Replicate Printed Wiring Board Impedance Measurement

The TDR method provides a simple means for determining PCB impedance and propagation delay characteristics for the target chosen for replicate printed wiring board. However, the actual data extraction from a test structure can be highly dependent on cursor positioning on the TDR pulse.

This section outlines the general instrument setup to obtain accurate results independent of probe type and test structure. Display adjustment should be completed to maximize measurement accuracy. The horizontal and vertical adjustments should be adjusted under probing conditions to fulfill the old library of printed circuit board reverse engineering.

The general recommendation is to continue adjusting both horizontal and vertical scaling until the launch edge is aligned with the first column and the reflected edge is aligned with the last. The vertical scaling should be adjusted to maximize ~50% of the screen between launch ledge and line under test. Screen should look similar to figure 13 under probing conditions for lower than 50Ω measurements.

In the ideal case, the waveform response area of interest should contain a large flat region to extract impedance data. As previously mentioned, depending on probe type, ringing and reflections will occur on the line of schematic diagram acquire from printed circuit board restoration. This will induce spikes in the waveform, as shown in Figure 8. Cursor positioning on a line that is not flat will result in some uncertainty in the measurement variance.

Selecting a position along the line after the ringing can minimize this uncertainty. Normally this will be towards the end of the trace where the line is open-ended. This is the most common and least accurate method that provides a good quick check but is very user-dependent and not repeatable for correlation work. Cursor positioning is usually a good way for lab measurements to determine ballpark impedance measurements suited to experience TDR users.

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