Builder’s Guide for AMD Athlon™ 64 Processor-Based
Desktops and Workstations
31684 Rev.
3.00 September
2004
2.
Disable Unused Clocks
Clock signals that have no load can have high levels of ringing that can lead to EMI problems. The
motherboard BIOS firmware should be programmed to detect and disable unused memory DIMM
and PCI clocks.
3.
Processor Heatsink Fan Cable Routing
A problem sometimes encountered with the processor heatsink DC fan cable is the large loop
formed in its routing to the motherboard connector. Shorten this cable length to reduce the loop
area as much as possible.
4.
Power Supply Cable Routing
Historically, the system power supply cable has been found to be very susceptible to picking up
EMI energy from within the system and coupling into the power supply and then onto the AC
power cord. Keep the power supply cable against the metal chassis and as far away from the
processor, memory DIMMs, and VRM components as possible. Fix this cable routing in place
with plastic cable ties.
5.
Other Internal Cable Routing
Cable routing inside the system should generally be routed along the metal chassis and away from
EMI sources such as the processor heatsink, clock modules, memory DIMMS, VRM components,
and high speed VLSI modules. Internal cables that connect to front I/O ports such as USB and
Audio are particularly sensitive. The use of a shielded cable or a ferrite core or both over these
internal cables can be effective at reducing EMI.
6.
Rear I/O Connector Shield
One common problem in many computer systems is poor electrical contact between the I/O
connector metal housings, the metal I/O connector plate, and the cut out in the system chassis
wall. This problem can be due to soft metal being used in the I/O connector plate or to an
insufficient number of spring-finger contacts. A solution is to use a hardened stainless spring steel
with a sufficient number of contact points to the I/O connectors and the wall of the system chassis.
Each I/O connector housing should have at least two contacts and as a general rule, there should
be a contact point at least every 1 cm between the I/O connector plate and the chassis. As a quick
remedy if this condition exists, a die-cut, conductive, foam gasket matching the I/O connector
pattern can be added to improve connector grounding to the chassis.
7.
Chassis Shielding
All chassis designs have gaps and seams to enable assembly and option installation. From an EMI
standpoint, however, some gaps are worse than others. The important dimension of a gap or seam
is the longest dimension. If you can slide a piece of paper for several inches along a seam, that
38
EMI Reduction Techniques
Chapter 5
