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22 July 2015

PAM4 Test Setups Vary With Applications

Figure 1: A high-level view of PAM4 use cases
In an earlier post, 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 the figure, would be sending an electrically modulated PAM4 signal from one chip to another. This would typically be an intra-PC board link.

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 ot 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.

Test Solutions

So what are the test setups that map to these three application profiles? For the first scenario of an electrical PAM4 signal moving from chip to chip, the test setup would comprise an oscilloscope (links in the figure denoted with a 1 are served by this setup). Typically, these chips will be mounted on an evaluation board with high-speed coaxial connectors. Thus, the output from the transmitter chip can be fed directly into an oscilloscope's input instead of into a receiver chip. The same setup would serve the PAM4 electrical signals used as inputs to the linear and coherent optical modulators. 

The next test solution would comprise an oscilloscope with an optical/electrical (O/E) converter (such as Teledyne LeCroy's OE695G), which converts optical intensity levels into voltage levels, or the opposite of the linear optical modulator. This setup would work well for those interested in the output side of the linear optical modulator in the middle application example shown in the figure.

Finally, for the 16-QAM, long-haul signals at the output of a coherent optical modulator, the test setup of choice would be an oscilloscope in tandem with a coherent optical receiver (an example would be Teledyne LeCroy's Optical Modulation Analyzer). Here, two signals are encoded into one light stream to modulate both amplitude and phase.

Next time, we'll look at the test challenges associated with PAM4 encoding.

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