling (the same effect is referred to as crosstalk in communications circles). Coupling is quite easy to understand and naturally leads to a more EMC friendly design if considered from the outset. Although, rather frustratingly, it also means that a perfect filter installation is impossible given that coupling can never be completely eliminated. Luckily the various national standards for conducted immunity and emissions that manufacturers are required to meet do not demand perfection - only an emissions profile within certain limits or immunity to an injected signal of certain power.


If we consider a filter’s primary function; prevention of conducted radio frequency interference (RFI) from leaving a piece of equipment and contaminating the mains supply we can start to build a mental model of what is happening. At Roxburgh EMC we think in terms of filters having a “clean” side and a “noisy” side. The clean side is the line side or the mains side. The noisy side is the load side or the equipment side. The filter’s job is to prevent the noisy side noise from appearing at the clean side and effectively keep noise created by a piece of equipment inside it. In this way the noise is prevented from interfering with the functioning of other devices connected to the same power supply.


Problems arise because the noisy side cables can induce RFI on clean side cables through capacitive and inductive coupling. If clean and noisy cables are routed together the noise from the noisy cables jumps to the clean cables. This effect can be so powerful that the filter is effectively bypassed and may as well not be present. This happens more often than one might expect as it can be so convenient to route clean and noisy cables together within the constraints provided by the design of the particular equipment. Here are 2 simple rules to consider when laying out cables and siting your filter. This will help you to get as much of the filtering performance you have paid for as possible;

1) Keep clean side and noisy side cables physically away from each other. Avoid running the cables in close proximity and if they must cross try to arrange this at right angles to minimise capacitive coupling

2) Keep the clean side cables as short as possible by siting the filter as close to the position where the power enters the equipment and physically pointing its line side towards the cable entry point.

There is obviously more to it than this and there are other effects to consider too. Here at Roxburgh EMC we have a number of technically qualified people who are happy to try and help. I am here for easy questions and for the complicated matters we have a number of highly experienced engineers.

Tom Downing
Product Manager
+44 (0)1724 273200

Article first published : 1|8|2018