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How does the TDR measurement of impedance work?
TDR requires two conductors running in parallel to identify the change in impedance. Any connection, cable/trace type, cable/trace break, or fault will cause a variation in impedance value. Each type of change will have a different effect on the TDR display.
How does TDR work when there is no fault?
Note that TDR will only observe the reflections if the injected pulse experiences any discontinuity in its path. If there is no discontinuity or no-fault, the pulse will get absorbed by the termination provided at the far end of the transmission line, hence no reflections.
How does reflection on a TDR circuit work?
A reflection from any discontinuity present in the signal path has a rise time equal to or slower than that of the incident pulse. The spacings between two discontinuities present on a circuit path determine how closely their reflection will be positioned on the TDR waveform.
What is the minimum rise time for TDR?
TDR impedance measurements come with a limitation; the minimum system rise time. The total rise time consists of the combined rise time of the driving pulse and the oscilloscope or sampler that monitors the reflections.
How does a TDR measurement instrument reduce random noise?
Modern TDR measurement instruments use a signal averaging technique to reduce the effects of random noise. But averaging reduces the processing speed of the TDR system. Interconnect components and the probe-to-DUT interface also generate reflections and can produce inductive reflections.
How does TDR accuracy depend on the VF?
TDR accuracy depends on the Velocity Factor (VF). It is good to test the trace from both ends. The output pulse level can be varied to assist in locating the discontinuities. Small trace path discontinuities and those at the far end of the transmission line will require a high pulse level.