PAM4 is a type of pulse amplitude modulation (PAM) technology. It is a mainstream signal transmission technology following non-return-to-zero (NRZ). As one of the multi-order modulation technologies, PAM is widely used in high-speed signal interconnection. The following figure shows the typical waveforms of NRZ and PAM4 signals. The right part of this figure compares the eye diagrams of NRZ and PAM4 signals, where an NRZ signal uses the single-pupil waveform and a PAM4 signal uses three-pupil waveform (three eye diagrams along the Y axis).