BT603 Users Manual
14
8. TDR
Testing
8.1.
Operation
Principles
When an electric pulse is transmitted from one end of a cable, this electrical signal travels through the cable, and its reflected signal is
generated where propagation characteristics (characteristic impedance) of the cable are changed; these points are defined as the
“events.” By measuring the shape of the reflected signals and the times they take to reach the tester, the types and the distance of the
events can be determined.
The changes in the characteristic impedance are often caused by open or shorted cables. Figure 9 shows the waveforms of the signals
measured by the tester when a pulse is initiated into two cable conditions. Position represents where the pulse signal applied to the
cable, and the Position represents where the reflected signal from open or shorted cables. In an open cable, both the initial pulse and its
reflected signal have the same polarity while the inverse polarity occurs in a shorted cable.
The interval between Position and corresponds to the time difference which is method to calculate the distance to the point of
impedance change. In this calculation, the velocity of propagation (VOP) is used; VOP is the speed of the electrical waves propagating
through a cable, which depends on the unique characteristics of each cable.
(Distance) = (Transmission time)
×
(9.84
×
108)
×
VOP (unit feet)
(Distance) = (Transmission time)
×
(3
×
108)
×
VOP (unit meter)
VOP is expressed as a percentage of the speed of light in vacuum. For example, when we say the VOP of a cable is equal to 66%, it
means that the signal propagates at 66% of the speed of light. The VOP value is affected by the type of a cable’s metal conductors, by its
insulation material, and by its structure. For communication cables, their VOP values are normally listed in one of their specifications
prepared by the cable manufacturers. Typical VOP values are between 30% and 90%.
