Digital Signal Processing Reference
In-Depth Information
2 inches
v
s
R
s
=
50
Ω
Z
o
=
50
Ω
Z
DUT
DUT
A
0-2
V
1.4
1.3
1.2
1.1
1
0.9
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Time, ns
Figure 3-44
TDR profile for Example 3-7.
Example 3-7
Calculate the equivalent
L
and
C
per unit length of the 2-in.
transmission line measured with TDR profile shown in Figure 3-44.
SOLUTION The impedance is calculated from the step voltage at 1.2 V, which
corresponds to the reflections from the transmission line under test.
v
r
=
1
.
2
−
1
.
0
=
0
.
2V
2
Z
0
Z
0
v
i
=
+
R
s
=
1V
=
Z
0
v
i
+
v
r
1
+
0
.
2
Z
DUT
v
i
−
v
r
=
50
·
0
.
2
=
75
1
−
The delay is calculated from the duration of the step:
1
τ
d
=
2
(
1
.
1ns
−
0
.
5ns
)
=
300 ps
The distributed inductance and capacitance values are calculated by solving the
impedance and delay equations simultaneously:
L
C
=
Z
0
=
75
τ
d
=
l
√
LC
=
300 ps
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