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What causes reflections in TDR?

Question :

What causes reflections in TDR?

Answer :

The reflections that a TDR displays and measures are caused by changes in the impedance of the path of the step (circuit board, cable, or integrated circuit). Any significant change in impedance will cause a reflection. As an example, if an open solder connection exists on a circuit board, you can see that change with TDR. TDR also displays changes in the conductor resistance. For example, if there is corrosion in a joint and there is high resistance at that point, this is seen by a TDR. TDR also displays changes in capacitance.

If you think of the TDR display in terms of bumps and dips, it tends to make interpretation a lot easier. A bump (upward deflection) indicates a higher-impedance event, such as an open (see Figure 13) or a reduction in line width. A dip (downward deflection) indicates a lower-impedance event, such as a short or an increase in conductor width. The time location of the high-impedance event or low-impedance event as well as the delta times is displayed on screen.

Figure 13

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FAQ ID 57681

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