13
Figure 7
Return loss failure with worst pair
7/8 reporting -2.0 dB margin @
4.9 MHz
The two parameters that provide the time domain information are HDTDX (High
Denition Time Domain Crosstalk) and HDTDR (High Denition Time Domain
Reectometry). As the name indicates, the HDTDX parameter shows the prole of
crosstalk happening along the link-under-test while HDTDR shows signal reections
along the link. Impedance changes cause signal reections that contribute to the
measured Return Loss value. If these reections become too big and the total amount of
energy reected exceeds the maximum amount allowed, the Return Loss test fails.
HDTDR diagnostics
In gure 7, we see a Return Loss failure at the remote end, so how do we go about
identifying the problem? First we identify the worst pair for return loss. In this example
the worst pair is pair 7/8. The worst pair result is 19 dB - that’s -2.0 dB Margin at 4.9
MHz. But if we examine the graph in more detail, we see that all the failures are below
50 MHz. This often suggests a cable issue.
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