Use an oscilloscope to find the cause of abnormal glitches

Abnormal glitches, non-monotonic edges, and metastable signals are some of the types of signal anomalies that often cause engineers to feel annoyed and sleepless. Troubleshooting anomalies is usually divided into three steps:

1. Observe and identify; confirm the existence of abnormality

2. Signal isolation; separates abnormal signals from good signals

3. Analyze the collection; look for clues to the root cause (such as frequency anomalies, unique patterns, or other indicators to diagnose the cause of the first anomaly.)

This application note complements one of the built-in automated demonstrations for the Keysight InfiniiVision 6000 X-Series oscilloscopes, Finding Coupled Signals Causing Glitches. This application note covers the following:

– Fast waveform update rates and why they are so important

– Hardware InfiniiScan Zone Touch Trigger and How It Can Speed ​​Up Your Signal Isolation

– Chroma-enabled segmented memory and how it can provide deeper analysis of your signal

– How 10-bit counters and adders, as well as counters triggered by specific events, can help you troubleshoot

Identify and isolate anomalies

If you suspect that there is an anomaly in your design, whether in product development, design verification or failure analysis, the first thing you need to do is to find the anomaly. Figure 1 shows an abnormal glitch mixed in with a good signal. This glitch causes intermittent operation failures in the design. If you use a slow waveform capture rate (trigger update rate) like a traditional digital storage oscilloscope, the first step of observation confirmation will take a long time. However, with the 6000 X-Series' 450,000 waveform capture rate per second (waveforms/second), you can see this abnormal glitch immediately. In terms of time saving, if you use an oscilloscope with a capture rate of 450,000 waveforms/second to display a glitch, it may take 10 seconds, but if you use an oscilloscope with a capture rate of 1,000 waveforms/second, it will take 75 minutes to display the same glitch!

Now that you have identified the glitch, you will want to isolate it from the good signal. Using advanced triggers is the fundamental way to isolate a signal in a modern oscilloscope. However, setting up advanced triggers requires expertise and can be challenging, depending on the complexity of the anomaly you are trying to isolate. With the 6000 X-Series oscilloscope’s unique hardware InfiniiScan Zone Touch Trigger, signal isolation is as simple as drawing a box around the signal or area of ​​interest and selecting whether the signal is “Must Cross” or “Must Not Cross.” The oscilloscope will only display waveforms that meet these qualifications. Figures 2 and 3 show examples of using the InfiniiScan Zone Trigger to isolate the anomalous glitch in our example signal. Because the 6000X InfiniiScan Zone Trigger is hardware-based, it can scan through triggers at speeds up to 160,000 waveforms per second. In comparison, software-based Zone Triggering can only view about 1,000 waveforms per second.

Figure 1. Fast update rates capture unusual glitches faster. Figure 2. Draw a box to set up an InfiniiScan zone touch trigger. Figure 3. Only waveforms that cross the box drawn in Figure 2 are captured and displayed.

Collect and conduct in-depth analysis to determine the source of the glitch

Once the anomaly has been isolated, the next step is to collect and analyze the relevant information and try to find the root cause of this glitch. Using the dual cursors on the 6000 X-Series oscilloscope's multi-touch screen, we can quickly measure the size of the glitch, which in the case of Figure 4 is approximately 40 ns as shown in the figure. Knowing the width of this glitch, we can now use a second method to isolate the glitch.

Figure 4. Using dual cursors to determine glitch size.

What we really want to know is whether this glitch occurs multiple times, and if so, how often. Among the advanced triggers in this case, pulse width trigger is a more ideal one. Pulse width trigger can work by setting the pulse width conditions of "greater than", "less than", or "between". The pulse width trigger setting shown in Figure 5 uses a pulse width of "less than 50 ns".

Figure 5. Setting the pulse width trigger

But how can we find out how often this glitch occurs? Segmented memory is a standard feature of the 6000 X-Series oscilloscopes that allows you to selectively capture and store important signal activity, or segments, without capturing trivial signal idle time. We give each segment a timestamp relative to the first trigger event. Segmented memory is an ideal solution because we suspect this glitch will occur very infrequently, separated by very long idle times. We will use segmented memory and pulse width triggering to find out how many times this glitch has occurred.

Figure 6. 50 glitches captured in segmented memory. The sidebar list shows relative time stamps.

Figure 6 shows the result of capturing 50 glitches. Using the scrollable sidebar event list, you can quickly find the timestamp of each segment. This list shows that the glitches are periodic, occurring every 42 ms, or at a frequency of 23.8 Hz.

In other words, you can determine that the potential root cause of this glitch is a coupled signal from the source at approximately 23.8 Hz.

Using segmented memory and colorimetric analysis together allows for further in-depth analysis. When the colorimetric display is activated, the segmented memory "segment analysis" function can overlap all segments. By showing a three-dimensional quantitative view of the waveform, the colorimetric display can provide how often a particular event of interest occurs, so find out what is happening as shown in Figure 7.

Figure 7. Chroma analysis of segmented memory can provide additional insight into the type of glitch you are dealing with.

This is an ideal solution for the abnormal waveform signal shown.

Also note that the segmented memory can capture 50 glitches in 2 seconds at 20 GSa/s. A traditional oscilloscope without segmented memory would require 40 Gpts of memory to perform the same length acquisition (2 sec / (1 pt / 20 GSa/s) = 40 Gpts).

10-bit counter/accumulator

The InfiniiVision 6000 X-Series provides an alternative method for finding the frequency of a particular glitch, using the built-in 10-bit counter and accumulator (option). The built-in 10-bit counter and accumulator can count the number of edges and the number of "trigger-qualified events". Figure 8 shows the use of the counter to measure the frequency of pulse width-qualified events. As expected, the counter found that the glitch occurred at a frequency of 23.8 Hz.

Figure 8. Using a 10-bit counter and accumulator to determine the frequency of events that qualify as triggers.

in conclusion

Oscilloscopes with slower waveform update rates can risk missing valuable information, but with the 450,000 waveforms/second update rate offered by the 6000 X-Series oscilloscopes, that risk is greatly reduced. Advanced triggering is a powerful event isolation tool, but hardware InfiniiScan Zones takes trigger usability to a new level. With InfiniiScan Zone triggering, you can be sure to trigger on all visible events.

In our example, after isolating the glitch, we determined that the suspected coupled signal occurred at a frequency of 23.8 Hz. We first analyzed it using the segmented memory function, and its time stamp showed that there was a 42 ms interval between any two adjacent glitches. We were able to confirm that the glitch was periodic because the results were consistent over the 2 seconds we captured. Without segmented memory, we would need an oscilloscope with a memory depth of more than 40 Gpts to capture the same length of time. Segmented memory combined with the color display can identify anomalous waveforms with low probability of occurrence. As an alternative troubleshooting method, we can use a 10-bit counter to monitor the trigger-qualified events and find that the glitch occurs at a frequency of 23.8 Hz.

With Keysight's InfiniiVision 6000 X-Series oscilloscopes, you can accelerate the process of finding root cause. With an acquisition rate of 450,000 waveforms/second, hardware InfiniiScan zone triggering, segmented memory with chroma display support, a 10-digit built-in counter, and an easy-to-use multi-touch display, you get the next generation of oscilloscope technology at a price that fits your budget.