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Making Measurements
'Dip' Events
Fault: A resistance fault will show as a dip on screen. The lower the value of
resistance, the lower the dip.
Short: A short (or 0 resistance fault) will show up as the lowest dip on the
screen. You cannot see events past a short.
Bridge Tap: A bridge tap will look like a resistance fault and an open. (A dip
followed by a peak.) The distance between the two events is the length of the
bridge tap. The start of a bridge tap looks like a resistance fault. Use the LT51
Resistance function to measure the resistance on the pair. If there is no
resistance, and you see a dip followed by a peak, you might suspect a bridge
tap. Also for a bridged tap, the absolute value of the dip is always greater than
the absolute value of the peak.
Blind spot
Reflections due to faults that are located very close to the LT51 RTXa-b
terminals, and hence occur before the TDR pulse has been completed, can still
be measured if the cable has an impedance close to 100 . This is due to hybrid
echo cancelling hardware built-in to the TDR circuit. However, when the cable
impedance is not close to 100 , a known length of cable (exceeding the TDR
pulses width) of the same type as being measured should be inserted between
the LT51 and the cable being tested. To locate the fault on the measured cable,
the length of the inserted cable needs to be subtracted from the result displayed
on the LT51 LCD.
LT51 User Manual
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