3.4. CROSSTALK-INDUCED NOISE
As explained in
Section 3.1
, crosstalk is the result of mutual capacitance C
m
in conjunction
with mutual inductance L
m
between adjacent conductors. The magnitude of the noise
induced onto the adjacent transmission lines will depend
Figure 3.2: Crosstalk-induced current trends caused by mutual inductance and mutual
capacitance.
on the magnitude of both the mutual inductance and the mutual capacitance. For example, if
a signal is injected into line 1 of
Figure 3.2
, current will be generated on the adjacent line by
means of L
m
and C
m
. For simplicity, let us define a few terms. Near-end crosstalk is defined
as the crosstalk seen on the victim line at the end closest to the driver (this is sometimes
called backward crosstalk). Far-end crosstalk refers to the crosstalk observed on the victim
line farthest away from the driver (sometimes known as forward crosstalk). The current
generated on the victim line due to mutual capacitance will split and flow toward both ends of
the adjacent line. The current induced onto the victim line as a result of the mutual
inductance will flow from the far end toward the near end of the victim line since mutual
inductance creates current flow in the opposite direction. As a result, the crosstalk currents
flowing toward the near and far ends can be broken down into several components (see
Figure 3.2
):
(3.8)
(3.9)
The shape of the crosstalk noise seen at the near and far ends of the victim line can be
deduced by looking at
Figure 3.3
. As a digital pulse travels down a transmission line, the
rising and falling edges will induce noise continuously on the adjacent line. For this
discussion, assume that the rise and fall times are much smaller than the delay of the line.
As described previously, a portion of the crosstalk noise will travel toward the near end of the
line and a portion will travel toward the far end. The portions traveling toward the near and
far ends will be referred to as the near- and far-end crosstalk pulses, respectively. As
depicted by
Figure 3.3
, the far-end crosstalk pulse will travel concurrently with the edge of
the signal on the driving line. The near-end crosstalk pulse will originate at the edge and
propagate back toward the near end. Subsequently, when the signal edge reaches the far
end of the driving line at time t = TD (where TD is the electrical delay of the transmission
line), the driving signal and the far-end crosstalk will be terminated by the resistor. The last
portion of the near-end crosstalk induced on the victim line just prior to the signal being
terminated, however, will not arrive at the near end until time t = 2TD because it must
propagate the entire length of the line to return. Therefore, for a pair of terminated
transmission lines, the near-end crosstalk will begin at time t = 0 and have a duration of 2TD,
or twice the electrical length of the line. Furthermore, the far-end crosstalk will occur at time t
= TD and have a duration approximately equal to the signal rise or fall time.