Test challenges for PAM4 signals: Page 2 of 3

February 08, 2016 //By David Maliniak
Test challenges for PAM4 signals
David Maliniak of Teledyne LeCroy considers the various test challenges associated with PAM4 signals.
level to the second level down (yellow traces). Even in a perfect world, these transitions do not cross the threshold at the same place as the others (green circle). Thus, the question is whether these transitions would affect clock recovery? Will they result in clock jitter?

The answer is "not really." If we assume roughly equal numbers of transitions to the left and right of the green circle, and many more transitions than either at the green circle, then the two yellow crossings to the left and right cancel each other out. Traditional clock recovery works fine.

Speaking of jitter, the proximity of those yellow crossings to the idealized crossing location at the green circle does raise questions. Referring to the small red arrows at the right side of the figure, if one is accustomed to analyzing NRZ signals, those gaps in time do indeed look like jitter.

How does one analyze jitter on a PAM4 signal? First, let's not lose sight of the ultimate purpose of analyzing jitter at all, which is to determine our predicted bit-error ratio. We want to know where we need to sample the signal to minimize bit errors (ideally, at the red crosses at the center of each of the three eyes). Thus, what we really care about is the eye width and the position of the sampler in the receiver. The number of trajectories that build up the eye aren't the issue, but rather the eye width at a given bit-error ratio as opposed to an in-depth analysis of what's going on at the crossings.

Another challenge in the PAM4 realm is that of noise tolerance. Instead of having the full amplitude range, we have 33% of that amplitude to work with. Thus, noise analysis becomes much more important. Just as we want to understand the eye width at a given BER to know where best to situate the sampler horizontally, we also

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