PCB Board Reverse Engineering for Propagation Velocity Test Coupon General Guidelines

Measurement of velocity or propagation delay after PCB Board Reverse Engineering is generally more difficult than impedance measurements. For velocity the structure delay is determined by measuring the difference in time it takes the pulse to propagate through the structure.

Points of measurement from Printed circuit board replication are not as simple as impedance, and accuracy will be extremely dependent on the probing technique. The most accurate delay measurements require advanced probing techniques utilizing controlled impedance microprobes with the TDR in TDT mode.

This improved accuracy comes at a cost in terms of equipment and measurement time. Selecting the method for measuring the velocity depends on accuracy desired, measurement time and cost from PCB board copying.

The simplest but least accurate method for measuring propagation delay is using TDR mode to measure delay between two identical test structures of different length. Propagation delay can then be calculated by dividing the different structure delay differences by the difference in length.

Accuracy will be dependent on probes and structure. Best case accuracy ~ +/- 8 ps/in for the recommendations below. If space permits, the above approach can be improved by inserting a third test structure of different length than the other two. To calculate the velocity, graph length vs. velocity from Pwb prototype Reverse Engineering, with a line drawn connecting the three points.

The intercept of the line with the axis gives the measurement error.

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