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Example:
TDR readings equal 700 feet and 500
feet from either end.
Actual cable distance equals
1000 feet.
700 + 500 = 1200
1000/1200 = Adjustment Factor = 0.833
700 x 0.833 = 584 actual (corrected length)
500 x 0.833 = 416 actual (corrected length)
NOTE: When measuring cable reels, cable coiled
on the reel can cause an error in the length read-
ing by as much as 2 to 5%.
3.6 Pulse Widths
Many TDRs have selectable pulse width settings. The
pulse width allows the TDR signal to travel down a
cable at different levels of energy and distances. The
wider the pulse width, the more energy is transmitted,
and therefore, the further the signal will travel down
the cable.
NOTE: Always start the fault finding procedure
in the shortest pulse width available, as the
fault may be only a short distance away. Use
the range or zoom and gain controls to locate
fault. If the fault is not located, adjust to the next
range or larger pulse width and retest. Keep
adjusting to the next larger pulse until the fault
is located.
Cable Loss
Cable has loss. A signal attenuates as it travels down
a cable. Some cables have greater loss or signal at-
tenuation than others. Because the pulse amplitude is
reduced by the loss in the cable, major faults at long
distances will appear to be of the same amplitude as
minor faults close to the instrument.
Attenuation affects the maximum length of cable that
can be tested. The greater the cable attenuation, the
more energy must be sent down the cable to test longer
lengths. To increase the amount of energy transmitted
into the cable, increase the pulse width. The Model
1205CXA has multiple pulse widths which the operator
can select to best accommodate the cable length being
tested. However, since the location of a fault is unknown,
it is best to start the testing procedure in the shortest
pulse and increase the pulse widths as the distance
being tested is increased.
