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input tap port and test the cable back to the next tap.
Double check that there is no AC on the line. Alterna-
tively, connect the lead to the output tap port and read
the following tap. Removing face plates is a lot easier
and quicker than working with connectors.
Warning: Make sure you do not test cable with
AC on the line.
4.2 Missing Signals, Corroded Splices, and Un
-
identified Cables
Missing Signals - In certain situations cables may go
bad for no apparent reason. For example a signal
may not be getting into a cable. Start by verifying the
problem is not within the transmitting equipment. If the
problem is not in the transmitting equipment then begin
to check the cable with a TDR. Check the cable from
both ends to find the fault.
After locating the fault on the waveform, measure out
to the distance of the fault, then check for anything un-
usual. For example, a new sign driven into the ground
or a new fence, look for the obvious problem. Then
repair the faulty part of the cable.
Corroded Splices - Within many systems, there are a
lot of cable splices. Many are old and their locations
are unknown. With most splices, it is just a matter of
time before they go bad. Use a TDR to locate corroded
splices that need to be repaired.
Identifying Cables - If no markings were used during
construction, one can go back with the aid of a plant
map and use the TDR to identify cables by their length.
This is a very efficient way to accurately identify and
mark cables.
4.3 Detecting Intermittent Faults
As sometimes happens, a cable may cause a problem
only when the wind blows, the rain falls, or infrequently
for no apparent reason. When this type of problem oc-
curs, the IFD function of the instrument can be a real
time saver.
The IFD mode can be used to monitor a cable for “in-
termittent” type problems. The instrument will monitor
the cable, waiting for the mysterious or elusive event
to take place. If a change in the waveform does occur,
the instrument will capture the change, not letting it
disappear. With the event captured, the distance can
be measured and the mystery solved.
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