Reading Between the Levels: Measuring PAM Signals

PAM-N signaling is increasingly common across high-speed serial interfaces, from Automotive Ethernet and data center interconnects to storage and general-purpose SerDes links, wherever higher spectral efficiency is required within constrained bandwidth. However, its multi-level nature means that conventional NRZ-based measurement approaches are no longer sufficient. This application note provides a structured, measurement-driven workflow for analyzing PAM-N signals on the R&S®RTP oscilloscope, using MGBASE-T1 PAM-4 as a practical black-box transmitter example. It guides users through the complete signal integrity evaluation sequence, including differential signal setup, CDR-based eye-diagram analysis with both software-based Advanced Eye and hardware-based Live Eye, transition-specific jitter decomposition, channel embedding using S-parameter models, receiver-side equalization, and SCPI-based automation. Advanced Eye provides the stable statistical view needed for detailed eye evaluation, per-eye measurements, and level-dependent analysis, while Live Eye complements this with a real-time view of signal stability, timing variation, and transient behavior as the signal is acquired. By following the workflow, engineers can move beyond a single composite eye diagram or aggregate jitter value and identify where signal degradation originates, whether from level-dependent eye closure, transition-specific timing behavior, channel loss, or equalization effects. The document is useful both as a step-by-step guide for engineers new to PAM-N measurements and as a reference for users targeting a specific part of the analysis workflow. Although MGBASE-T1 PAM-4 is used as the working example, the measurement logic can be transferred to other PAM-N transmitter investigations by adapting the signal configuration and analysis setup to the interface under test.