Question

a) How wiring and enclosure design is linked to radiated EMI?

b) What are good rules to design an enclosure to prevent radiated EMI? Consider case material, thickness and holes.

Answer 1

Answer A).

Wires/Cables:-

When lines and cables need to pass into or out of a unit, the cables can be screened to prevent any radiation of the signals being carried or pick up of external signals. However when screened cables are needed for electromagnetic compatibility EMC applications, the screen must be bonded tot he equipment signal ground as soon as it enters the unit, otherwise unwanted signals may be radiated or picked up and this would compromise the EMC compliance.

The electromagnetic compatibility, EMC performance is of electronic equipment today is a great importance and as a result it is necessary to design for EMC. In order to enable the unit to pass its EMC testing and be placed on the market, it is necessary for it to conform to the directives and regulations in force. For a unit to be successful, it is necessary for it to be designed to provide a high level of electromagnetic compatibility, EMC performance and reduction of EMI.

Enclosure:-

Although screened enclosures may not be an option that is preferred from a cost viewpoint, placing the unit in a conductive enclosure that is grounded will significantly improve the performance. All filtering can then be undertaken at this interface and the conductive wall will provide a barrier to radiation, thereby improving both the emissions and susceptibility elements of the EMC performance.

Where cost and possibly aesthetics are important it is possible to spray the inside of cabinets with conductive paint, although the level of screening provided will not be nearly as good as if a fully conductive metal case is used. Where high levels of EMC performance are required care should be taken to choose a case where the continuity of the screen is not breached. The case should ideally be made of as few elements as possible. At each joint there will be the possibility of radiation passing through. Where joints to occur they should be as tight as possible and they should have good continuity between them.

Some metal cases using a prefabricated style of construction with anodised aluminium panels do not offer good EMC performance, although they are aesthetically more pleasing than some RF tight cases. A balance has to be made dependent upon the performance required and the EMC tests that need to be undertaken.

Answer B) :-

• 1 – Never route signals over split reference planes!

• 2 – Keep current loops as small as possible.

• 3 – Decoupling: use low-inductance capacitors/traces AND planes.

• 4 – Use ground planes on PCB for shielding.

• 5 – Route high-frequency signals adjusted to a plane.

• 6 – Control rise- and fall-time.

• 7 – Add ceramic capacitors close to every pin of a connector.

• 8 – Fill top and bottom layer with circuit GND and matelize the PCB edges.

• 9 – Add stitching vias around high-speed signal vias.

• 10 – Connect circuit GND to chassis at IO area.

• 11 – Lay cables along chassis (GND/earth).

• 12 – Do NOT use shield as signal conductor for low-frequency signals.

• 13 – Lay only one end of the cable shield to ground for low-frequency signals.

• 14 – Lay both ends of the cable shield to ground for high-frequency signals.

• 15 – Minimize loop area of signals in cables.

Answer 2

Answer:

a) Link Between Wiring/Enclosure Design and Radiated EMI

• Wiring:

• Poor cable routing (e.g., long, parallel traces) acts as antennas, radiating electromagnetic noise.

• High-frequency signals or abrupt current changes (e.g., from switching circuits) exacerbate EMI.

• Enclosure:

• Gaps, seams, or unshielded openings allow EMI leakage.

• Conductive materials (e.g., steel, aluminum) block EMI, while non-conductive materials (e.g., plastic) offer no shielding.

Key Point:
$\boxed{{\displaystyle \text{Wiring acts as EMI radiators; enclosures block or leak EMI based on design.}}}$

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b) Rules to Design an EMI-Resistant Enclosure

1. Material Selection:

• Use conductive metals (aluminum, steel) for Faraday cage effects.

• Avoid plastics unless coated with conductive paint/film.

2. Thickness:

• Thicker metal (>1 mm) better attenuates high-frequency EMI.

3. Holes/Apertures:

• Minimize size/number of holes to reduce EMI leakage.

• Use EMI gaskets or conductive mesh over vents.

• Follow λ/20 rule: Hole diameter < 1/20th of EMI wavelength (e.g., <1.5 cm for 1 GHz).

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4. Grounding:

• Ensure low-impedance ground connections to dissipate EMI.

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5. Cable Penetrations:

• Use shielded cables with ferrite beads at entry points.

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Key Point:
$\boxed{{\displaystyle \text{Use conductive, thick enclosures with minimized apertures and proper grounding.}}}$