30925 Rev. 3.04 February 2004 Builder’s Guide for AMD Opteron™ Processor-
Based Servers and Workstations
Chapter 6 EMI Reduction Techniques 43
These electromagnetic interference (EMI) reduction techniques can be implemented with
relatively short lead-times at the final system-assembly stage. Proper electromagnetic
compatibility (EMC) and motherboard design techniques are assumed.
The effectiveness of all the EMI-reduction techniques varies from system to system. This chapter
is provided as a guideline only, to help identify and close the common EMI energy paths that
allow radiated emissions to escape from the chassis enclosure.
6.1 EMI Emission Standards
EMI emissions from a computer system must be controlled and kept below regulatory limits.
Radiated EMI emissions are measured with an antenna, typically 10 meters away from the
computer system under test. There are different EMI standards for systems marketed in the United
States and Europe, and all standards are continually updated. Typically, most computers must
meet FCC Class “B” for the US and CE Class “B” EMI requirements to be sold in Europe.
6.2 AMD Opteron™ Processor-Based System Builder
EMI Reduction Techniques
This document describes system-level EMI reduction techniques based on past successful problem
resolution of EMI radiated emissions. All the techniques described may be able to be implemented
at the final system assembly stage with relatively short lead-time. This document does not contain
long lead-time techniques involving motherboard re-layout or chassis sheet metal redesign. Proper
up-front electromagnetic compatibility (EMC) motherboard design techniques are assumed. For
more information on these techniques, please refer to the AMD Athlon™ 64 FX and AMD
Opteron™ Processors Motherboard Design Guide, order# 25180.
The effectiveness of the following EMI reduction techniques varies among different computer
systems. EMC engineering tests must be performed to determine how effective each of the
following EMI reduction techniques is for a particular system.
This list of EMI reduction techniques is numbered in the recommended order of evaluation and
relative simplicity. Each item is described in detail for clarity.
1. Spread Spectrum Clocking
AMD processors are designed to run with spread spectrum clocking enabled. Ensure that the
motherboard BIOS has enabled the spread spectrum feature of the system clock generator.
Enabling the spread spectrum setting often lowers frequency amplitudes by more that 5 dB.