Mohr CT100B Series - Operating Instructions; Connecting to the Cable or Device-Under-Test (DUT)

4. Operating Instructions
If you would like a detailed explanation of TDR measurement theory and applications before using
the CT100B to test cables, please read Section 7, TDR Measurement Theory, or go to
www.mohrtm.com.
4.1. Connecting to the Cable or Device-Under-Test (DUT)
The first and most important decision to make when testing a cable or device-under-test (DUT) is
how to connect to the DUT. Because the connection to the DUT acts as a filter for the test signal
into and out of the DUT, it is very important that the connection have the highest bandwidth and
least aberration possible.
Controlled impedance connections are recommended whenever possible. Controlled impedance
connections have conductors with uniform geometry and dielectric properties that do not change
significantly during a test session or between test sessions. Examples of controlled impedance
connections include coaxial-coaxial adapters, coaxial-twisted-pair adapters, and fixed-pitch TDR
probes.
Alligator clip and clip lead adapters with flexible wire test leads should be used only as a last
resort. Alligator clip and clip lead adapters are not made for high-frequency operation, are severely
bandwidth-limiting, and introduce severe aberration and measurement uncertainty into the TDR
trace. Figure 4.1 shows a comparison test of a coaxial cable using clip lead adapters (red, top) and
a controlled-impedance adapter (black, bottom). The clip lead adapter TDR trace is severely
degraded.
MOHR is able to supply controlled-impedance adapters for almost every type of cable and
connector.
CT100B TDR Cable Analyzers Operator’s Manual 39