PEX 8648-AA RDK Hardware Reference Manual, Version 1.2
Copyright © 2010 by PLX Technology, Inc. All rights reserved
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4.2
Board Layout Information
4.2.1 Trace Routing Design Rules
Impedance of the PCI Express differential pairs is 91 ohms ±5%. The differential pairs are routed as edge-
coupled stripline pairs.
4.2.2 Power
De-Coupling
Power de-coupling is provided by two means – plane capacitance provided by the PCB layer stackup and discrete
de-coupling capacitors. Plane capacitance filters noise above approximately 100 MHz. The footprints for the
discrete de-coupling capacitors are designed such that the inductance between the pad and plane is reduced by
careful via placement. (Refer to
Figure 24
.)
The PEX 8648 switch power balls are individually de-coupled by 0201-size, 1,000-pF capacitors, with larger-value
capacitors providing additional de-coupling around the perimeter of the package. (Refer to
Figure 25
.)
Figure 24. De-Coupling Capacitor Footprints
