MODELING AND BUDGETING OF TIMING JITTER AND NOISE - Advanced Signal Integrity for High-Speed Digital Design - page 545

and data rate, which often determines the amount of voltage and timing noise
allowed on a link. Accordingly, the chapter is centered on the concept of bit
error rate, and we relate each of the analysis techniques described in the chapter
to the BER to provide a useful design methodology.
Following the introduction to bit error rate, we discuss sources of timing
variation, known as jitter , that degrade the eye width, and describe the method
creating a jitter budget. We follow that with a description of the sources of
voltage noise that degrade the eye height, and describe a method for analyzing
and creating a system noise budget. We then present the peak distortion analysis
(PDA) technique for comprehending voltage and timing noise in determining the
worst-case received eye from the pulse response of a high-speed interconnect
channel. In providing a method for determining the maximum jitter and voltage
noise, PDA accounts for intersymbol interference (ISI) caused by losses and
reflections, and for degradation of the data eye due to crosstalk.

13.1 EYE DIAGRAM

Most high-speed designs use the eye diagram to evaluate system performance. We
show an example eye diagram for a 10-Gb/s 100-bit data sequence in Figure 13-1.
An eye diagram is constructed by slicing the time-domain signal waveform into
sections that are a small number of symbols in length, and overlaying them.
The horizontal axis of the eye diagram represents time and is typically one or
two symbols wide, while the vertical axis represents the amplitude of the signal.
Figure 13-2 illustrates the eye diagram construction process for both a “perfect”
eye and one that is distorted by losses and/or reflections.
As the figures show, distortion of the signal causes the data eye to close.
Conceptually, we want the eye to be as “open” as possible, as a larger eye opening
implies that we have more margin to the voltage and timing requirements. From a

Unit Interval

250

200

150

100

50

Eye height

0

Eye width

− 50

− 100

− 150

− 200

− 250

70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230

Time (ps)

Timing variation (jitter)

Figure 13-1 Example received eye diagram for a 10-Gb/s 100-bit data sequence.