Decision Feedback Equalization

Figure 1: DFE filter output is based on
a linear combination of previous bit
decisions

In debugging high-speed serial links, one must be cognizant of various forms of equalization that might be used in the link to compensate for signal degradation in the channel. Inter-symbol interference (ISI), attenuation, impedance mismatches, and insertion losses can all contribute to this loss of signal quality. To combat these effects, designers implement techniques such as continuous time linear equalization and feed-forward equalization.

The Causes of Ground Bounce and How To Avoid It

Figure 1: This cross section of a 100-MHz microstrip
transmission line shows us how a return path should look

We've been discussing the topic of ground bounce on digital I/O lines as well as an effective way to diagnose and analyze it. We've also run through a detailed example of how to measure it using a quiet-low I/O driver as a sense line. Now, let's take a step back and examine the root causes of ground bounce, and also discuss some best design practices for avoiding it altogether.

A Walk-Through of Ground-Bounce Measurements

Figure 1: The trigger pulse from the
MCU is one clock cycle in width

In earlier posts in this series, we've explained what ground bounce is and how it happens. We have also taken a deeper dive into the use of I/O drivers to implement sense lines that let us better quantify and analyze what kind of ground-bounce hit our system is taking. Now, let's look at a detailed example of how to measure and diagnose ground bounce.

More on Quiet-Low I/O Drivers and Ground Bounce

Figure 1: To configure an I/O driver as a quiet-low line, its
output is connected directly to Vss on the die

Ground bounce can plague digital I/O lines with bit errors and turn your hair grey trying to uncover the cause in the process. But there is a trick you can use to make the analysis a little easier: using a quiet-low I/O driver as a sense line to reveal the existence, and magnitude, of ground bounce in your system.

About Ground Bounce and How to Measure It

Figure 1: Shown are five I/O drivers
within a package driving signal
lines on a PC board

Designing and/or troubleshooting a system with, say, an MCU driving signals across transmission lines, can be an interesting exercise in patience and diligent sleuthing. Perhaps you're seeing an inordinate amount of bit errors at the receive end of I/O lines but having some difficulty nailing down the source. In many cases, the problem is ground bounce, an issue that can be tough to diagnose and cure. Let's begin an examination of the ground-bounce phenomenon by explaining how it arises and then outlining an approach for finding it.

Feed-Forward Equalization

Figure 1: FFE creates a number of delayed versions of the
input signal that are then added back to the signal with
proper weights

In addition to continuous time linear equalization (CTLE), another means of improving signal quality at the receiver end of a high-speed serial data link is known as feed-forward equalization (FFE). In terms of implementation, FFE is not unlike the pre-emphasis filtering that is done on the transmitter side. An FFE implementation looks for all intents and purposes like a digital finite impulse response (FIR) filter.

Continuous Time Linear Equalization

Figure 1: A CTLE implementation at the receiver end of a
serial-data channel seeks to boost higher frequencies while
not boosting noise any more than necessary

We've been looking at the broad topic of debugging high-speed serial links, and in that context, we're also touching on ways to improve signal performance on the receiver side. One of those ways is to implement continuous time linear equalization (CTLE).