3M™ Advanced Systems Tester 900AST Series
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
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.
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