System EMC Control
- Final Line of Defense -

Even optimally implemented lines of defense at the component, PCB and cable level may not solve all EMC compliance problems. Consequently, the design of a system enclosure is usually the last line of defense.

Note that if none of the earlier lines of defense are incorporated, design for compliance at the system level can be a very difficult and expensive proposition.

Suggestions:

• Always solve EMC problems at earlier lines of defense first.
• Solve EMC problems at the sub-system levels first, but be aware that New EMC problems can arise when sub-systems are assembled into larger systems. In EMC work the whole may be greater than the sum of the parts.
• Shielding techniques and technologies are available.
• Proper grounding of shields and extruding cables is imperative.
• Enclosures can resonate at critical frequencies, which may exacerbate EMI.
• Enclosure EMI-testing techniques are useful to gain knowledge of where your design can be optimized for compliance.

See Shielding For Electronics, Inc. for examples:

http://www.shieldingforelectronics.com/

The Process

The basic process you need to implement is:

1. Have available expert and working knowledge of good compliance design practices.
2. Incorporate proper noise suppression technologies and techniques at each line-of-defense:
   1. Regulatory - Use loosest requirements required for intended market.
   2. Component and circuit design - design expertise plus simulation*.
   3. PCB - layout expertise plus simulation*.
   4. Cables - design expertise plus simulation*.
   5. System - system expertise plus enclosure testing.
3. Do preliminary EMC testing as soon as possible, when you have something to test, and as often as required to become compliant. This "fail early and often" test and debug methodology is valuable, since theory and simulation are not perfect.
4. Complete the final EMC testing and product validation.
5. Maintain controlled records on all design decisions made and all testing completed.

* Simulation may only get you in the ballpark, and it tends to be more useful in complicated designs.