TDR Fundamentals
Step Reflection Testing
9-8
The velocity of propagation can be determined from an experiment on a known
length of the same type of cable for example, the time required for the incident
wave to travel down and the reflected wave to travel back from an open circuit
termination at the end of a 120 cm piece of RG-9A/U is 11.4 ns resulting in a v
p
= 2.1 x 10 cm/sec. Knowing v
p
and reading T from the oscilloscope determines
D. The mismatch is then located down the line.
Analyzing Reflections
The shape of the reflected wave is also valuable since it reveals both the nature
and magnitude of the mismatch. Figure 9-6 shows four typical oscilloscope
displays and the load impedance responsible for each.
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