DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 302

common-mode propagation velocities are approximately equal to the odd- and
even-mode velocities if the interpair coupling is weak. Odd- and even-mode
velocities are defined in Section 4.3.1:

ν differential ≈ ν odd (7-6a)

ν common ≈ ν even (7-6b)

To quantify the voltage propagating in the differential and common modes, the
following definitions are often used:

= V − V

V dm (7-7a)

V + V

2

V cm = (7-7b)

where V dm is the voltage propagating in the differential mode, V cm is the volt-
age propagating in the common mode,

V represents the voltage propagating

on line 1 (the same as v D + used in the two-signal-conductor example), and V

represents the complementary signal propagating on line 2 ( v D − used in the

two-signal-conductor example).

7.6 DRAWBACKS OF DIFFERENTIAL SIGNALING

Differential signaling is a powerful tool used to design high-speed digital buses
that dramatically reduces the amount of common-mode noise seen at the receiver,
which allows higher data rates to be realized. However, differential signaling is
not a panacea. Most obviously, a differential bus consumes significantly more
area on the printed circuit board than does a single-ended bus. Sometimes, dif-
ferential buses are so large that they force designers to use printed circuit boards
with more layers, which increases cost. The increased number of signals drives
package, socket, and connector pin counts to higher levels, which complicates
designs and further increases system costs. Also, the impedance tolerance of
differential pairs tends to be lower than single ended because the variations in
etching and plating profiles of the conductors affect mutual inductance and capac-
itance and, consequently, differential impedance. Furthermore, small asymmetries
in the differential pair can have a large impact on signal integrity if not controlled
properly.

7.6.1 Mode Conversion

In Section 7.4 we showed how common-mode noise coupled onto a differen-
tial pair causes voltage to propagate in the even mode, which is rejected by
the receiver assuming that the common-mode rejection ratio is high enough.
Another mechanism that causes even-mode voltage to propagate are phase errors