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

Operation

TX Null

Without the "TX Null" control, the transmitted pulse would
be visible at the beginning of the trace, swamping any
re flections 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" re flections to be seen.
N O T E : In some cases, it may not be possible to
completely null the transmitted pulse.

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.
•
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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