7. TDR Measurement Theory
Figure 7.2. A short cable fault shows a downward step edge at the location of the fault.
Short or open faults measured through long lengths of cable (hundreds of feet) will show long,
shallow reflections on the TDR trace because the cable preferentially attenuates higher frequencies
in the test signal, degrading the rise or fall time of the reflected fault. Figure 7.3 demonstrates a
TDR trace with an open fault at the end of an 824 ft. RG-6 coaxial cable with long, shallow
reflected rise caused by cable attenuation of high-frequency components of the step-pulse.
Figure 7.3. An open cable fault at 824 ft.
Faults with reactive components such as capacitance and inductance appear as either dips below or
bumps above the characteristic impedance of the cable, depending on whether they are in series
with the conductor or represent a fault to ground.
Normal connectors will show up as an impedance discontinuity simulating a small fault. Depending
on the type and quality of the connectors, the expected impedance variation will differ, as shown in
CT100B TDR Cable Analyzers Operator’s Manual 97
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