Ac And Dc Coupling; Figure 4-11: Microstrip Edge-Coupled Differential Pair; Figure 4-12: Stripline Edge-Coupled Differential Pair; Figure 4-13: Ac-Coupled Serial Link - Xilinx RocketIO User Manual

PCB Design Requirements
Tight coupling of differential traces is recommended. Tightly coupled traces (as opposed to
loosely coupled) maintain a very close proximity to one another along their full length.
Since the differential impedance of tightly coupled traces depends heavily on their
proximity to each other, it is imperative that they maintain constant spacing along their full
length, without deviation. If it is necessary to separate the traces in order to route through
a pin field or other PCB obstacle, it can be helpful to modify the trace geometry in the
vicinity of the obstacle to correct for the impedance discontinuity (increase the individual
trace width where trace separation occurs).
Figure 4-11
differential impedance.

AC and DC Coupling

AC coupling (use of DC blocking capacitors in the signal path) should be used in cases
where transceiver differential voltages are compatible, but common mode voltages are not.
Some designs require AC coupling to accommodate hot plug-in, and/or differing power
supply voltages at different transceivers. This is illustrated in
UG024 (v1.5) October 16, 2002
RocketIO™ Transceiver User Guide
and Figure 4-12 show examples of PCB geometries that result in 100Ω
W S
1
Trace
E = 4.3 H
r
Reference Plane

Figure 4-11: Microstrip Edge-Coupled Differential Pair

Reference Plane
W S
1
Trace
Dielectric
E = 4.3
r
Reference Plane

Figure 4-12: Stripline Edge-Coupled Differential Pair

TX

Figure 4-13: AC-Coupled Serial Link

www.xilinx.com
1-800-255-7778
W = 6.29 mil (0.160 mm)
1
W
2 W = 6.29 mil (0.160 mm)
2
S = 10 mil (0.254 mm)
H = 5.0 mil (0.127 mm)
Trace
Z = 55.3Ω
01
Z = 55.3Ω
02
Z
0DIFF
Dielectric
W = 3.0 mil (0.076 mm)
1
H
1
W = 3.0 mil (0.076 mm)
2
W S = 6.85 mil (0.174 mm)
2
H = 10.0 mil (0.254 mm)
1
H = 10.0 mil (0.254 mm)
2
Trace
Z = 64.8Ω
01
Z = 64.8Ω
02
Z
0DIFF
H
2
Figure
C
1
Z
0DIFF
C
2
UG024_23_020802
= 100Ω
UG024_22_041902
= 100Ω
UG024_22a_041902
4-13.
RX
R
93