EMI Shielding: Emissions and Immunity

Any device used to block the RF signal between its source and a receiver is an electromagnetic interference (EMI) shield. Such a shield will react differently depending on its design variations (i.e. material, dimensions, etc.). The measure of this ability to attenuate RF is shielding effectiveness (SE), which is expressed in decibels (dB), the ratio of field strength on one side of the shield to the other side. The figure below shows the relationship between shielding effectiveness (in dB), the amount of attenuation, and attenuation percentage.

The switching speeds of today's high speed electronic components create high frequencies which escape from (emissions), or enter (immunity), any openings in electronic enclosures. Interference control standards are mandated by various regulatory agencies in most industrialize markets to control the unwanted transmission or receiving of such high frequencies. In addition, it may be preferred engineering practice to control these emissions to assure proper equipment performance in both systems and groupings of electronic equipment such as in medical, instrumentation, process controls or other diagnostic situations.

When access panels, doors, ventilation, displays, etc. are required in an otherwise solid enclosure, precautions must be taken to eliminate the transmission of radio frequencies through such openings or slots.

Shielding gaskets assure tight RFI/EMI sealing of electronic enclosures containing high speed microprocessors.