EMC Question of the Week: August 10, 2020

TDR response (impedance vs. time)

A TDR is connected to the input of a 50-Ω cable and measures the waveform indicated on the right. The cable is terminated with a 150-Ω resistor and a parasitic

  1. series inductance
  2. shunt inductance
  3. series capacitance
  4. shunt capacitance


The best answer is "d". When the wavefront first hits the termination, the shunt capacitance looks like a short (0 Ω). Once the capacitance becomes fully charged, the impedance is that of the 150-Ω resistor. The RC time constant associated with the exponential charging of the capacitor would be the value of the capacitance times the parallel combination of 50 and 150 ohms (37.5 Ω). A time constant of 4 ns indicates a capacitance of about 100 pF.

A series capacitance would have resulted in an impedance that jumped from 50 Ω to 150 Ω then rose to infinity. A series inductance would have jumped from 50 Ω to infinity then decayed to 150 Ω. A shunt inductance would have decayed from 50 Ω to 0 Ω.

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