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Electromagnetic Compatibility: Keeping the Signal Peace

Key Takeaways

  • Electromagnetic compatibility is the state where device operations do not influence one another.

  • Layout designers and engineers can adopt best practices to prevent or confine EMI.

  • It’s critical to thoroughly evaluate all aspects of a device for EMI.

Square-pyramidal foam used in EMI/EMC testing

Electromagnetic compatibility uses ancheoic chambers with structures like these to stop signal reflection.

People prefer smaller, lighter, and more robust electronics for most applications. For the end-users, smaller form factors mean greater portability, freeing devices previously tied to outlet power. Manufacturers can save money on materials and pack more features into a smaller space, incentivizing the production and marketing of a board. While PCB and IC manufacturing has made incredible strides to miniaturize electronics, smaller and more powerful systems present a greater likelihood of electromagnetic interference (EMI).

EMI is one of the primary considerations during PCB layout, assembly, and testing, and for a good reason: compliance testing costs tens of thousands of dollars. EMI-conscious design for manufacturing (DFM) aims to pass these expensive electromagnetic compatibility tests on the first attempt, but doing so requires a holistic approach to PCB box builds.

Types of EMI Coupling

Common mode

A coupling referenced to ground where current flows in the same direction.

Differential mode

A coupling referenced to ground where current flows in opposite directions.

Common impedance

Coupling between two networks of the same impedance. This can be through ground, power distribution, or other networks.


Coupling between two wires. Can be capacitive or inductive in nature.

Where Is Electromagnetic Compatibility Found in Designs?

Electromagnetic compatibility reduces the susceptibility or ability to influence nearby electronics from conductor coupling or field radiation. Notably, the strength of coupling and radiation increase with higher circuit parameters like voltage, current, and signal propagation speeds. Adhering to electromagnetic compatibility requirements is difficult because best practices and educated guesses dominate the field. However, this is not for lack of trying: EMI is generally unsolvable by analytical methods, instead requiring numerical approximations. This heuristic technique is also highly complex, requiring extremely sophisticated field solvers to model the field strength within a circuit.

Fortunately, EMI appears in some common areas during troubleshooting. Engineers and testers can simplify discovery time by beginning diagnostics here:

  • Switching circuits - Even at low voltages, opening inductive circuits generates values in the thousands of volts alongside frequencies of several MHz. At higher operating voltages, switching disconnectors can produce signals with nanosecond rise times that pose a significant hazard to delicate components (e.g., microprocessors). 

  • RF circuits - Radio waves are high-speed and useful waveforms for various communications tasks. Due to the growing preponderance of communication-enabled devices, the average electronic device experiences greater exposure to electric waves of several V/m. To prevent interference, designers will want to consider additional shielding options (whether the whole enclosure or targeted to the circuit at the assembly level).

  • Grounding - Electrical charges that can’t quickly dissipate through grounded equipment can travel and destroy sensitive electronics instead. In the right environmental conditions, a human body can carry a charge of several kV; an electrostatic discharge (ESD) event will travel with a rise time on the order of nanoseconds. There is a twofold issue: end-user products must contain design elements that protect the device and user from accidental discharge events, while testing setups during product development must properly ground technicians.

Engineering Compliance in Electronics

Electromagnetic compatibility is often an iterative component of board design, although it doesn’t necessarily have to be. Even as designs evolve and components change, designers will want to adopt three primary methods to curb design EMI:

  • Suppression - Power circuitry requires limiting disturbing waveforms through different filtering techniques. For example, a parallel configuration of an RC network on an AC coil or a flyback diode on a DC coil will reduce the EMI generated within these nets.

  • Coupling - Coupling is an extensive EMI topic, but for layout designers, the most critical application will be the placement of noisy or fast-switching components and the distance between traces. For placement, heed manufacturers’ layout guidelines and separate high-speed digital components from low-speed analog. Designers will want to discuss design rules with fabricators before entering into manufacturing, but generally, keep single-ended traces from running parallel for extended distances, especially in the case of high-frequency or high-power lines.

  • Shielding - Alleviating EMI is often a post hoc manufacturing solution during prototyping, as design constraints may preclude optimal layout strategies. Shielding provides an easily integrable method to confine interference to its source or protect sensitive circuitry. 

Investigating Individual Elements of a Device

EMI can manifest at any system level or design stage, so the best rectification is to analyze each system element separately. This divide-and-conquer strategy ensures that no stone goes unturned in intricate electrical systems:

  • PCB - The layout designer will have the greatest influence here, as they will be responsible for component placement and routing. Identify the most likely sources or combinations of circuits that could couple and separate them as much as possible; if there is little impact on signal integrity, separating aggressive and susceptible EMI circuits to opposite sides of the board is an excellent strategy.  A ground flood around noisy traces combined with via fencing can also greatly reduce coupling.

  • Component selection - Component choices can further mitigate EMI after accounting for function and performance considerations. Surge arrestors, diodes, filter networks, and shielding options grant designers many avenues to correct EMI without infringing on the underlying design intent.

  • Cabling - External interfacing is a potential entry or exit point for EMI, often overlooked. Multiple sheathing and shielding options are available for cables that prevent external and internal coupling between bundled wire groups.

Instill Compatibility Early in Designs With Cadence Solutions

Electromagnetic compatibility is one of the cornerstones of DFM and device reliability; electronics that fail to “play nice” with one another result in service disruption or performance degradation. Confirmation of compliance requires expensive testing, but design teams can use best practice methods to identify common sources of EMI. Simulation models for troublesome EMI are invaluable for designers; Cadence’s PCB Design and Analysis Software design suite gives teams an unparalleled ECAD environment. When it’s time to move to layout, OrCAD PCB Designer offers best-in-industry speed and functionality.

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