Read part 1 and part 2 of this series for an introduction to PAM4 and the test setups for real-world applications.
PAM4 encoding offers the advantage of doubling the bit rate in a serial data channel, doing so by increasing the number of voltage levels from two to four. It's a fairly complex modulation scheme, so it should be no surprise that it presents some test and measurement challenges.
Imagine, if you will, a plain-vanilla NRZ eye diagram, and an ideal one at that. It would have no jitter, just a finite rise and fall time.
The first thing that often comes up in discussions of PAM4 signals is: How does one recover a clock? In the case of an NRZ signal, the purpose of clock recovery is to pinpoint the crossover point, or the place at which the signal crosses the threshold (see the green circle at center left of the Figure 1).
Figure 1 The major test challenges posed by PAM4 signals
With a PAM4 signal, it's a similar situation except for the fact that there are four voltage levels instead of two. The two intermediary levels mean a lot more signal transitions: there's the one from the bottom level to the next one higher, and from that level to the third, and from the third to the top (blue traces c, b, and a, respectively, in the figure).
The blue trace b crosses at the same place as the black traces that cover the full amplitude range (at the green circle). Some transitions do not cross any threshold at all (blue traces in top and bottom thirds), but there are likely not as many of those as there are transitions that do cross thresholds. So considering these transitions, one may assume that traditional clock recovery would be adequate.
But then there are the transitions from the bottom level to the second level up, and from the top