Velocity Factor - Megger TDR1000/2 Guide De L'utilisateur

Operation
TX Null (where fitted)
Without the "TX Null" control, the transmitted pulse
would be visible at the beginning of the trace,
swamping any reflections within the pulse length
(the dead zone). The 'TX Null' circuit matches the
characteristic impedance of the cable under test
to produce an equivalent pulse. Subtracting this
equivalent pulse from the transmitted pulse effectively
removes the dead zone from the display and allows
"near end" reflections to be seen.
NOTE: In some cases, it may not be possible to
completely null the transmitted pulse.
Unbalanced
Balance adjusted

Velocity Factor

The velocity factor is used by the instrument
to convert the measured time for a pulse to be
reflected, into a distance. It can be displayed as a
ratio of the speed of light (eg 0.660 = 66% of the
speed of light), or as a distance per microsecond in
ft/us or m/us.
If the exact length of cable is known and the
reflection from the cable end is visible then an
accurate velocity factor can be determined:
•
Locate the reflection caused by the end of
the known length of cable with the instrument
set on the shortest possible range to see the end
of the cable.
•
Locate the start of this reflection as described
in the Operation section of this manual.
•
Adjust the velocity factor until the correct
cable length is shown.
Balance adjusted
•
Note the VF value for future reference.
The measurement of the distance to the fault can
now be made with more confidence. The ability of
the instrument to accurately measure the distance
to a cable feature relies on the velocity factor
being correct. Any errors in the velocity factor are
directly related to distance measurement errors.
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