DTX Series CableAnalyzer
Technical Reference Handbook
3-28
ELFEXT (Equal Level Far-End Crosstalk) Test
While NEXT is measured at the same end as the signal
source, FEXT (far-end crosstalk) is measured at the far end.
Because all far-end crosstalk signals travel the same
distance, they experience the same amount of attenuation,
as shown in Figure 3-14. This means that all crosstalk signals
contribute equally to noise at the far end. This is different
from near-end crosstalk. At the near end, crosstalk
occurring closer to the source contributes more to noise
than crosstalk occurring farther from the source
(Figure 3-20).
Because of attenuation, FEXT on longer cables is less than
FEXT on shorter cables of the same type. Subtracting the
effects of attenuation normalizes the results for length and
produces ELFEXT (equal level far end crosstalk) values. Since
ELFEXT does not depend on length, it is used instead of
FEXT to evaluate cable performance.
Because all far-end crosstalk signals travel the same
distance, they tend to add up in phase. Therefore, high
ELFEXT is critical when two or more wire-pairs carry signals
in the same direction. 1000BASE-T carries bi-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.
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