Fluke DTX Series - PSNEXT (Power Sum Near End Crosstalk) Test; PSACR (Power Sum Attenuation to Crosstalk Ratio) Test; ELFEXT (Equal Level Far-End Crosstalk) Test

DTX Series CableAnalyzer
Technical Reference Handbook
3-26
PSNEXT (Power Sum Near End Crosstalk) Test
PSNEXT results show how much each cable pair is
affected by the combined crosstalk from the other pairs.
PSNEXT is the difference (in dB) between the test signal
and the crosstalk from the other pairs received at the
same end of the cabling. The tester uses the NEXT values
to calculate PSNEXT. Higher PSNEXT values correspond to
better cabling performance.
PSNEXT results are typically a few dB lower (worse) than
worst-case NEXT results.
PSACR (Power Sum Attenuation to Crosstalk Ratio)
Test
PSACR values indicate how the amplitude of signals
received from a far-end transmitter compares to the
combined amplitudes of crosstalk produced by near-end
transmissions on the other cable pairs. PSACR is the
difference (in dB) between PSNEXT and attenuation
(insertion loss). The tester uses the PSNEXT and
attenuation results to calculate PSACR values. Higher
PSACR values mean received signals are much larger than
the crosstalk from all the other cable pairs. Higher PSACR
values correspond to better cabling performance.
PSACR is the difference (in dB) between each wire pair’s
attenuation (insertion loss) and the combined crosstalk
received from the other pairs. The tester uses the PSNEXT
and attenuation values to calculate PSACR values.
PSACR results are typically a few dB lower (worse) than
worst-case ACR results.
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 3-20. 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 3-14).
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-