CROSSTALK - Advanced Signal Integrity for High-Speed Digital Design - page 149

i 1

+

v 1 C g

C M

−

i 2

+

v 2 C g

−

Figure 4-4 Coupled capacitor circuit.

Examination of equation (4-12) reveals that the effective capacitance of line 1
when it is driven in isolation is equal to the sum of the capacitance to ground and
the mutual capacitance between lines 1 and 2. Conceptually, (4-12) says that the
voltage signal applied to line 1 must charge up both the capacitance to ground and
the capacitance between lines. Therefore, the sum of the capacitance to ground
plus the mutual capacitance gives the total capacitance of line 1. Equation (4-13)
indicates that signal in line 1 will impress an unwanted signal (i.e., crosstalk
noise) on line 2 through the mutual capacitance. These effects are represented
conceptually by the capacitance sketch for the three-conductor PCB (two signals
plus a ground) shown in Figure 4-5.
We can also further our insight into the impact of mutual capacitance by
analyzing the even-mode ( v 1

= v 2 ) and odd-mode ( v 1 =− v 2 ) cases. For the

even mode, since v 1 = v 2 , we assume that the rise times (or fall times) are equal,

and therefore that dv 1 /dt

= dv 2 /dt = dv/dt . Substitution into (4-10) and (4-11)

leads to

dv
dt

i 1 = i 2 = C g (4-14)

For the odd mode we use dv 1 /dt =− dv 2 /dt = dv/dt , and find that

= C g + 2 C M

dv
dt

i 1 =− i 2 (4-15)

C m

1 2

C g C g

Figure 4-5 Coupled capacitances on a printed circuit board.