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

08 November 2018

How Tip Inductance Impacts a Probing System's Bandwidth

Shown are FFT plots of a 10-MHz, fast-edge square wave reaching the oscilloscope via direct coax connection  (orange-yellow plot) and 10x passive probe fitted with a coax tip adapter (straw-colored plot)
Figure 1: Shown are FFT plots of a 10-MHz, fast-edge square
wave reaching the oscilloscope via direct coax connection
(orange-yellow plot) and 10x passive probe fitted with a coax
tip adapter (straw-colored plot)
If you're using 10x passive probes with your oscilloscope, it's important to understand the bandwidth of your probing system and how it's affected by various methods of probing the signal of interest. There's a relatively easy way to determine this parameter by probing a fast-edge, 10-MHz signal from a square-wave generator. Doing so can also instruct us in the effects of tip inductance on the probe's bandwidth.

01 November 2018

How Equalization Works in 10x Passive Probes

The adjustable equalization circuit on the oscilloscope end of the coaxial cable compensates for the 10x passive probe's inherent low-pass filter characteristics
Figure 1: The adjustable equalization circuit on the oscilloscope
end of the coaxial cable compensates for the 10x passive
probe's inherent low-pass filter characteristics
We've been discussing 10x passive probes and their inner workings; our last post covered all the ways in which a 10x passive probe is apt to be a liability. They'd be basically unusable for any measurements at all but for one attribute: their equalization circuit (Figure 1). Without it, the 10x passive probe makes a pretty good low-pass filter, but the equalization circuit counters that with a high-pass filter to balance things out.

24 October 2018

Secrets of the 10x Passive Probe

The 10x passive probe  becomes a better measurement tool when we understand its limitations
Figure 1: The 10x passive probe
becomes a better measurement
tool when we understand its
limitations
We began this series of posts on oscilloscope probes by putting them in perspective: Probes have a number of different jobs to do, including serving effectively as both a mechanical and electrical interface. Despite having electrical attributes of their own, we want them to grab our signal of interest, but we don't want them to affect that signal in any way.

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.