Differential Signaling - Understanding Signal Integrity - page 165

become distorted when they combine with residual energy remaining on the line
caused by reflections from earlier symbols.
Any such interference between symbols [ intersymbol interference (ISI)] alters
the shape of the waveform appearing at the receiver [14-16] and is easily observed
with an eye diagram. Fortunately, signal processing techniques have been devel-
oped that allow proper signal reception of very high-speed signals, even on lines
where ISI and losses are high.

13.2.4 How Are the Effects of Loss Corrected?
We have seen in previous chapters how transmission line losses increase as fre-
quency increases, and we also saw that a signal with fast rise times have a high-fre-
quency content. Since high-data rate signaling requires sharp, fast rise time pulses,
it is apparent how transmission line losses can become significant when signaling at
high data rates. In fact, even in the absence of crosstalk or other kinds of noise, in
high-speed signaling it is quite likely for frequency dependent losses to distort the
pulses enough to prevent the link (the hardware and software connection between
a transmitter and receiver) from operating properly.
It is very common to use an equalizer on high-speed links to improve the qual-
ity of the received signal [11, 12, 14, 15]. The equalizer can either be a passive
circuit [13, 17] or a precompensation circuit built into the transmitter. With the
precompensation technique [10, 12, 18-20], the transmitter monitors the data pat-
tern that it is sending and intentionally predistorts the signal to compensate for the
transmission line loss.
When done properly, the predistortion applied by the transmitter and the dis-
tortion caused by the transmission line interact in such a way as to produce in a
much cleaner signal at the receiver. An example of the benefits of a properly equal-
ized net is shown with the eye diagram appearing in Figure 13.5.
The eye in Figure 13.5(a) is difficult to find and is an example of unaccept-
able signal integrity. Activating equalization at the transmitter as shown in Figure
13.5(b) dramatically reduces jitter and improves ISI. The eye is clearly defined and
is large enough to permit reliable link operation.

13.3 What Are the Electrical Characteristics of a Differential
Transmission Line?

In Chapter 9 we saw how two traces that were carrying signals exactly out of phase
with one another operated in the odd mode. Because this describes the way in which
ideal differential signals switch, differential signaling operates in the odd mode.
This suggests that to form a diff-pair the odd-mode impedance should be set to
the desired impedance. However, as shown in (13.1), the differential impedance is
actually twice the odd-mode impedance. This profoundly affects the way in which
differential transmission lines are created and terminated.

ZZ = 2

(13.1)

diff oo