Cdnext (Common-Mode To Differential-Mode Near-End Crosstalk) - Fluke Versiv Series Technical Reference Handbook

Versiv Series Cabling Certification Product Family
Technical Reference Handbook
CDNEXT (Common-Mode to Differential-Mode Near-End
Crosstalk)
CDNEXT is the difference in amplitude (in dB) between a
common-mode voltage applied to a pair and the differential-
mode voltage measured on other pairs at the same end of the
cabling. Higher CDNEXT values correspond to larger differences
between the amplitudes of the common-mode and differential-
mode voltages, which means better cabling performance. Since
CDNEXT occurs mostly in the connecting hardware, the
measurement indicates the quality of the design and
workmanship of the jacks and connectors in the installation.
At the time this manual was released, no test standards required
CDNEXT measurements.
Figure 66 on page 146 shows the CDNEXT plot.
CM to DM Conversion in Cables and Connectors
Twisted pair cabling readily picks up common-mode noise from
external sources such as motors and fluorescent lights. But not
much of this common-mode noise converts to differential mode
in the cabling because of the construction of the pairs. The wires
are very close together, and the twisting equalizes the noise
signal picked up on each wire, so that very little differential
voltage is generated.
But conversion to differential mode readily occurs in RJ45
connectors. Typically, the wires are untwisted in the plugs. Some
wires can be close to individual wires in other pairs for a
significant distance, as shown in Figure 65. This spacing causes
common-mode voltage on some pairs to be converted to
differential-mode voltage on other pairs. The problem is most
severe between pairs 4 and 3 and pairs 2 and 3.
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