EMC Question of the Week: April 12, 2021

TDR output waveform drops abruptly to 0 then exponentially returns to 50 ohms with a time constant of 2 nanoseconds.

The TDR waveform in the figure indicates a discontinuity in a 50-Ω transmission line. Which of the following most closely models the discontinuity?   

  1. 8 μH series inductor
  2. 80 pH shunt inductor
  3. 8 μF series capacitor
  4. 80 pF shunt capacitor


The best answer is “d.” The voltage drops to zero at the discontinuity indicating there is a shunt capacitance. The RC time constant associated with the exponential rise back to 50-Ω is 2 nanoseconds. R is 25 Ω (50 in parallel with 50). Solving for C yields 80 pF.

A series inductor would have initially looked like an open circuit with a reflection coefficient of +1 and the waveform would have abruptly risen instead of falling. A shunt inductor or a series capacitor would not allow an abrupt change in the waveform. The series capacitor would exponentially rise from 50 Ω to an open circuit. A shunt inductor would exponentially fall from 50 Ω to 0 Ω. 

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