10
TS100 Cable Fault Finder
Users Guide
Time Domain Reflectometry
(TDR) Technology
Note
This section goes deeper into the theory of
operation. You can skip this section and still use
the tester effectively by reading the other parts
of this manual. However, it is worth reading this
section if you want more insight into how the
tester works.
One of the keys to understanding how the TS100
works is to first understand that a pair of wires has a
fixed impedance as long as the wires of the pair are
kept in the same geometrical relationship to each
other. A pair of wires (either standalone or within a
multi-wire cable) is designed to have a constant wire-
to-wire impedance. If the physical relationship of the
wires in the pair is altered during the wire run, then
there will be a change in impedance at the point
where the physical relationship changes. For example,
if one or both wires of the pair are broken (open), or
they are shorted to each other, or they become
sufficiently separated from each other, their impedance
will change. The TS100 looks for these changes in
impedance. If the impedance change is large enough,
(such as that caused by a break in one of the wires of
the pair), the TS100 will detect the impedance change
and will display the length of the wire up to the
impedance change.
From the previous information, it should be easy to
deduce that the TS100 can measure the length of a pair
of un-terminated wires, because, the open circuit at the
far end causes a very large impedance change.
The TS100 Cable Fault finder uses Time Domain
Reflectometry (TDR) to determine the length of the
target cable. A TDR, much like RADAR, sends a pulse
down the pair of wires. Part of that pulse reflects off
any impedance variations in the pair of wires. All of the
reflections, together with the original pulse, combine
to make an electrical signal (TDR waveform) that has
various flat and bumpy sections that represent the
start, the impedance changes, and the end of the cable.
The size and shape of the flat and bumpy sections
depend on the distance to the impedance changes and
the magnitude of the impedance changes.
For example, two runs of 12/2 ac wire joined with a
splice will have a TDR waveform with 2 flat sections
separated by a bump. The two flat sections represent
the lengths of the two sections of wire. The small bump
in the middle represents the small impedance change
at the splice point. The large bump at the end
represents the large impedance change at the end of
the wire run (see Figure 4).
TDR technology examines this TDR waveform (see
Figure 4), looking at the sizes of the flat sections and
the bumps. The software decides which of the
elements of the waveform is most representative of the
common problems encountered in the wiring
industries and reports the distance to that element.In
the case of the waveform in Figure 4, the TS100 will
report the distance to the end of the wire run and will
ignore the small bump in the middle because it is too
small to be considered a problem.
If more than one problem exists on the cable, the
software in the TS100 Cable Fault Finder only reports
the nearest problem.
The actual result of the measurement is the TIME to the
fault. The software in the tester converts the measured
time to a length by multiplying the time by the speed
of the electrical signal in that particular cable. That
speed is represented as a percentage of the speed of
light and is called the Velocity of Propagation (VOP).
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