Operation
14
Pulse Widths
As the RANGE and IMPEDANCE matching of the
TDR are adjusted so the duration of the transmitted
pulse changes. The pulse widths range from 7ns to
3µs to overcome signal attenuation and enable the
instrument to see further down a length of cable. The
greater the range selected on the TDR, the wider the
transmitted pulse.
The accuracy of the “Distance to fault” is not affected
by the length of the pulse. However, if two or more
features exist close together (excluding open or short
circuits), then the second or subsequent feature may
be partially masked by the reflection from the first
fault. Hence, for potential multiple features, the
instrument should be used with the shortest suitable
range, and so the smallest pulse width, that can see
both features.
For output pulse characteristics, refer to output pulse
data in the TDR Specification at the end of this guide.
Techniques
To improve on the accuracy of the measurement,
numerous techniques can be used, depending on the
situation encountered. Not every situation can be
described, but the following points are effective and
the most common and easily implemented methods.
Test the cable from both ends
When fault finding a cable it is good practice to take
measurements from both ends. Particularly in the case
of open circuit faults, when the true end of the cable is
not visible. If the measurement is made from both
ends, then the combined answer should equal the
expected length of the cable. Even when the true end
of the cable is visible on the display, the reflections
after the fault may be too obscure to analyse clearly. In
this case, measurement from both ends yields a
clearer picture as well as improved accuracy.
It is also good practice to follow the cable route with
a cable tracer, as not all cable runs will be straight. It
can save a great deal of time if the exact route of the
cable is known as faults are often found at these
points and can be accredited to third party
intervention.
Care and Maintenance
Other than replacing the batteries, the instrument has
no user serviceable parts. In case of failure it should be
returned to your supplier or an approved
AVO INTERNATIONAL repair agent.
Cleaning the instrument should only be done by wiping
with a clean cloth dampened with soapy water or
Isopropyl Alcohol (IPA).
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