What is the Most Challenging Cause of Electromagnetic Interference?
There are so many issues that make EMI difficult, it’s hard to pick just one.
A few candidates that immediately come to mind are: printed circuit board radiation, enclosure shielding, input power filtering, crosstalk, and grounding. There are many more.
But without a doubt, cable-connected circuits are the most ubiquitous challenge.
Ask any EMI expert, the biggest cause of EMI test failures is excessive noise on electrical cables. Both circuit-induced noise and noise induced by the electromagnetic environment.
NOISE ON CABLES
Cables are the primary mechanism for radio frequency energy to get out of a device and the main way it gets in.
So why don’t we hear more about EMI on cables?
Probably because it’s a difficult topic and it’s not well understood by most engineers. It is a lot easier to visualize a digital clock trace on a printed circuit board than it is to visualize residual harmonics of a switching power supply on a power cable. Electrical engineers tend to think in the time domain, but EMI is almost entirely described in the frequency domain.
Problematic interference on cabling can be caused by minutely small noise levels. Plus, cabling can interact with circuitry in complex ways that are not obvious, even to experienced engineers.
EMI EXPERTS KNOW
Subject matter experts in EMI collaborate to create industry standards that define EMI requirements. Those experts understand that cable connected circuits are central to controlling interference. In every electromagnetic interference specification, in every industry, nearly all EMI tests either measure noise on the cables or induce noise on the cables.
It’s not that there’s something inherently bad about cables. After all, electrical cables are essential. They get power into the device and they let one device communicate with another. The problem with cables is that they are quite efficient at picking up noise and transporting to where it isn’t wanted. Cables provide a convenient path for noise to get into and out of the device.
IT ISN’T SIMPLE
Knowing that many EMI problems are directly related to cables, you might think that controlling EMI would be a simple matter of shielding the cables.
Unfortunately, shielding only solves part of the problem. Shielding can reduce fields coupled between the cabling and the environment, which helps control radiated emissions and radiated immunity. But shielding does nothing to control unintentional, undesirable current and voltage conducted along the cabling.
CE (conducted emissions) tests limit how much noise a device can emit on is power and signal lines. Conversely, CS or CI (conducted susceptibility or conducted immunity) tests inject noise on power lines and signal cables to ensure the device is susceptible to interference produced by other equipment on the lines. Shielding does little or nothing to improve performance for CE and CS/CI tests.
Complicating matters further, cabling tends to be electrically long (long compared to wavelength of the interference signals they carry). So cabling is quite efficient at picking up and giving off noise. Cables support standing waves and the resulting resonances can amplify otherwise tolerable noise to levels that cause upset or even damage.
Accept that circuits connected to electrical cables are a major cause of EMI test failures and are susceptible to interference from noise injected on and coupled to cable conductors. Then adopt a plan to design control measures into the product.
A good prevention strategy is the key to a successful EMI control program.
a. Take action early in the design phase of every project to understand design tradeoffs that affect EMI.
b. Review each cable-connected circuit to assess its emissions and its susceptibilities.
c. EMI analysis software is essential, http://www.emisoftware.com. Interactions between circuits, EMI filtering, cabling, and the environment are complex.
d. Design in contingency plans so that if testing reveals a problem you have options for mitigation without major redesign.
e. Perform developmental testing. It is much easier to make changes during the design process than to fix a test failure during final certification testing.
EMI Analyst™ software suite is designed specifically to address the most challenging cause of electromagnetic interference, cable-connected circuitry.
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