Fluke DTX-1800, Technical Reference Handbook, Page: 115 / 398

Certifying Twisted Pair Cabling
Twisted Pair Autotest Results
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3
amd94f.eps
Figure 3-22. Far-End Crosstalk (FEXT)
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 ACR-F (attenuation to crosstalk ratio, far end)
values. Since ACR-F 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
ACR-F 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 ACR-F is a critical parameter for
1000BASE-T certification.
Like ACR-N, ACR-F represents a signal-to-noise ratio for the
cabling. Higher ACR-F 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 ACR-F
values correspond to better cabling performance.
Far-end
crosstalk
Signal
source
Crosstalk near
the input
Crosstalk farther
from the input