caused by any changes in the imped-
ance of the cable such as those caused by splices and connectors in the
path, shorts between conductors, splits in cable pairs, load coils, water
intrusion, and any type of damage that affects the impedance of the line.
The EZ-TESTER Series TDRs display the distance to potential faults
with a graphical display, using a horizontal line representing the distance
along the cable being tested. Cables without flaws generate a flat trace.
When faults are detected, a bump on the trace represents an increase in
impedance, and a dip represents a decrease in impedance with the leading
edge of the bump or dip representing the relative distance along the line to
the fault. The open end of a cable being tested shows up as a large bump
on the trace. A dead short in a cable shows up as a large dip on the trace.
Faults of lesser degree generate traces with smaller bumps and dips. In
many cases, the nature of the fault may be distinguished by subtle differ-
ences in the shape of the bumps and dips.
Factors Affecting Accurate Measurements
Factors that affect the accuracy of a TDR measurement include
the type of dielectric used in the cable, the gauge of the conductors, the
temperature of the cable, the length and shape of the electrical pulse being
A P P L I C A T I O N N O T E
a publication of
MEETING YOUR TESTING NEEDS TODAY AND TOMORROW
Publication Number TTS3-0901
1
Typical TDR Display
Practical Tips for Using Metalic Time
Domain Reflectometers
(The EZ Way) transmitted by the test set, and the distance the pulse
travels. The accuracy of information provided to the
test set during setup directly affects the accuracy of the
measured distance.
The distance between the TDR and a potential fault
is computed by measuring the time delay between the trans-
mission of a pulse and the return of reflected energy. The
speed at which the pulse passes down and back along the
cable is primarily determined by the manufactured character-
istics of the cable and is called the Velocity of Propagation
(VOP). The computed VOP uses the speed of light as the
reference, with the speed of light being 1 or 100%. A cable
with a VOP of .75 can transmit a pulse at at a speed of 75%
of the speed of light. Different cables are manufactured with
varying electrical characteristics caused by differences in
gauge, insulating materials (dielectric), and spacing between
the conductors. Each variation affects the cable's electrical
impedance which, in turn affects the VOP for that cable.
External factors at the time of measurement, such as the
temperature of the cable, also affect the VOP and must be
taken into account when a TDR measurement is made. The
test set provides a setup screen to enable the user to enter
this information.
Cable Characteristics Setup Screen
The accuracy of TDR measurements depends on
the accuracy of information entered into the test set during
setup. The percentage of error in the VOP entry will result in
an equal percentage of error in the computed distance
between the test set and the fault. To assist the user in
making an accurate measurement with the TDR, the test set
automatically computes the VOP as the user selects the
appropriate cable characteristics on the setup screen.
Range and Resolution
During the period of time that each pulse is being
transmitted by the TDR, the receive section of the test set
is blind to reflected energy. The blind zone is represented
by the flat top of the trace on the left side of the TDR
screen as shown in the circled enlargment of the trace
below. Following the flat section, the trace drops and
curves to the right as the transmitted pulse charges the
line.
TDR Blind Zone
The pulse width is designed to be as short as
possible for each range setting to minimize the blind zone,
however, as the range setting is increased to cover a
longer distance along the cable, a wider pulse is necessary
to be sure there is enough energy to allow the reflection to
return to the test set. The longer range setting also results
in a longer blind zone. In the EZ-TESTER Series, the
shortest range setting is 750 feet with a blind zone of about
40 feet. The maximum range of the test set is 32,000 feet
with a blind zone of about 850 feet.
After the test set has displayed a trace, the user
must mark the leading edge of a bump or dip with a movable
2 cursor, represented by a vertical line. The test set deter-
mines the distance to the fault by measuring the position of
the cursor. The ability of the test set to clearly display the
leading edge of the bump or dip is referred to as the resolution.
The shorter the pulse, the better the resolution. The longer
distance the pulse must travel, the greater the distortion
of the pulse. Therefore, the best resolution of the display
is obtained, by using the lowest range setting possible.
In circumstances when long distance tests are necessary,
diminished resolution may prevent the test set from
detecting two faults that are close together. When a fault
exists on the line, some of the energy from the pulse will
be reflected back to the test set and some of the energy will
continue farther down the line, possibly allowing the test
set to detect and display a second fault. Depending on
the severity of the first fault, so much energy may be
reflected that the second fault will not be detected.
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Tips for Using a TDR
1. Whenever possible, always obtain the manufacturer's specifications for the cable being tested before departing the
shop for the test site. Accurate information is essential to accurate distance measurements. Cable conductor gauge and VOP
are available from the manufacturer and can often be found on cable spools. Refer to local documentation for cable length so
that the far end bump isnt mistaken for a fault.
2. Determine the velocity of propagation (VOP) as accurately as possible before performing the CABLE
CHARACTERISTICS setup. If the VOP is not known, test a known length of an identical cable type. Experiment with the VOP
setting and perform TDR measurements on the sample cable until the test result matches the known length of the cable, then use
the VOP setting on the actual cable being tested.
3. Temperature of the cable being tested affects the VOP. During the CABLE CHARACTERISTICS setup, be sure to
include as accurate a temperature value as is practical to improve measurement accuracy.
4. Always break the cable into individual sections wherever possible and test each section separately. Measurements
on short sections are often more accurate than measurements made on long sections.
5. Regardless of the length of each section, always start testing with the shortest range setting to improve detection of
small faults close to the testing point. The shorter pulse widths have the shortest blind zone and allow detection of close-in faults
that may be missed at a higher range setting. When the short range tests indicate no potential problems, increase the range
settings one at a time.
6. Whenever possible, test each section from both ends. Potential faults close to the far end may not have been
detected due to signal losses, or may be inaccurately measured because of decreased resolution due to long pulse length. By
moving to the far end and repeating the tests, those faults may show up more clearly since shorter range setting may be used.
Testing from both ends may compensate for an incorrect VOP setting. A single fault that is detected from both ends of the cable
may appear to be at two different locations on the line. In this event, compute the midpoint between the two measurements and
inspect that position on the cable for the fault.
7. After locating and repairing a fault, always retest the section. The first fault may have masked a smaller fault farther
down the line. 4
8. When a bump or dip is detected, position the cursor to the leading edge, then use the zoom feature to center the
cursor. Increase the display gain to sharpen the leading edge, and reposition the cursor for a more accurate measurement.
9. Very minor faults at shorter ranges may be more detectable with longer pulse widths. If a 2,500 foot cable appears
normal with the tester set to the 2,700 foot range, retest the cable at the higher range settings to generate a stronger reflection.
10. If a fault is suspected within the 40 foot blind zone at the 750 foot range setting, and is not accessible for visual
inspection, attach a good section of the same type cable using a good quality connection, and retest the cable. The added section
of cable will occupy the blind spot and effectively move the fault onto the usable portion of the trace.
11.
Use your technical experience and common sense to recognize discrepancies between test results and your
knowledge of the cable being tested. If the displayed distance to a fault is 1000 feet and maintenance has recently been performed
at 950 feet, the location of the fault and the location of the maintenance may be the same.
- - NORMAL LINE
The trace for a normal line begins with the initial plateau shape of the blind zone,
followed by the drop as the pulse charges the line. The remainder of the trace is
flat until the open end of the cable generates the characteristic bump. The
sharpness of the bump can be increased by increasing the range setting and
gain of the display. It is decreased by longer distances, shorter range setting,
lower gain, and by the presence of faults between the test set and cable end.
SHORTED LINE - -
A shorted line is displayed as a sharp dip in the trace. In this case, circuit