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Chapter 3
Measurements
29
Chapter 3
Measurements
TDR BASICS
Locating Cable Faults
The CE4000 TDR has two cable test modes, Auto and Manual.
Select the desired icon on the Main menu for the preferred mode
of operation. Press the MENU key at any time during operation
and the unit will return to the Main Menu screen (Figure 3-1).
Figure 3-1
The CE4000 operates on the same principle as most TDRs. A
test pulse, typically called the incidence pulse is transmitted
into the cable under test. The pulse travels down the cable at
a speed that is controlled by the properties of the cable. The
speed is called the VOP or velocity of propagation and is
measured as a percentage or decimal fraction of the speed of
light. When the pulse reaches a discontinuity in the cable
some of the energy (all of the energy if the cable is open or
shorted) is reflected back up the cable toward the original
source. The TDR measures the time required for the round
trip of the reflected pulse from the TDR, from which it was
launched, to the fault and back to the TDR. The distance to the
fault is then calculated by the formula: