What Is Electromagnetic Interference (EMI)?

Electromagnetic interference (EMI) is any unwanted electromagnetic energy that disturbs the normal working of an electronic circuit. Every device that switches current produces some electrical noise. It becomes interference when that noise reaches another circuit strongly enough to corrupt a signal, reset a processor or drown out a radio link. When the noise is in the radio-frequency range it is often called radio frequency interference (RFI).

Every case of EMI has three parts: a source that produces the noise, a coupling path that carries it, and a victim circuit that is sensitive to it. Remove any one of the three and the problem goes away, which is why engineers tackle EMI at the source, on the path and at the victim at the same time.

The effects range from the merely annoying to the dangerous. A display may flicker, a serial link may drop characters, an analogue sensor may read wrong, an Ethernet port may lose packets or a computer may reboot when a nearby contactor switches.

Any fast change in voltage or current can radiate or conduct noise. In industrial and vehicle installations the usual sources are:

Conducted EMI travels along wires: power cables, signal leads and shared ground connections. It dominates at lower frequencies, roughly below 30 MHz, and is controlled with filters, ferrites and careful grounding. Radiated EMI travels through the air as an electromagnetic field, with cables and slots in an enclosure acting as unintended antennas. It dominates at higher frequencies and is controlled with shielding and layout.

Noise can also couple at short range: capacitively between conductors that run close together, or inductively between loops of wire carrying changing current. That is why signal cables should never be bundled tightly with motor or power cables. EMI is also described by duration: continuous noise from a running drive, or transient bursts and surges from switching, lightning and ESD.

EMI is the problem; electromagnetic compatibility (EMC) is the goal. A product is electromagnetically compatible when it neither emits enough noise to disturb other equipment nor is disturbed by the noise it will meet in its intended environment. EMC testing therefore has two halves: emissions, which limit the noise a product gives out, and immunity, which checks it keeps working when noise is applied.

Which limits apply depends on where the equipment goes. The table lists the standards most often seen on industrial computer datasheets.

In the UK and EU, EMC is a legal requirement before a product can carry the UKCA or CE mark, under the Electromagnetic Compatibility Regulations 2016 and the EMC Directive. In the US, the FCC sets emission limits but not general immunity requirements, so immunity testing there is driven by industry and customer specifications.

EMI is far cheaper to design out than to fix on site. The main techniques work on the source, the path or the victim.

Rail, vehicle and military installations are some of the harshest EMI environments, with traction drives, ignition systems and radios packed close together. Our railway computers are designed for EN 50155 rolling stock, and most list EMC to the rolling stock EMC standard EN 50121-3-2, and the Premio JCO-1000-ORN Jetson computers and Premio RCO-3000-RPL rugged fanless PCs meet EMC to EN 50121-3-2. For defence platforms, our military computers are designed to MIL-STD-461 emission and susceptibility limits.

A compliant computer is only part of the answer. The cables, power supply and installation decide whether the whole system stays within limits, so EMC has to be planned as a system rather than read off one datasheet.

We specify the computer, isolated I/O and cabling for the EMC environment of each project as standard, and can advise on shielded connectors and power filtering for the installation.