Digital Signal Processing Reference
In-Depth Information
Figure 13.3
A digital signal analyzer such as this Tektronix DSA 8200 [7] is used to display a dif-
ferential data stream. (Courtesy of Tektronix, Inc. Used with permission.)
All rising edges lay within
this region
Timing
uncertainty
(jitter)
Latest falling edge
Highest V
ih
Earliest falling edge
Lowest V
ih
Mask
Earliest rising edge
Highest
V
il
Lowest
V
il
Latest rising edge
All falling edges lay
within this region
Figure 13.4
The earliest and latest appearances of the rising and falling edges and their amplitudes are
simultaneously displayed in this eye diagram. (
From:
[13]. Used with permission.)
In reality jitter will prevent the pulses edges from perfectly lining up, and reflec-
tions and losses will prevent each pulse from having the same logic low (
V
il
) and
high levels (
V
ih
). These things reduce the horizontal and vertical eye opening and,
if severe enough, can prevent proper operation.
13.2.3 What Is Intersymbol Interference?
We saw in Figure 8.2 how transmission line losses cause a pulse to shrink in height
(amplitude) and for its base to grow in width. This widening at the base is particu-
larly serious because if it grows large enough, successive pulses (symbols) will blend
together, making it difficult to distinguish between each one. Symbols can also
