Intel Xeon Processor Multiprocessor Platform Design Guide

109
System Theory
10.3 Crosstalk
Crosstalk is caused through capacitive and inductive coupling between networks. Crosstalk can be
backward or forward. Backward crosstalk creates an induced signal on a victim network that
travels in a direction opposite that of the aggressor's signal. Forward crosstalk creates a signal that
travels in the same direction as the aggressor's signal. On an AGTL+ bus a driver on the aggressor
network is not necessarily at the end of the network. Therefore, it sends signals in both directions
on the aggressor's network. The signal propagating in each direction causes crosstalk on the victim
network. This effect is illustrated in Figure 10-2, which shows a driver on the aggressor network
and a receiver on the victim network. Figure 10-3 shows two aggressors on each side of the victim.
Additional aggressors are possible in the z-direction if adjacent signal layers are not routed in
mutually perpendicular directions. Because coupling coefficients decrease rapidly with increasing
separation, it is rarely necessary to consider aggressors that are at least five line widths separated
from the victim. Additionally, there is crosstalk internal to the IC packages, which can also affect
the signal quality.
Figure 10-2. Propagation on Aggressor Network
Figure 10-3. Aggressor and Victim Networks
Victim
A
ggressor
Signal Propagates in both
directions on aggressor line
.
chipset
CPUCPU
Victim
Agent 1
Aggressor
Aggressor
Aggressor
Aggressor
Agent 5