Balance Control
Without Balance Control (see point 3 in the User Controls and
Display section) the transmitted pulse would be visible at the
beginning of the trace, swamping any reflections within the
pulse length (the dead zone). The balancing circuit attempts to
match 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
and allows cable features much closer in to be detected.
Alternatively, using the [L1-L2] & [OFF] 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.
NOTE: In many cases, it will be impossible to completely null
the transmitted pulse.
Velocity Factor
The velocity factor is the scalar that is used to convert the
measured time interval into an actual length of cable. It can be
displayed in one of two ways: a ratio of the transmitted pulse
speed to the speed of light, or as a distance per microsecond.
When it is displayed as the distance per µs (either m/µs or
ft/µs) the velocity factor will be indicated as half the speed of
the pulse in the cable. This is because the pulse in fact has to
go along the cable to the cable feature and back again which is
twice the distance to the feature.
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. If the exact length of a piece
of cable of the same type as the C.U.T is known and the
reflection from the cable end is visible then a more accurate
value can be determined:
1. 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.
2. Locate the start of this reflection as described in the
Operation section of this manual.
3. Adjust the velocity factor until the correct cable length is
shown.
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
percentage errors in the velocity factor are directly proportional
to distance measurement errors. Hence, the TDR2000 uses the
velocity factor to three decimal places to reduce any errors.
Pulse Widths
The TDR2000 pulse widths range from 20ns to 16µs to
overcome signal attenuation and enable the instrument to see
further down a length of cable. In distance terms for the size of
the transmitted pulse, this represents a transmitted pulse from
as small as 4.0m to 3199m! (This assumes a velocity factor of
0.667.) Without Balance Control, this would be an enormous
dead zone, but with the instrument correctly balanced, faults
can be seen well within the pulse width.
9
Instrument Features:
