RocketIO™ Transceiver User Guide www.xilinx.com 117
UG024 (v3.0) February 22, 2007
PCB Design Requirements
R
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 Figure 3-15.
Capacitors of value 0.01 μF in a 0402 (EIA) package are suitable for AC coupling at
3.125 Gb/s when 8B/10B encoding is used. Different data rates and different encoding
schemes may require a different value.
DC coupling (direct connection) is preferable in cases where RocketIO transceivers are
interfaced with other RocketIO transceivers or other Mindspeed transceivers that have
compatible differential and common mode voltage specifications. Passive components are
not required when DC coupling is used. This is illustrated in Figure 3-16.
Figure 3-13: Microstrip Edge-Coupled Differential Pair
Figure 3-14: Stripline Edge-Coupled Differential Pair
UG024_22_042903
Trace Trace
Reference Plane
Dielectric
W
2
W
1
S
H
W
1
= 6.29 mil (0.160 mm)
W
2
= 6.29 mil (0.160 mm)
S = 10 mil (0.254 mm)
H = 5.0 mil (0.127 mm)
Z
01
= 55.3Ω
Z
02
= 55.3Ω
Z
0DIFF
= 100Ω
E
r
= 4.3
UG024_22a_042903
TraceTrace
Reference Plane
Reference Plane
Dielectric
W
2
W
1
S
H
2
H
1
W
1
= 3.0 mil (0.076 mm)
W
2
= 3.0 mil (0.076 mm)
S = 6.85 mil (0.174 mm)
H
1
= 10.0 mil (0.254 mm)
H
2
= 10.0 mil (0.254 mm)
Z
01
= 64.8Ω
Z
02
= 64.8Ω
Z
0DIFF
= 100Ω
E
r
= 4.3
Figure 3-15: AC-Coupled Serial Link
C
1
C
2
UG024_23_042503
TX
RX
Z
0
= 100Ω Differential
Product Not Recommended for New Designs
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