Basic Cable Testing
Locating NEXT and ELFEXT Problems
7
7-15
The vertical scale represents the magnitude of the crosstalk detected. The crosstalk
levels shown on the plot are adjusted to compensate for cable attenuation. Without
this adjustment, the peak on the right side of the plot (farther from the test tool)
would appear much smaller. The adjusted plot helps you identify crosstalk sources
because you can use the vertical scale to measure crosstalk magnitudes plotted at
any distance from the test tool. You can also compare the relative magnitudes of
crosstalk peaks to determine the largest sources of crosstalk on the cable.
The vertical scale increases logarithmically. The scale units are arbitrary. A level
of 50 represents a crosstalk magnitude that is close to causing a cable failure. The
level of 100 is approximately 20 times greater than the level of 50. A level of 100
represents an extremely high level of crosstalk that is typically due to split pairs.
Cables or other hardware that cause crosstalk levels of 100 or above are considered
unusable. Crosstalk levels near 0 are considered to be inconsequential.
The HDTDX plot from a cable that failed a NEXT test may show one or more
peaks of crosstalk greater than a level of 50. A failure can also be caused by a
crosstalk level less than 50 if the level is sustained over a substantial distance of
the cable.
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