You need to test, we're here to help.

You need to test, we're here to help.

10 October 2018

Putting Probes in Perspective

Probe, cable, and oscilloscope form a system that makes or breaks the accuracy of signal acquisitions
Figure 1: Probe, cable, and oscilloscope form a system
that makes or breaks the accuracy of signal acquisitions
Few aspects of using an oscilloscope are as important as the probe: after all, the probe forms both the mechanical and electrical interfaces between the device under test (DUT) and the oscilloscope itself. To feed a signal into an oscilloscope, we're limited to a coaxial connection. Thus, we need a geometry transformer that picks up the signal of interest from the DUT and transfers it to the oscilloscope's coaxial connection.

13 September 2018

Decision Feedback Equalization

DFE filter output is based on a linear combination of previous bit decisions
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.

30 August 2018

The Causes of Ground Bounce and How To Avoid It

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

29 August 2018

A Walk-Through of Ground-Bounce Measurements

The trigger pulse from the MCU is one clock cycle in width
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.

16 August 2018

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

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

07 August 2018

About Ground Bounce and How to Measure It

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

23 July 2018

Feed-Forward Equalization

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