Generate PAM4 signals for receiver compliance testing

September 26, 2016 // By Alexander Katsman, Adsantec
56 Gbit/sec 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/sec PAM4 signal runs at 28 Gbaud/sec compared to a 56 Gbit/sec NRZ signal that runs at 56 Gbaud/sec. 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.


To generate a PAM4 test signal, we designate one of the pattern generator/active amplifier pairs as the MSB, which provides an output amplitude twice as large as that from the other pair (designated as LSB). The resulting signal will meet all the electrical PAM4 signalling and transmitter performance parameters as specified in the standards for different symbol/data rates. Figure 3 shows the physical setup diagrammed