DTX Series CableAnalyzer
Technical Reference Handbook
3-24
Return Loss
Return loss is the difference between the power of a
transmitted signal and the power of the signals reflected
back. The signal reflections are caused by variations in
the cable’s impedance. Figure 3-18 shows some common
sources of reflections that create return loss.
High return loss means the cabling reflects very little of
the transmitted signal back to the source. High return
loss is especially important for high-speed systems, such
as Gigabit Ethernet. The bi-directional (full-duplex)
transceivers used in these systems use directional couplers
to distinguish between incoming and outgoing signals.
The couplers may interpret strong reflected signals as
incoming data, resulting in data errors.
A return loss plot indicates how well a cable’s impedance
matches its rated impedance over a range of frequencies.
Figure 3-19 describes the return loss plot.
For return loss failures, the testers diagnostic screens
(
J Fault Info) may show more than one possible cause
for the failure. In this case, you can use the HDTDR
analyzer results to further diagnose the problem. See
Chapter 4 for details.
Signal
source
Kinks and other
distortions
ConnectionsVariations in materials
and construction
Reflected
signals
amd93f.eps
Figure 3-18. Sources of Return Loss
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