Next - Fluke DSP-4100 User Manual

DSP-4000 Series
Users Manual

NEXT

The test tool measures NEXT (near-end crosstalk) by applying a test signal to one
cable pair and measuring the amplitude of the crosstalk signals received by the
other cable pairs. The NEXT value, expressed in decibels, is computed as the
difference in amplitude between the test signal and the crosstalk signal measured
at the same end of the cabling. Higher NEXT values correspond to less crosstalk
and better cabling performance.
All signals transmitted through cabling are affected by attenuation. Because of
attenuation, crosstalk occurring at the far end of the cabling contributes less to
NEXT than crosstalk occurring closer to the signal source. To verify proper
cabling performance, NEXT is measured from both ends of the cabling.
FEXT and ELFEXT
FEXT (far-end crosstalk) is the difference between the amplitude of a far end test
signal applied to one wire pair and the resulting crosstalk at the near end on a
different wire pair. Like NEXT, FEXT is expressed in decibels, and higher FEXT
values correspond to better cabling performance.
ELFEXT (equal level far-end crosstalk) is the difference between the FEXT and
the attenuation of the disturbed wire pair. Like ACR, ELFEXT is a type of signal-
to-noise ratio that indicates the transmission quality of a cable link. The name
"Equal Level FEXT" refers to the fact that all FEXT signals are attenuated equally.
Figure 7-9 illustrates this important difference between NEXT and FEXT.
Crosstalk signals received far from a transmitter contribute little to NEXT because
they are attenuated over a long distance. But all FEXT signals are attenuated by
the same amount, so they all contribute equally to crosstalk at the far end.
Because all FEXT signals travel the same distance, they tend to add up in phase.
This means that the signals always add up to a worst-case FEXT value.
There can be a difference between the NEXT and FEXT of a link, particularly in
the connecting hardware. This difference is due to the nature of the capacitive and
inductive currents that cause crosstalk. At a signal source (the near end) these
currents can subtract. If the currents subtracted at the near end, they add up at the
far end. Thus, a connector that attains high NEXT by balancing the two currents
may have very poor FEXT performance.
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