Instrument Features; Velocity Factor; Pulse Widths - Megger Tdr2000/2 Benutzerhandbuch

INSTRUMENT FEATURES

Tx Null
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.
No Tx Null
NOTE: In some cases, it will be impossible to completely null the
transmitted pulse.
Alternatively, using the L1-L2 mode, where L2 is connected to a known
good length of the cable under test, L2 is used instead of the balancing
circuit to automatically null the transmitted pulse. In this case only the
difference between the good line and the bad line will be displayed.

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.
12
Adjusted Tx Null
If the velocity factor of the cable under test (C.U.T) is not known, it may
be accurately established by:
Testing a known length of cable, locate the reflection for the end
of the cable on the display. Use the shortest possible range setting.
Use the ZOOM button to accurately position the distance cursor.
Adjust the velocity factor of the TDR using the VF key (4) until the
TDR displays the correct length of cable.
Make a note the VF value for future reference.
The table of velocity factors in the HELP pages of the instrument is a
rough guide and in practice, the settings are subject to many variable
factors. By using the previous procedure on a known length of cable, a
more accurate VF can be established.
The measurement of the distance to the fault can now be made with
more confidence that the measurement will be correct. 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 proportional to distance measurement errors. Hence, the
instrument uses the velocity factor to three decimal places to reduce any
errors.

Pulse Widths

As the RANGE of the TDR is adjusted (Key 11) so the duration of the
transmitted pulse changes. Each of the instrument ranges has a default
pulse width. The pulse widths vary from 7 ns to 16 µs. The greater the
range set on the TDR, the wider the transmitted pulse.
At short range, a short pulse width will have good resolution. As the test
range increases, cable attenuation prevents distant faults from being
seen.