Lastly, somewhat new for PAM4 in contrast to NRZ is the notion of linearity. In NRZ, there are only the two amplitude levels to worry about while PAM4 gives us these two intermediary amplitude levels. Are the distances a, b, and c the same so that the levels are evenly spaced and we have maximum openings for all three eyes? Does a = b = c? That's what is meant by "linearity" in the context of PAM4.
That's a quick rundown of the test and measurement challenges raised by the advent of PAM4 signaling.
Read parts 1 and 2 of this series here: