PAM4, with its four-level signal modulation — compared to two-level signal modulation (PAM2), most commonly referred to as NRZ — avoids the signal degradation caused by the increased bandwidth. PAM4 succeeds by transmitting two bits per symbol. For a given data rate, it cuts the bandwidth in half as opposed to NRZ. For example, a 56 Gbit/s PAM4 signal runs at 28 Gbaud/s compared to a 56 Gbit/s NRZ signal that runs at 56 Gbaud/s. Here, we need to distinguish the symbol rate (referred to as the baud rate) from the data rate to make the comparison. Figure 1 shows the difference between PAM4 and NRZ signals.
While a PAM4 signal experiences more ISI (intersymbol interference) than a PAM2 signal at a given baud rate, it experiences much less at a given data rate. That's because PAM4 sends two bits per baud. This minimization of ISI at a given data rate on bandwidth-limited channels such as electrical backplanes is the main motivation for the switch to PAM4.
Generate PAM4 test signals
Figure 2 shows block diagram of a test setup that can generate differential PAM4 signalling. It uses two serial digital-pattern generators, two active programmable pre(de)-emphasis amplifiers, two passive microwave combiners, and two phase-matched cables after the combiners.