Extended Protocol
High-speed signal testing relies on specialized protocols to verify circuits where dc coupling is absent. Using ac boundary scan allows engineers to check the integrity of capacitive-coupled nets by detecting the transitions of a pulse rather than a steady state. The technique applies specifically to differential pairs and other high-frequency paths that standard dc methods cannot penetrate.
Pulse Mechanism
Implementation involves the use of a transmitter that toggles the signal at a frequency high enough to pass through the coupling capacitor. The receiver on the other side of the component monitors for the expected transition, confirming that the physical path is intact. Because the signal is transient, the timing of the capture must be synchronized with the launch of the edge.
This timing window is necessary for accuracy because a late capture would only see a settled dc level, leading to a false failure indication in the test log.
Failure Mode
Open circuits at the capacitor pad or within the dielectric material result in a lack of pulse detection. Short circuits are identified when a signal appears on a neighboring net that should be isolated. This method identifies defects in data lanes that operate at gigabit speeds where traditional static testing fails to provide coverage.
