At which point on the TDR waveform should I take my impedance measurement to achieve the best accuracy?
First of all, a more accurate way to determine the impedance is to read the value off of the impedance profile waveform, rather than TDR waveform. The TDR waveform impedance readouts may be distorted by the multiple reflections in the DUT, but these multiple reflections can be de-embedded by Z-line algorithm in IConnect TDR and VNA Software.
If it is believed that the multiple reflections in the DUT are negligible, the TDR waveform impedance readout will still quite often be affected by the reflections due to the probe interface to the DUT. The DUT interface reflections dictate that the impedance reading must be taken as far as possible from the DUT measurement interface, whereas the transmission losses demand that the reading be taken as close as possible to the DUT interface. We suggest that the optimal measurement would include readout after a point where the DUT interface reflections are minimized, and would be computed as an average value over time
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