Making Measurements
LT51 User Manual
60
TDR Measurement
You can use the TDR Measurement to determine the length of a cable and to
precisely locate faults such as Shorts, Breaks or Opens, Bridged Tap, Load
Coils, Splices or Splits. During the TDR measurement, the LT51 continually
applies a Pulse to the RTXa-b terminals. As the pulse travels down the pair
(connected to the RTXa-b terminals), any impedance change or fault
encountered will result in some of the pulse being reflected back to the LT51.
These reflections together with the launch pulse are recorded by the LT51 as a
trace and displayed on the LCD, so that the distance between the events can be
measured:
Connect the pair to be tested to the RTXa-b terminals.
Press the key, to select the TDR measurement of the LT51.
Auto-TDR
When the key is pressed, the LT51 will automatically search for the first
impedance discontinuity (open, short, bridged tap upto 3km, PVF=67%) and
place the marker at the start of the event, displaying the distance to the event. It
will automatically search all ranges from the lowest to the highest, and zoom the
waveform to show all features in the display.
Zo Impedance
An impedance of
50 or 75
can be set (via or keys) for TDR.
TDR Range
The TDR Range needs to be set to cover the expected length of the cable being
tested:
Press the [f1] soft-key to select
TDR Range.
Turn the knob clockwise to increase, or anti-clockwise to reduce, the
TDR
Range.
Press the key (or any key) to clear the dialog box.
NOTE: The values for TDR Range are affected by the PVF setting.
PVF (Propagation Velocity Factor)
For the LT51 to display the trace with accurate distance information, the correct
PVF of the cable being measured must be set:
Press the [f2] soft-key to select
PVF.
Turn the knob clockwise to increase, or anti-clockwise to reduce,
PVF.
Press the key (or any key) to clear the dialog box.
.
NOTE: Changing the PVF will automatically update the TDR range, distance
resolution (centre of bottom bar) and distance display on the LCD.
Summary of Contents for UMDT51A
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