USB 3.0 Hub Design Guide
© 2015 Genesys Logic, Inc. - All rights reserved.
Page 9
GLI Confidential
A differential pair should
avoid
discontinuation in the reference
plane
, such as
splits
and
voids
. When a
signal need
changes layers
, must keep the
same reference plane
and the
ground stitching via
should be
placed by
closing to the signal via
. A
minimum of 2 stitching via
per pair of signals is recommended. Never
route a trace so that it straddles a plane split. (See
Fig. 2.4)
Power Plane
Ground Island
L3 Vcc layer
Differential pair at Bottom layer
Signal Via
Place Stitching Vias close to Signal Via
(Space within <= 50 mils)
Place Stitching Vias to Ground layer along edge of Ground island
Plane void
Bottom Signal
Top Signal
Layer3 Power Plane
Layer3 Ground Island
Figure 2.4
2.3.6
Signal Return Path
An incorrect signal return path is one of the most common sources for noise coupling and EMI problems.
Figure 2.5 Return Current tries to follow the Signal Path
Source
Sink
Signal: Source to Sink
Signal Return Path: Sink to Source