747-400
MAINTENANCE MANUAL
SHORT
O
N
O F F
O F F
O F F
ac 20.000 ft
SHORTED CABLE
OPEN
O
N
O F F
O F F
O F F
ac 20.000 ft
CUT CABLE
EXAMPLES OF CABLE FAILURE CONDITIONS
Time Domain Reflectometer Displays
Figure 202 (Sheet 2)
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747-400
MAINTENANCE MANUAL
POSITIVE IMPEDANCE
O F F
O F F
O F F
O F F
FRAYED CABLE
NEGATIVE IMPEDANCE
ac 20.000 ft
O F F
O F F
O F F
O F F
CRIMPED CABLE
EXAMPLES OF CABLE FAILURE CONDITIONS
ac 20.000 ft
Time Domain Reflectometer Displays
Figure 202 (Sheet 3)
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747-400 MAINTENANCE MANUAL
(c) If it is necessary, turn the NOISE FILTER clockwise to average-out some of the noise.
NOTE: This will increase the time necessary for the TDR to
____
give a waveform output.
(d)
Make sure these failure conditions do not show. 1) A drop in the waveform, this shows a short circuit in the applicable cable. 2) A rise in the waveform, this shows an open circuit in the appplicable cable.
3) Hills and valleys in the waveform, these can show bends, kinks, frays and water in the applicable cable.
(e)
If you see a failure condition in the applicable cable, turn the <>POSITION control.
(f)
Put the cursor on the leading edge of the rising or falling pulse.
(g)
Read the distance in the top right corner of the LCD.
NOTE: This value is the distance from the start of the
____ applicable cable to the cause of the failure.
(h)
If you do not use a 50 ohm terminator, you will see an open circuit at the end of the applicable cable.
(i)
Make a note of the distance to the open circuit. 1) Use a jumper to cause a short circuit on the other end of
the applicable cable. 2) Examine the waveform shown on the LCD. 3) Make sure the waveform now shows a short circuit where the
open circuit was.
NOTE: The length of the applicable cable is the distance
____
shown on the LCD.
4) If the waveform continues to show an open circuit at the same distance from the TDR, this is a failure.
NOTE: The distance to the open is the distance to the
____
failure.
S 972-004
(3) Measure the reflection coefficient:
NOTE: The reflection coefficient (p) is a measure of the impedance
____ change at a point in the applicable cable. It is used to find if the failure condition is important or not.
(a)
Adjust the VERT SCALE to make the waveform one division high.
(b)
Turn the <>POSITION control until the line-cursor is on the failure condition.
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747-400 MAINTENANCE MANUAL
(c) Read the reflection coefficient directly off the LCD above the VERT SCALE control. 1) A good waveform with no failure conditions (flat) will have
a reflection coefficient of zero. 2) Small impedance changes from 10mp to 100mp can possibly be a cable connector and are usually not important. 3) Large impedance changes of +/- 1000mp show an open or a short circuit in the applicable cable.
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