Intel Xeon Processor Multiprocessor Platform Design Guide

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Mechanical and EMI Design Considerations
7.2.2 Terminology
Electromagnetic Interference (EMI) - electromagnetic radiation from an electrical source that
interrupts the normal function of an electronic device.
Electromagnetic Compatibility (EMC) - the successful operation of electronic equipment in its
intended electromagnetic environment.
7.2.3 Brief EMI Theory
Electromagnetic energy transfer can be viewed in four ways: radiated emissions, radiated
susceptibility, conducted emissions and conducted susceptibility. For system designers, reduction
of radiated and conducted emissions is the way to achieve EMC compliance. Susceptibility is
typically not a major concern in the desktop PC environment although it may be more important in
an industrial environment.
The main component of EMI is a radiated electromagnetic wave, which consists of both electric
(E-fields) and magnetic (H-fields) waves traveling together and oriented perpendicular to one
another. Although E- and H-fields are intimately tied together, they are generated by different
sources. E-fields are created by voltage potentials while H-fields are created by current flow. In a
steady state environment (where voltage or current is unchanging), E- and H-fields are also static
and of no concern to EMI. Changing voltages and currents are of concern since they contribute to
EMI. If a dynamic E-field is present then there must be a corresponding dynamic H-field, and vice
versa. Motherboards with fast processors will generate high-frequency E- and H-fields from
currents and voltages present in the component silicon and signal traces.
Two methods exist for minimizing E- and H-field system emissions: prevention and containment.
Prevention is achieved by implementing design techniques that minimize the ability of the
motherboard to generate EMI fields. Containment is used in a chassis environment to contain
radiated energy within the chassis. Careful consideration of board layout, trace routing and
grounding may significantly reduce the motherboards radiated emissions and make the chassis
design easier.
7.2.4 EMI Regulations and Certifications
Original Equipment Manufacturers (OEMs) ensure EMC compliance by meeting EMI regulatory
requirements. System designers must ensure that their computer systems do not exceed the
emission limit standards set by applicable regulatory agencies. Regulatory requirements referenced
in this document include:
United States Federal Communication Commission (FCC) Part 15 Class B
International Electrotechnical Commission's International Special Committee on Radio
Interference (CISPR) Publication 22 class B limits
The FCC rules are viewed to require any OEM who sells an “off-the-shelf” motherboard in the
United States to pass an open chassis test. Open chassis testing is defined as removing the chassis
cover (or top and 2 sides) and testing for EMI compliance (although permitted emission levels are
allowed to be higher). Removing the cover greatly reduces the shielding provided by the chassis
and increases the amount of EMI radiation. The purpose of this regulation is to ensure that system
boards have reasonable emission levels since they are one of the main contributors to EMI.