Fluke DSP-100 User Manual page 128

DSP-100/2000
Users Manual
Reflections from Opens
An open, or break, in the cable represents an abrupt increase in the cable's
impedance. The impedance of an open is nearly infinite. In an open cable, a
signal's energy is not dissipated by a terminating impedance, so the signal
bounces back towards the source. This reflection appears at the source with the
same amplitude and polarity as the original signal, as shown on the next page in
Figure 7-9. By measuring the amount of time taken for the reflected pulse to
return, the test tool can determine the location of the open in the cable.
Figure 7-9. Signals Reflected from an Open, Shorted, and Terminated Cable
7-16
Cable
Transmitted Pulse
Transmitted Pulse
Transmitted Pulse
No Reflected Pulse
End of Fault
Open
Reflected Pulse
Short
Reflected Pulse
Terminator
(Matches cable
characteristic
impedance)
gc35f.eps