EMC

EMC design

A TV tuner card showing many small bypass capacitors, and 2 CCA shields -- a metal box shielding the coax inputs and another metal box shielding the S-Video connector

Main article: EMC problem (excessive field strength)

Electromagnetic noise is produced in the source due to rapid current and voltage changes, and spread via the coupling mechanisms described earlier.

Since breaking a coupling path is equally effective at either the start or the end of the path, many aspects of good EMC design practice apply equally to potential emitters and to potential victims. Further, a circuit which easily couples energy to the outside world will equally easily couple energy in and will be susceptible. A single design improvement often reduces both emissions and susceptibility.

[edit]Grounding and shielding

Grounding and shielding aim to divert EMI away from the victim by providing an alternative, low-impedance path. Techniques include:

• Shielded Housings.

In order to access the components, a housing is typically made in sections (such as a box and aq lid); often an RF gasket is used at the joint between sections, to reduce the amount of interference that can leak through the joint.

RF gaskets come in various types. One type is based on a waterproof flexible elastomeric base with chopped metal fibers dispersed into the interior or long metal fibers covering the surface or both. ("oriented wire gasket", "woven mesh gasket", etc.)

RF fingerstock comprises springy metal "fingers" and gives the best electrical shielding, but is not water tight.

• Shielded Lines.
• Grounding or earthing schemes such as Star Earthing for audio equipment, or Ground planes for RF.

[edit]Other general measures

• Decoupled Cable Entries (Line filter, Signal filter) using RF chokes, or RC elements.
• Transmission line techniques for cables and wiring, such as balanced differential signal and return paths, and impedance matching.
• Avoidance of Antenna Structures, such as loops of circulating current, resonant mechanical structures, unbalanced cable impedances or poorly grounded shielding.