SYSTEM ANALYSIS USING RESPONSE SURFACE MODELING - Advanced Signal Integrity for High-Speed Digital Design - page 621

to 0 . 2733. Repeating the process to find the worst-case corner for eye width,

we find that the worst-case corner is R TT =

−

60 , R Tx = 40 , and Z diff = 84

for the 0.508-m length. At an equalization setting of − 0 . 2467, the worst-case eye

width is greater than 74 ps, which meets the spec.
Before moving on, we note that an alternative approach to finding the
worst-case corner would be to look at the worst-case observations from the
original experiment. They will often provide insight into the system trends and
response sensitivities.

Contour Plots Another technique for visualizing the behavior of the system
that provides insight into the interactions between variables is the contour plot.
We show an example in Figure 14-6, in which the impedances are set at the
worst-case values that we determined previously. As the figure shows, there is
a clear interaction between the equalization setting and the length of the system
in determining the eye height. The longer the trace length, the more equalization
we need in order to maximize the eye height. (This is, of course, what we expect
for a lossy transmission-line pair.) The contour plot confirms our conclusion that
we want the equalization value to be set at − 0 . 26 for the maximum-length case.

−

It also shows that even though a setting of 0 . 26 is not optimum for shorter

lengths, the eye widths still improve as the length decreases.

Eye Height (mV)

Predicted Eye Height Contour Plot

− 0.1

105-110
100-105
95-100
90-95
85-90
80-85
75-80
70-75
65-70
60-65
55-60
50-55
45-50
40-45
35-40
30-35
25-30
20-25

− 0.12
−

0.14

− 0.16

0.18

−

− 0.2

− 0.22
−

0.24

− 0.26

− 0.28

−

0.3

Length (m)

Figure 14-6 Contour plot at the worst-case corner for the example system.