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9.7.4.1
Distance from Magnetics Module to RJ45 (Distance A)
The distance A in
between the magnetics module and the RJ45 connector should be kept to less than 1 inch of
separation. The following trace characteristics are important and should be observed:
Differential Impedance: The differential impedance should be 100 Ω. The single ended trace
•
impedance may be approximately 50 Ω; however, the differential impedance may also be
affected by the spacing between the traces.
•
Trace Symmetry: Differential pairs (such as TDP and TDN) should be routed with consistent
separation, and with exactly the same lengths and physical dimensions (for example, width).
Warning:
Asymmetric and unequal length traces in the differential pairs contribute to common mode noise.
This may degrade the receive circuit's performance and contribute to radiated emissions from the
transmit circuit. When the 82562ET must be placed further than a couple of inches from the RJ45
connector, distance B may be sacrificed. Keeping the total distance between the 82562ET and RJ45
as short as possible should be a priority.
Note: Measured trace impedance for layout designs targeting 100 Ω often results in lower actual
impedance. OEMs should verify actual trace impedance and adjust their layout accordingly. When
the actual impedance is consistently low, a target of 105 Ω –110 Ω should compensate for second
order effects.
9.7.4.2
Distance from Intel® 82562ET to Magnetics Module (Distance B)
Distance B should also be designed to be less than 1 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 may 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.
9.7.5
Terminating Unused Connections
In Ethernet designs, it is common practice to terminate unused connections on the RJ45 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, and couples capacitively to the ground plane, creating the required 1500 pF
of capacitance. The signals may be routed through 75 Ω resistors to the plane. Stray energy on
unused pins is then carried to the plane.
Design Guide
®
®
Intel
Pentium
M Processor and Intel
Figure 123
should be given the highest priority in board layout. The distance
®
E7501 Chipset Platform
I/O Controller Hub 3 (Intel
®
ICH3-S)
177
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