Fluke DSP-2000 - Manual (Page 130)

DSP-100/2000
Users Manual
7-18
Cable Termination
Because signal reflections can distort the shape of communication signals, the
unused ends of cable segments must be terminated to prevent reflections. The
terminating device is a resistor with a value equal to the cable’s characteristic
impedance. A signal reaching the terminator is neither reflected nor passed: the
signal is absorbed and dissipated by the terminating resistance.
Because the test tool relies on signal reflections to determine cable length, the tool
cannot measure the length of properly terminated cables.
Interpreting the TDR Plot
The TDR plot has a horizontal scale that represents distance and a vertical scale
that represents the percentage of reflection relative to the original signal, as shown
in Figure 7-10.
gc36c.eps
Figure 7-10. A TDR Plot
Notice that the reflection percentages can be positive or negative. A positive value
indicates that the polarity of the reflection is the same as the polarity of the
original signal. As discussed earlier, positive reflections are caused by abrupt
increases in the cable’s impedance, such as those caused by mismatches in cable
types, poor connections, or breaks in the cable.