EMC Question of the Week: January 26, 2026

A TDR output that spikes at 15 nanoseconds and returns to 50 ohms with a time constant that is approximately 0.8 nanoseconds

The TDR output waveform shown in the figure indicates a discontinuity 1.5 meters down the line. The discontinuity appears to be a  

  1. short circuit
  2. open circuit
  3. series inductance
  4. series capacitance

Answer

The best answer is “c.” The discontinuity initially looks like an open circuit and within a few nanoseconds the impedance returns to 50 ohms. This mimics the behavior of a series inductance. This could be a connector or a cut in the shield that focuses the currents on one conductor or increases the spacing between the two transmission line conductors.

The time constant (τ=L/R) associated with the exponential decay is approximately 0.8 ns. This corresponds to an inductance of about 80 nH. 

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