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4.8 Application Guide (General Hints for TDR)
Reflections can be classified to fall into two groups:
• Regular reflections
• Reflections from faults (irregular reflections)
Regular reflections
Even faultless pairs may produce reflections caused by inherent
discontinuities such as joints or cable type changes.
Reflections from faults
A faulty pair produces regular reflections and, in addition, reflections from
the fault. Because of the attenuation losses, the reflection from a fault
long way down the cable may be much smaller than a regular reflection
from a nearby discontinuity.
A suitable method to distinguish regular and irregular reflections is to
compare the faulty cable with a good one. By using the L1-L2 method,
the regular reflections caused by the common features of the two pairs
will be canceled but the reflections due to faults will remain unchanged.
In telephone cables there are several pairs of conductors. The physical
length of the pairs depends on their position in the cable. The length is
increasing with the distance of the layer from the center. Consequently,
the physical length of pairs can be longer than the cable length, and the
propagation velocity (V/2) may be different for different layers. Therefore,
in case of comparative tests, the two pairs compared should be in the
same layer.
If there is more than one fault, the first one may reflect so much from the
pulse energy that the subsequent fault may not be seen. Therefore,
having located and eliminated the first fault, the cable section following
the fault should be tested again.
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