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Electromagnetic Interference
- by Matt Stevenson

EMI what is it and how do I know if I have it?

EMI is another TLA (three letter acronym that the PCB industry is notorious for) for Electromagnetic Interference. Every electronic device generates EMI at some level, so yes, your design definitely has some amount of EMI. When current flows through wires, traces or circuits some of the energy is propagated through the air in the form of electromagnetic radiation as well as within your closed design creating disturbance voltages throughout the conductors in your device. EMI is more prevalent when operating at higher frequencies (>50MHz).

Why should I care about EMI?

There are two main problems with EMI. The first is how much EMI your device is  generating and radiating to the environment. There are regulations on the amount of interference that a device can generate due to FCC guidelines, military requirements and other international regulations as well. These have been put in place for personal safety and to maintain an electronic device's ability to carry out its function long term. The other issue with EMI is a device’s inherent ability to operate in the presence of EMI, whether that is EMI created within the device or from external sources, too much exposure to interference can negatively impact the functionality of the device.

Limiting the EMI in a PCB layout can be an effective method for reducing the emissions, meeting FCC regulations and keeping your design signals clean and working as expected. Here are some things to keep in mind when you are planning the lay out of your PCB to help limit EMI in your designs.

Here are a four items to consider at the planning and schematic stages of your design that will set you on the right path.

Choosing SMD components over through hole parts. Generally the leads on the through hole parts create higher levels of inductance and thus opportunity for EMI.

Design with multilayers adding a ground plane on the layer directly below the external signals (2 and n-1). The presence of these planes in close proximity to the signal effectively will reduce the return path, keep signals clean and reduce EMI emission. Connecting decoupling or bypass capacitors to these planes offer another effective technique for reducing EMI due to the short and logical return paths

Limiting the operating current and or the rise times of the signals will help to reduce larger fluctuations in current offering lower EMI emission rates.

Matching the impedance on signals. This is a fairly critical practice of design especially at higher signal speeds that will reduce the opportunity for signal reflection, harmonics, ringing, and overshooting digital signals - all of which increase the EMI radiation.

Don’t let yourself be surprised by EMI it can create issues and errors with your projects that could result in delays, budget overrun and missed deadlines. Consideration of these choices at the pre-planning stage of your PCB design can help save you time and effort in the prototyping and testing phases.