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TDR Example 1
Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
Given the following:
Board material:
Dielectric thickness:
Copper thickness:
Dielectric constant:
The formula for the characteristic impedance is:
87
Z
=
-------------------------
o
ε
+
1.41
r
For a line width, w = 0.1 inch, the characteristic impedance of the line is
calculated to be 51 Ω. A board was fabricated as shown in Figure 11-8 to the
dimensions specified above. Figure 11-8 and Figure 11-9 show the incident and
reflected waveforms observed with the TDR. The vertical scale is calibrated
both in terms of the voltage and the reflection coefficient, ρ. Equation 3 can be
rearranged to determine the characteristic impedance of the line:
1
+
0.01
Z
=
-------------------
line
1 0.01
–
where:
Z
= characteristic impedance of the line under test
line
and
Z
= impedance of the known line.
reference
The 50 Ω reference point is shown in Figure 11-9. The mean level of the
reflected waveform due to the line has a ρ = + 0.01. Substituting values into
equation 15 permits calculation of the line impedance:
1
+
Z
=
-------------------
line
1 0.01
–
which agrees closely with the calculated value.
11-22
Norplex Type G-10
h = 0.062 inch;
t = 0.0014 inch;
ε
= 5.3.
r
5.98h
ln
-------------------
0.8w
+
t
•
Z
reference
0.01
•
50 ohms
=
51 ohms
(14)
(15)
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