In Example 16.3, suppose that the amplitude of the reflected wave equals 99% of the amplitude of the forward-propagating wave. What is the impedance of the fault if the intrinsic line impedance is Z0 = 200 Ω?
EXAMPLE 16.3
Time domain reflectometry is a method of testing that relies on reflections from mismatched loads to locate the load. This is very useful in locating short circuits or cuts in inaccessible lines such as underground cables. A pulse is sent on the line and its reflections are recorded on a screen or chart. The distance between every two pulses is twice the time it takes to propagate to the fault. If the speed of propagation is known for the line, the exact location of the fault can be found. From the magnitude, shape, and sign of the signals, it is also possible to evaluate the type of fault (short, low or high impedance, open) before repair. This can save considerable time and labor, especially if cables are buried.
considerable time and labor, especially if cables are buried. A lossless underground telephone cable has inductance per unit length of 1μH/m and capacitance of 25 pF/m. The cable has developed a fault and it is required to locate the fault and identify its nature. The time-domain reflectometer reading looks as in Figure 16.9.
(a) Find the distance of the fault from the source.
(b) What kind of fault does the cable have?
