Optical PAM4 technology is going mainstream, being incorporated into Ethernet standards such as IEEE 802.3bs and 802.3cd (400G, 200G, 100G and 50G speeds). The characterization, compliance, and troubleshooting of PAM4 signals differs significantly from earlier NRZ (non-return-to-zero, PAM2) signaling. With PAM4, the focus shifts from mask tests to Transmitter and Dispersion Eye Closure Penalty Quaternary (TEDCQ) for optical signals (and Signal to Noise Distortion Ratio or SNDR for electrical). You'll need to learn how to perform these tests.

Despite increased complexity and a need for new measurement approaches, the move to PAM4 was inevitable to keep pace with ever-increasing network bandwidth demands. Standards bodies such as IEEE and OIF reached the conclusion that the most practical means to achieve higher throughput would be to increase bit density by doubling the number of bits per unit interval (UI), which PAM4 does.

If you need an introduction to PAM, then continue down this page. If you're familiar with how PAM4 works, then jump to page 2: NRZ and PAM4 optical measurements.

PAM4 primer

As the name implies, PAM4 uses pulse amplitude modulation, meaning that information is passed by modulating the pulse amplitude with no regard to phase. PAM4 uses 4-level signaling and transmits two bits per unit interval (UI) also called a symbol, and thus uses half the bandwidth compared to traditional PAM2-NRZ coding (Figure 1).

Figure 1 PAM2-NRZ and PAM4 baseband signaling and eye diagrams show two and four amplitudes, respectively.

The move to four-level signaling leads to three eye diagrams per UI. Table 1 compares the characteristics of PAM4 and NRZ signals. With PAM4, speeds are expressed in terms of symbol rate (baud). The UI period is 1/symbol rate. Because PAM4 codes two bits per symbol, a symbol rate of 25 GBaud transfers 50 Gbits/s.

PAM4 and NRZ
Table 1 PAM 4 doubles the data rate compared to NRZ coding.

Although the standards are still evolving, the IEEE 802.3bs and IEEE 802.3cd committees are on their way to embracing PAM4 signaling. Standards highlighted by the red box in Table 2 have particular relevance for optical PAM4 with distances ranging from under 100 m to 10 km.

Table 2 The latest datacom standards are adopting PAM4 at 25+ GBaud. (Source: Tektronix)

[Continue reading on EDN US: NRZ and PAM4 optical measurements]

Kan Tan is a Principal Engineer at Tektronix. Prashanth Thota is a system engineer at Tektronix.

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