5. TDR Measurement Theory
The reflection coefficient is the ratio of the amplitude of the reflected portion of the test signal
to the amplitude of the incident test signal. The reflection coefficient (Gamma, Γ) is related to
the impedance change (
Z
) at a given point in a cable according to:
Γ =
Z
L
−
Z
S
Z
L
+
Z
S
Where
Z
L
is the impedance of the load (e.g., the device under test [DUT]) and
Z
S
is the source
impedance of the TDR, typically 50 ohms but potentially other values if an impedance matching
adapter is being used. The reflection coefficient uses units of rho (
ρ
) or millirho (m
ρ
).
Note that the reflection coefficient (Γ) is a complex variable with unique amplitude and phase
for each frequency. However, when measuring reflection coefficient directly from a TDR trace,
the measured value is the average reflection coefficient over all frequencies in the incident and
reflected test signal.
The CT100B is also able to measure and display frequency-domain complex reflection coefficient
values in the context of S
11
return loss and cable loss plots such as those produced with Vector
Network Analyzer (VNA) and Frequency Domain Reflectometer (FDR) equipment. This is
discussed further on page 76.
5.3. Common Types of TDR Cable Faults
Open faults appear as a pulse upward in the TDR trace (Figure 5.1) because the fault reflects
all of the incident step energy in-phase with the test signal.
Figure 5.1.
An open cable fault shows an upward step edge at the location of the fault.
Short faults appear as a pulse downward in the TDR trace because the fault reflects all of the
incident step energy 180
◦
out-of-phase with the test signal (Figure 5.2).
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 5.3
demonstrates a TDR trace with an open fault at the end of an 824 ft. RG-6 coaxial cable with
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CT100B TDR Cable Analyzers Operator’s Manual
