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The VOP of a cable can change with temperature,
age and humidity.
It can also vary from one manu-
facturing run to another.
Even new cable can vary
as much as +/- 3%.
There are several ways to determine the correct
VOP. The first is to simply refer to the VOP card
provided with the instrument. Second, consult the
manufacturer for the correct VOP of that specific
cable. A third way is to actually determine the VOP
from a known cable length. Measure a known cable
length, the longer the cable, the more accurate the
VOP will be. Correctly place the cursors of the TDR
on the output pulse and the reflected pulse (end) of
the cable. Change the VOP setting until the “Distance
Between Cursors” displays the known length. You
have now determined the VOP of the cable.
Reducing VOP error:
When trying to pinpoint a fault,
the most common technique used to reduce VOP
error is to test the faulty cable from both ends. The
procedure is as follows:
Determine the path of the cable. With a measuring
wheel or tape, measure the exact length of the
cable being tested. Set the VOP according to the
manufacturer’s specifications, test the cable from
one end and record the fault distance reading. Next,
using the same VOP setting, test from the opposite
end of the cable and again record the fault distance
reading. If the sum of the readings is the exact length
of the cable that was measured, the VOP is correct
and the fault has been located.
However, if the sum of the two readings is more than
the measured distance, reduce the VOP setting and
retest.
If the sum of the two readings is less than
the measured distance, increase the VOP setting
and retest, but the operator must also consider the
possibility of
two
faults. Keep changing the VOP
settings until the distance readings total the known
length.
The same result can be obtained mathematically.
Take the actual cable length and divide by the
sum of the two TDR readings obtained by the tests
from each end. This produces an adjustment fac-
tor. Next, multiply each of the TDR readings by the
adjustment factor. The result will be the corrected
length readings.