Generate PAM4 signals for receiver compliance testing

September 26, 2016 //By Alexander Katsman, Adsantec
56 Gbit/s links are moving to PAM4 signalling as a way to overcome channel bandwidth issues. As this modulation technique takes hold, you'll need to generate variety of signals to test receivers under varying distortion conditions. Stress testing a receiver can help you understand a receiver’s weaknesses and measure its tolerance to various signal impairments.

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.

Figure 1: PAM4 modulation (right) uses four amplitude levels and thus can transmit two bits per symbol as where NRZ (PAM2) sends one bit per symbol.

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.

Figure 2: Block Diagram of PAM4 signal generator setup shows the differential output needed to drive a receiver.

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