Chapter 3
Calibration, Configuration, and Special Operations
3-50
Evaluating a TDR Graph
When evaluating the TDR graph, the first consideration should be the average
impedance of the cable being tested:
• A graph with sharp dips or spikes can identify anomalies.
• For subtle problems, interpretations should be based on benchmark data
obtained from the analysis of many cable segments.
When anomalies are detected in the cable pair, the cursor is automatically placed
on the first anomaly. Press the Arrow keys to position the cursor on the anomaly
of interest. Impedance and distance values will be displayed on the screen.
The following troubleshooting tips identify typical cable conditions and their
corresponding screen plots.
Condition TDR Plot Indication
Open, Near End Upswing develops early. In comparison to good cable
pairs, this pair appears shorter in length.
Short, Near End Downswing develops early. In comparison to good
cable pairs, this pair appears shorter in length.
Open, Far End Fully developed upswing at the Far End.
Short, Far End Fully developed downswing at the Far End.
Split Pair 20% to 30% rise in relative impedance at the split with
a corresponding drop in impedance where the pair is
reconnected.
Cable segment with higher than
nominal impedance
Peak in the level area of the plot.
Cable segment with lower than
nominal impedance
Dip in the level area of the plot.
Level Plot, much higher than the
known cable NVP
Wrong cable type selected, or wrong cable type
installed.
No distinct upswing or
downswing at the Far End
Matched terminator attached to the cable. The pair
appears to have a very long length.
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