Fluke DSP-2000 - Manual (Page 129)

Basic Cable Testing
Time Domain Reflectometry (TDR)
7
7-17
Reflections from Shorts
A short represents an abrupt decrease in the impedance between the two
conductors in a cable. A short is caused when the insulation surrounding a cable’s
wires is damaged, allowing the wires to touch each other. The result is a near-zero
impedance connection between the conductors.
A short also causes signal reflections, but in a manner opposite of an open. In a
shorted cable, the signal’s energy is not dissipated because the short’s impedance
is close to zero. The signal is reflected back to the source, where it appears with
the same amplitude but a polarity opposite of the original signal, as shown in
Figure 7-9.
Reflections from Other Discontinuities
Reflections are also caused by impedance discontinuities that measure somewhere
between infinite and zero impedance. These discontinuities can be caused by
mechanical stress that damages cable wire or insulation without causing a
complete open or short. They can also be caused by cable mismatches and faulty
contacts at connectors or punch-down blocks.
A cable fault that has an impedance higher than the cable’s characteristic
impedance reflects a signal that has the same polarity as the original signal. If the
fault is not a complete open, the amplitude of the reflected signal will be less than
the original signal.
If the fault’s impedance is lower than the cable’s characteristic impedance, but is
not a complete short, the reflected signal will have the opposite polarity and less
amplitude than the original signal.