Certifying Twisted Pair Cabling
Twisted Pair Autotest Results 3
3-27
directional signals on all four wire pairs, so ELFEXT is a
critical parameter for 1000BASE-T certification.
Like ACR, ELFEXT represents a signal-to-noise ratio for
the cabling. Higher ELFEXT values mean that data signals
received at the far end of the cabling are much larger
than crosstalk signals received at the far end. Higher
ELFEXT values correspond to better cabling performance.
NEXT and ELFEXT performance tends to be similar in
cable, but may differ greatly in connecting hardware.
Some connectors achieve good NEXT performance by
balancing the inductive and capacitive currents that
cause crosstalk. Since these currents are 180° out of phase
at the near-end of the cabling, they cancel out, which
eliminates crosstalk at the near end. However, currents
that cancel at the near end add up at the far end,
causing far-end crosstalk and poor ELFEXT performance.
Figure 3-21 describes the ELFEXT plot.
Far-end
crosstalk
Signal
source
Crosstalk near
the input
Crosstalk farther
from the input
amd94f.eps
Figure 3-20. Far-End Crosstalk (FEXT)
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