I/O Controller Hub 2
R
178 Intel
®
Pentium
®
4 Processor / Intel
®
850 Chipset Family Platform Design Guide
9.9.4.4.2 Distance from Intel
®
82562ET to Magnetics Module
Distance B should also be designed to be less than one inch between devices. The high-speed
nature of the signals propagating through these traces requires that the distance between these
components be closely observed. In general, any section of traces that is intended for use with
high-speed signals should observe proper termination practices. Proper termination of signals can
reduce reflections caused by impedance mismatches between device and traces. The reflections of
a signal may have a high frequency component that may contribute more EMI than the original
signal itself. For this reason, these traces should be designed to a 100 Ω differential value. These
traces should also be symmetric and equal length within each differential pair.
9.9.4.5 Reducing Circuit Inductance
The following guidelines show how to reduce circuit inductance in both back planes and
motherboards. Traces should be routed over a continuous ground plane with no interruptions. If
there are vacant areas on a ground or power plane, the signal conductors should not cross the
vacant area. This increases inductance and associated radiated noise levels. Noisy logic grounds
should be separated from analog signal grounds to reduce coupling. Noisy logic grounds can
sometimes affect sensitive DC subsystems such as analog to digital conversion, operational
amplifiers, etc. All ground vias should be connected to every ground plane; and similarly, every
power via, to all power planes at equal potential. This helps reduce circuit inductance. Another
recommendation is to physically locate grounds to minimize the loop area between a signal path
and its return path. Rise and fall times should be as slow as possible. Because signals with fast rise
and fall times contain many high frequency harmonics, that can radiate significantly. The most
sensitive signal returns closest to the chassis ground should be connected together. This will result
in a smaller loop area and reduce the likelihood of crosstalk. The effect of different configurations
on the amount of crosstalk can be studied using electronics modeling software.
9.9.4.6 Terminating Unused Connections
In Ethernet designs it is common practice to terminate unused connections on the RJ-45 connector
and the magnetics module to ground. Depending on overall shielding and grounding design, this
may be done to the chassis ground, signal ground, or a termination plane. Care must be taken when
using various grounding methods to insure that emission requirements are met. The method most
often implemented is called the “Bob Smith” Termination. In this method a floating termination
plane is cut out of a power plane layer. This floating plane acts as a plate of a capacitor with an
adjacent ground plane. The signals can be routed through 75 Ω resistors to the plane. Stray energy
on unused pins is then carried to the plane.
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