Fluke Networks CertiFiber Pro Users Manual - Manual (Page 132)

Versiv Cabling Certification Product Family
Technical Reference Handbook
110
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. The Autotest includes the CDNEXT test
for troubleshooting purposes only.
Figure 56 on page 111 shows the CDNEXT plot.
CM to DM Conversion in Cables and Connectors
Twisted pair cabling can readily pick 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 55. 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.
Newer Connectors Reduce Mode Conversion
Newer Cat 6A connector designs compensate for the inherent
unbalance of RJ45 connectors, and have much lower mode
conversion than older designs. Links constructed with these
connectors have better EMI rejection and also tend to have better
alien crosstalk performance.