5. TDR Measurement Theory
The purpose of this section is to familiarize the operator with the basic theory of time-domain
reflectometry measurement theory in preparation for using the CT100B instrument.
5.1. Time-Domain Reflectometry (TDR)
TDR is a form of closed-circuit radar in which a transient test signal is injected into a
device-under-test (DUT) such as a cable, and reflected voltages are measured over time to
construct a TDR waveform or “trace”. Assuming a transmission line such as a cable with
uniform geometry, the test signal propagates with a characteristic velocity such that the time
can be related to distance. The changes in reflected voltage in a TDR trace with respect to time
correspond to distance from the test port to impedance changes.
Cable or connector faults are regions where the measured reflection coefficient and associated
impedance are outside of manufactured specification. Cable faults are almost always broadband
with preferential attenuation of higher frequencies, and indicate one or more of the following
problems:
1. Change in the geometry of the conductors with respect to one another,
2. Change in the dielectric properties of the insulator, and/or
3. Partial or complete interruption in one or both of the signal conductors.
Cable faults are important because they degrade signal quality. Short or open faults completely
disrupt signal transmission. In many ways such “hard” faults are helpful because they are easy
to diagnose. However, more subtle partial or “soft” faults cause portions of the signal energy to
be reflected and/or delayed, contributing to noise. This can show up as reduced bandwidth in
microwave/RF cable systems and increased bit error rates in digital cable systems.
Multiple small faults that are individually insignificant with respect to noise may contribute in
an additive fashion to exceed the overall “noise budget” of the cable assembly. Because TDR
typically measures the transmission line with sensitivity well below the bit error threshold of
most communications systems, it is useful to find these “insignificant” faults and deviations
from manufactured specification that may contribute to signal integrity problems.
5.2. Reflection Coefficients
The amplitude of reflected voltage at a particular location in a TDR waveform is determined by
the reflection coefficient at that location. The CT100B displays reflection coefficient on the right
hand side of the screen with units of millirho (m
ρ
) and relative reflection coefficient with prefix
of ∆m
ρ
.
CT100B TDR Cable Analyzers Operator’s Manual
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