Design for EMC: grounding, shielding, filtering, layout and cables
Practical principles for designing electronic products that pass EMC tests first time: PCB layout, grounding, shielding, filtering, cabling and early checks.
EMC is much cheaper to design in than to fix. The principles below apply to most products. They are rules of thumb: treat them as a checklist and confirm with measurement.
1. Think in terms of current loops
Every signal current returns to its source. The loop area formed by the signal path and the return path controls both radiation (large loops radiate) and pick-up (large loops receive). Keep loops small, and give high-frequency currents an easy, direct return path, normally under the signal on a solid reference plane.
2. PCB layout
- Use a solid ground (reference) plane in a multilayer board and avoid splitting it under fast signals; a gap in the plane forces the return current to detour and creates a loop.
- Put decoupling capacitors right at the power pins of fast ICs, with short wide connections (the connection inductance, not the capacitor, usually limits the benefit). Use several values to cover different frequency ranges; mind capacitor self-resonance.
- Partition the board by function (noisy digital, switching power, sensitive analogue, I/O) and put filters where signals leave a partition.
- Keep fast clock and edge traces short, do not route them near board edges or connectors, and avoid routing them across plane splits.
- Place the connectors together on one board edge, with filtering and protection at the connectors, and connect connector shells to the chassis with low inductance.
- Slow the edges where function allows (series resistors, drive-strength settings). Energy of a digital edge reaches up to about 0.35 / rise time, so a 1 ns edge puts significant content up to several hundred MHz.
- For switching converters, minimise the hot loop (switch, diode or synchronous switch, input capacitor), control the switch-node area and consider snubbers or slower gate drive.
3. Grounding
- At low frequencies (below about 1 MHz) a single-point ground avoids ground loops; at high frequencies (above about 10 MHz) multiple low-impedance connections are needed (a plane or many bonds). Mixed designs use a hybrid approach.
- A wire is an inductor at RF: a rule of thumb is about 10 nH per centimetre, so a 5 cm pigtail has an impedance of about 31 Ω at 100 MHz. A short, flat or wide connection is much better.
- Connect the chassis and cable shields to the enclosure over 360°, not through a wire “pigtail”.
- Keep safety earth (PE) requirements in mind: EMC grounding must never compromise protective earth.
4. Shielding
- A shield works by reflection and absorption (and multiple reflections). Thin conductive sheets work well for electric fields and plane waves at high frequency; low-frequency magnetic fields need high-permeability materials or distance and loop reduction.
- Apertures matter more than material. A slot or hole with its longest dimension l leaks; the shielding effectiveness of an aperture is roughly 20·log₁₀(λ / 2l) dB (when l is below λ/2), so a 100 mm slot gives about 20 dB at 150 MHz and almost no shielding near 1.5 GHz.
- Use conductive gaskets, overlapping seams, many small screws or fingers instead of few, honeycomb vents for ventilation, filtered connectors and shielded cables for penetrations.
- Skin depth of copper is about 66 µm at 1 MHz and about 6.6 µm at 100 MHz, so even thin copper or sprayed or plated coatings can shield at high frequencies, provided seams and joints are bonded.
The aperture formula is a first estimate (a textbook result); actual performance depends on slot shape, the number of apertures and internal resonances.
5. Filtering
- Put a filter at every boundary where a conductor leaves a shielded or noisy area: mains inlet, I/O connectors, DC supply to a sub-assembly.
- A power-line filter has differential-mode elements (X capacitors, series inductance) and common-mode elements (a common-mode choke, Y capacitors to earth). Y capacitance is limited by touch/leakage-current rules in the safety standards.
- Mount the filter at the entry point, with its metal case or ground tightly bonded to the chassis; wiring between the filter and the entry should be short and not run next to the filtered output.
- Filter performance depends on the source and load impedances; catalogue insertion loss is measured in a defined impedance system, and results in a real product may differ.
- Ferrite beads and clamps add series impedance for common-mode noise at 10–500 MHz; they are useful, but a layout fix is normally cheaper and more robust.
6. Cables
- Cables are the main antennas of a product (coupling paths): reduce common-mode current on them first.
- Use twisted pairs for differential signals, balanced drivers for long links, and shielded cables with 360° shield terminations for high-speed or sensitive signals.
- Separate noisy (power, motor) cables from sensitive (sensor, audio) cables, cross them at right angles when they have to cross.
- Keep cable lengths to the minimum needed and note that cable length changes the resonance and therefore the emission spectrum.
7. Protection against transients
- ESD, EFT and surge need a defined protection concept: a robust chassis connection, TVS diodes or varistors at ports, series resistance, and creepage/clearance. See ESD, EFT/burst and surge.
- Firmware can help: watchdogs, input debouncing, data checks, retries.
8. Check early and often
- Design review against the checklist above and the standards that will apply.
- Near-field probes on the prototype to find hot spots; a current probe on cables to find common-mode current; a spectrum analyser and a simple artificial network for a pre-compliance look.
- Pre-compliance tests at a laboratory or a small in-house setup before the formal test; fix, repeat.
- Keep the same configuration (cables, loads, firmware) for the formal test as in the checks, and document it.
Common mistakes
- Treating EMC as the last step before launch.
- Adding a filter without checking the ground connection of the filter.
- Slots, gaps and unfiltered connector pins in a “shielded” enclosure.
- Pigtail shield connections.
- A split ground plane under fast traces.
- Fixing with ferrites on the wrong place (they treat the symptom, not the loop).
Further reading
- Henry W. Ott, Electromagnetic Compatibility Engineering (Wiley, 2009); Clayton R. Paul, Introduction to Electromagnetic Compatibility (Wiley, 2006); Tim Williams, EMC for Product Designers (Newnes): standard textbooks for the principles above.