Fluke 990DSL, User Manual

Page: 185 / 242

TDR Testing and Fault Location
What Happens During a TDR Test: An Overview
6
6-3
What Happens During a TDR Test: An Overview
You can use the TDR test to determine the length of a cable and to precisely
locate faults along the cable. When making a TDR measurement, the
CopperPro applies a balanced, high-frequency drive pulse to the test pair. Any
fault in the cable causes some of this pulse to reflect back to the CopperPro.
The tester captures these reflections (also called echoes), which can be caused
by the faults described later (see “Reflections”). The CopperPro measures the
amount of time it takes for the pulse it transmitted to return, then uses that time
to calculate the distance to the source of the reflection (the fault).
The elapsed time between the transmission of a pulse and its reflection is
measured by a highly accurate clock source. Based on the velocity of
propagation (VOP) of the pulse, the CopperPro converts the time to distance.
The type of cable you are testing determines the VOP, which is defined as the
ratio of the speed of the pulse in the cable to the speed of light. For example, a
cable with a VOP of 0.64 transmits a pulse at 64% of the speed of light.
Length Measurements and VOP
To obtain accurate length measurements, you must choose the right cable type
or supply the correct VOP of the cable that you are testing. The more accurate
the VOP, the more accurate the CopperPro’s measurement of the distance to
the source of the reflection (the fault). The CopperPro stores VOP values for
standard cable types in its memory. Be aware that actual VOP values can vary
(see “Determining the VOP” later in this chapter). Therefore, if accurate length
measurements are critical to your testing process, you should specify
actual
VOP values when you set up the test. Most cable manufacturers supply VOP
information in their specifications.
Reflections
Reflections are caused by discontinuities in a cable’s characteristic impedance,
such as gauge changes, splices, faults, the cable’s end, or series network
elements. Positive (in-phase) reflections result from sudden increases in cable
impedance, such as those due to opens, load coils, or resistive splices.
Negative (out-of-phase) reflections result from decreases in cable impedance,
such as those due to resistive faults (for example, shorts, grounds, and crosses),
bridged taps, and build-out capacitors. A cable that has no faults or network
elements and is properly terminated in its characteristic impedance generates
no reflections.