Test Challenges for PAM4 Signals

The major test challenges posed by PAM4 signals

PAM4 encoding offers the advantage of doubling the bit rate in a serial data channel, doing so by increasing the number of voltage levels from two to four. It's a fairly complex modulation scheme, so it should be no surprise that it presents some test and measurement challenges.

Why High Oscilloscope Sampling Rates Matter

Figure 1: Here is an example of aliasing that results from
sampling a signal at less than the Nyquist rate of 2f_max

A key to accurate measurements with an oscilloscope is to ensure that the instrument maintains a high sampling rate. This applies to most measurements; conversely, for many measurements, accuracy may suffer as sample rate decreases. In the worst case, some signal components may be "aliased," meaning that the true signal shape is corrupted by the addition of bogus signal components that arise from undersampling of real signal components.

Debugging I2C Buses in Embedded Systems

Figure 1: Debugging of I²C on a Teledyne LeCroy
WaveSurfer 3000 oscilloscope

Embedded systems became ubiquitous decades ago and are now found in everything from mobile devices to vehicles to the traffic lights that control their movements. These days, they're typically based on microcontrollers and perform some specific task(s) within a larger system, such as controlling your car's ABS system. They may or may not have any sort of user interface, and can range widely in terms of complexity and functionality.

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 Fundamentals of PAM4

PAM4 doubles the number of bits in serial data transmissions
by increasing the number of levels of pulse-amplitude modulation,
but does so at the cost of noise susceptibility

As our society's hunger for data grows—not only more data, but more data delivered faster—older modulation schemes based on NRZ-type encoding grow increasingly inadequate. We need to get data from point A to point B as efficiently as possible, whether that means between chips on a PC board or from one end of a long-haul optical fiber to the other. A modulation scheme that's gaining favor in many quarters is PAM4, and in this post we'll look at the basics of PAM4 before turning to the test and analysis challenges it poses.

Using Persistence Mode and Exclusion Trigger

Figure 1: By using exclusion triggering, the oscilloscope
is prevented from triggering on the normal signal shape.
The trigger is set to capture only pulses with widths
different by at least 15 μs from the typical 325 μs.

Any piano player is well acquainted with the right-most pedal on their pianos, known as the sustain or open pedal. Pressing that pedal while playing will lift all of the instrument's dampers away from the strings, allowing them to ring freely until their vibration dies out or the pedal is released.

On a digital oscilloscope, the persistence display mode is a little like the sustain pedal on a piano. When persistence display is selected, the oscilloscope will trigger, display the signal trace, then trigger again and add another trace to the display, and so on.

Don't Leave Oscilloscope Performance on the Table

Figure 1: In this screenshot, four signals are displayed
on a single grid. Each signal is only using
64 counts of its ADC, which amounts to 6-bit resolution.

As test and measurement companies add to their products' capabilities, digital oscilloscopes serve a larger and more sophisticated set of measurements as vendors have added to their capabilities. And even though many of those additions come at the behest of the user community, many oscilloscope users don't even scratch the surface of what their instrument can do.