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3M
™
Advanced Systems Tester 900AST Series
12
generally cause small upward peaks due to a change
in impedance. To determine if an upward event is an
impedance change or a main event fault you may go
to the other end of the coaxial cable and disconnect
it. Next, take a jumper and short the inner conductor
to the outer shield. If you see a large downward
deflection in the trace the small peak is impedance
change. Conversely, if you can’t see the short or
downward deflection you have an open circuit in the
cable.
Target Contact
The first peak or dip on the screen is usually the
“target contact” which occurs at the point where
the 900AST tester connects to the circuit under test.
Since impedances of the different types of coaxial
conductors vary significantly, the initial event caused
by the 100-ohm TDR launch circuit contacting
the target can usually be ignored. Remember that
distance zero (0”) begins at the end of the test leads
for the TDR function. Should the dip after the initial
peak move significantly below the line you may have
a short at the shield termination point on the end
of the cable. Move to the other end of the coaxial
conductor and connect the tester leaving the first end
open. The end of the trace should produce an upward
deflection at the end of the cable. A downward
deflection may indicate a short.
Open Circuit
Any open circuit will show up as a peak on the
screen. A complete open will be the tallest peak you
will see. You cannot see events past a complete open.