CROSSTALK - Advanced Signal Integrity for High-Speed Digital Design - page 167

t = ½ t d ⋅ I

Aggressor: Z 0 , t d , I
Near-end × talk pulse at t = ½ t d ⋅ I

Victim : Z 0, t d , I

Far-end × talk pulse at t = ½ t d ⋅ I

Figure 4-18 Propagation of incident aggressor signal and coupled noise pulses.

the aggressor signal, or it may have opposite polarity. The forward coupled wave
then begins to propagate toward the far end of the victim line ( z = l ). We also
have a backward coupled wave that is a function of the sum of the capacitively
and inductively coupled currents. The backward traveling noise will have the
same polarity as the aggressor signal, which is a direct result of the summing of
the inductive and capacitive coupling. The backward crosstalk noise propagates
toward the near end of the victim line, where it is immediately detectable.
As the incident wave on the aggressor propagates toward the far end, it contin-
ues to couple energy over to the victim line. As Figure 4-18 shows, the forward
crosstalk pulse on the victim line propagates alongside the aggressor signal. Since
coupling continues along the length of the line, the amplitude of the far-end noise
pulse grows as it propagates along the length of the coupled pair. The backward
crosstalk pulse propagates back toward the near end ( z = 0).

The coupling of energy to the victim line continues as the aggressor propagates
along the line until it reaches the far end at time t = τ d l , where τ d is the signal
propagation delay per unit length and l is the line length. Alternatively, we
could define the time as t = l/ν p , where ν p is the propagation velocity of the
signal. At that point, assuming that we have a matched termination, the coupling
ceases since the aggressor does not generate reflected waves. As Figure 4-19
demonstrates, arrival of the far-end crosstalk noise occurs simultaneously with
arrival of the aggressor signal. Since the far-end crosstalk travels along with the
aggressor signal, the crosstalk noise pulse grows in amplitude but does not grow
in width. Coupling occurs only during the signal transition, so the width of the
forward coupled pulse will be approximately equal to the rise (or fall) time of

t = t d ⋅ I

Z 0 , t d , I

Near-end × talk pulse at t = t d ⋅ I

Z 0 , t d , I

Far-end × talk pulse at t = t d ⋅ I

Figure 4-19 Propagation of coupled noise pulses as the aggressor reaches the far end
(z = l) .