Why do I need to have a reference step waveform when acquiring an impedance profile?

Why do I need to have a reference step waveform when acquiring an impedance profile?

The reference waveform provides you with the information about the incident step at the DUT in the TDR measurement system. This information is required in order to be able to deconvolve the incident step from the TDR response and obtain the true DUT impedance profile. A measurement of a short or an open termination at the DUT reference plane will provide a good reference short. In most cases, the parasitic inductance of a short will be lower than the fringing capacitance of an open, and because of that a reference short measurement is typically a preferred method for obtaining the reference step waveform.