Bedienungsanleitung Fluke networks TS 100 Cable Fault Finder (Seite 14 von 20) (Englisch)

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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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