7. TDR Measurement Theory
The purpose of this section is to familiarize the operator with the basic premise of time-domain
reflectometry measurement theory as part of using the CT100B instrument.
7.1. 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 at precise elapsed
times 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 constant velocity and 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 (e.g., a deformed
cable),
2. Change in the dielectric properties of the insulator (e.g., moisture in the insulation), 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 our TDR
typically measures the transmission line with more sensitivity than 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.
7.2. Reflection Coefficients
The amplitude of reflected voltage at a particular location in a TDR waveform is recorded as the
reflection coefficient at that location. The CT100B displays reflection coefficient on the right side
of the screen with units of millirho (mρ).
CT100B TDR Cable Analyzers Operator’s Manual 95
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