Datasheet

SZZA016B
7–237 Basic Design Considerations for Backplanes
Figures 6 and 7 show the effects of the term
1 )
ǒ
C
d
ńC
o
Ǔ
Ǹ
on Z
o
and t
pd
by plotting the
normalized effective impedance and t
pd
in terms of distributed capacitance divided by the
characteristic capacitance, C
d
/C
o
.
0.5
0.4
0.2
0.1
0
0.9
0.3
0123456
0.7
0.6
0.8
1
78910
11 12 13 14 15
C
d
/C
o
NORMALIZED IMPEDANCE
Z
o(eff)/
Z
o
Figure 6. Effective Impedance vs C
d
/C
o
2.5
2
1
0.5
0
1.5
0123456
3.5
3
4
78910
11 12 13 14 15
C
d
/C
o
NORMALIZED t
pd
t
pd(eff)/
t
pd
Figure 7. Effective t
pd
vs C
d
/C
o
An easy-to-follow calculation using Figures 6 and 7 is based on a 50-Ω line (Z
o
) with a t
pd
of
180 ps/in. (7.09 ns/m) used in a system where the C
d
/C
o
ratio is 3. From Figure 6, the C
d
/C
o
of 3 yields an effective impedance of 0.5 times the characteristic impedance, or
Z
o(eff)
= 0.5 × 50 = 25 Ω. Figure 7 shows that for the same C
d
/C
o
ratio of 3, the transmission-line
effective propagation delay has doubled and t
pd(eff)
= 2 × 180 = 360 ps/in. (14.18 ns/m).