EMC Design-Grounding And Shielding-circuit Board Pcb

- Feb 18, 2017-

EMC Design-Grounding And Shielding-circuit

Board Pcb

EMC design

A TV tuner card showing many small bypass capacitors and three metal shields: the PCI bracket, the metal box with two coax inputs, and the shield for the S-Video connector

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.

Grounding and shielding

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

  • Grounding or earthing schemes such as star earthing for audio equipment or ground planes for RF. The scheme must also satisfy safety regulations.

  • Shielded cables, where the signal wires are surrounded by an outer conductive layer that is grounded at one or both ends.

  • Shielded housings. A conductive metal housing will act as an interference shield. In order to access the components, such a housing is typically made in sections (such as a box and lid); an RF gasket may be used at the joints to reduce the amount of interference that leaks through the joint. RF gaskets come in various types. A plain metal gasket may be either braided wire or a flat strip slotted to create many springy "fingers". Where a waterproof seal is required, a flexible elastomeric base may be impregnated with chopped metal fibers dispersed into the interior or long metal fibers covering the surface or both.

Other general measures

  • Decoupling or filtering at critical points such as cable entries and high-speed switches, using RF chokes and/or RC elements. A line filter implements these measures between a device and a line.

  • 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.

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