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.

Device Analysis in Switch-Mode Power Supplies

Figure 1: Setup for analysis of switching losses

in a switch-mode power supply's MOSFET

Our survey of testing switch-mode AC-DC power supplies started by looking at the variety of measurements one might make on these devices and why differential probes and amplifiers are often the best choice over passive probes. Subsequently, we examined the key sources of error in power-supply measurements and how to minimize them. Now it's time to start taking some measurements with an eye toward device analysis, particularly the switching transistor in a switch-mode supply.

Reducing Errors in Switch-Mode Power Supply Measurements

Figure 1: Skew between voltage and current probes

results in power measurement errors

Almost all portable electronic devices, and lots of non-portables, come with switch-mode power supplies. These range from common "wall warts" to the larger brick-sized supplies that power a laptop. We've taken a look at the typical measurements one might make on a switching power supply and at why single-ended measurement techniques should take a back seat to differential approaches. Now, let's see what steps we can take to ensure that our measurements on power supplies are accurate.

Testing Techniques For Switch-Mode Power Supplies

Figure 1: A simplified schematic of
a switch-mode power supply circuit

On its journey from wall socket to the device being powered, power typically passes through a switch-mode power supply, where the AC signal is rectified into DC before it reaches the device. After that, the DC signal (often 5 V) is passed on to DC-DC converters on the device's PC board for feeding various voltages to branches of the device's power-delivery network. Let's look at some of the measurement techniques and considerations relative to testing switch-mode power supplies.

Testing Challenges in Motor Drive Systems (Part III)

Figure 1: An example of PWM for

a single power semiconductor

As noted in an earlier post, variable-frequency motor drives (VFDs) display a good amount of variation in terms of architectures and topologies. Another differentiator between VFDs is their application of pulse-width modulation (PWM) techniques.