In an earlier article, we surveyed the basic properties of PAM4 signals. Now, we will examine some of the ways in which PAM4 is finding application in the real world and what test and measurement setups might look like for those applications.
The simplest application, as seen at the top of Figure 1, would be sending an electrically modulated PAM4 signal from one chip to another. This would typically be an intra-PC board link.
Figure 1 A high-level view of PAM4 use cases
Another use case for PAM4 is to send an electrical signal from a chip into a linear optical modulator (see the figure, center left). The modulator changes the electrical signal levels into optical intensity levels, or brightness levels, generated by a laser. An electrical PAM4 signal has been transformed into an optical PAM4 signal.
Conversions of electrical PAM4 to optical PAM4 through a linear optical modulator are often found in data centers, where large amounts of data are being transferred from one server or one building to another. Companies such as Cisco, Google, and Facebook might use PAM4 in this way.
A third scenario involves two electrical PAM4 signals (see the figure, lower left). Rather than being fed to a linear optical modulator that converts electrical levels to optical brightness levels, the two PAM4 signals are inputs to a coherent optical modulator, which converts the two signals to the amplitude and phase of an optical signal that's being modulated.
Referring again to the lower left of the figure, we see two electrical PAM4 signals as inputs to the coherent optical modulator. That's two signals with four potential levels, resulting in 16 possible combinations of those two signals. That gives us a coherent 16-QAM signal which is seeing application in next-generation long-haul optical communications.
So what are the test setups that map to these three application profiles? For the first scenario of an electrical PAM4