Fluke CableAnalyzer DSX-5000, Technical Reference Handbook, Page: 154 / 700

Versiv Series Cabling Certification Product Family
Technical Reference Handbook
118
Return Loss
Return loss is the difference (in dB) 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 48 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 49 shows
the return loss plot. The tester puts the cursor at the worst
margin.
For return loss failures, you can use the HDTDR analyzer plot to
locate the cause of the failure. See “The HDTDR Analyzer” on
page 194.
Note
Because the 3 dB rule applies to return loss results,
the result is usually an . See “About dB Rules” on
page 150.