Proper Oscilloscope Triggering

Introduction

A common problem experienced by novice oscilloscope users is the inability to effectively capture signals we’re trying to measure. We’ve all experienced how frustrating (and sometimes embarrassing) it can be when we can’t get the data we need quickly, spending too much time fumbling along because we’re unsure how the triggering functions on our oscilloscopes work.

Often,  we resort to pushing the auto-trigger function with the hope that this feature will miraculously pick up the signal we wish to measure. We hope that by using this feature,, we can get the data we need quickly and move on with the rest of our lives. But what if the auto-trigger function doesn’t work? If this sounds like something you are experiencing (or have experienced), please read on.

Importance of Proper Oscilloscope Triggering

Proper oscilloscope trigger control (triggering) is one of the most important controls to understand when using an oscilloscope. Time spent learning more about setting up and properly triggering an oscilloscope is time well spent.

Proper oscilloscope triggering is essential because it allows you to see a clear, steady image of repetitive waveforms. And it also allows you to accurately capture the waveform you need for single types of events.

Advanced Trigger Controls on Modern Oscilloscopes

There are two main trigger settings or controls found on modern oscilloscopes. The first is set trigger point/trigger threshold control, and the second is trigger slope. These features allow you to trigger on either the positive slope (rising edge) or the negative slope (falling edge) of a waveform at a precise location on the waveform.

Trigger Conditions

There are many trigger conditions allowing you to customize the parameters to trigger on. For example, you can select to trigger on a pulse-width of a certain amount (greater than, less than, equal to, pattern, edge, etc.).

You can also select from a variety of protocols to trigger on. You can use these protocol triggers to trigger waveforms at the packet level (I²C, SPY, UART, USB, etc.). In these cases, the scope triggers when it identifies a certain packet and its characteristics.

Trigger Menu

The trigger menu on most modern oscilloscopes contains the following features:

• Edge
• Video
• Pulse
• Slope
• Overtime
• Alternate

These options help you trigger on certain characteristics of the waveform you’re trying to capture.

Edge Triggering

Edge triggering is where the trigger point intersects the signal. It is where it is beginning to record. Any data before the trigger point is considered pre-trigger data, and any after the trigger point is considered post-trigger data.

As mentioned previously, most modern oscilloscopes can trigger on either a rising or falling edge.

Note: Edge triggering works equally well with sine waves, square waves, triangle waves, etc.

Modes

There are typically two different trigger modes: Auto and Normal.

Auto mode means that the scope will trigger even when it is not in a trigger setting (the scope still triggers even if the trigger point is set well above or below the amplitude of the waveform).

In normal mode, the trigger stays on the last waveform that was on the screen and it does not trigger until the signal falls within the range of the waveform.

Pulse Triggering

With pulse triggering, the scope can trigger square or pulse-shaped waveforms with edge triggering (i.e., on the rising edge), but another way the scope can trigger on this type of waveform is based on the width of the pulses. Again, here we can select equal to, not equal to, greater than, not greater than, pulse-widths in which to trigger on.

Some Practical Points

Spend a few minutes reading the triggering section of the instruction manual for the scope you’re using before attempting to capture any waveforms. A little up-front knowledge beats guessing on how best to run the trigger features of the scope.

Think about the waveform you want to capture. You’ll want to understand its characteristics so that you can tell the scope what characteristic you want to trigger on.

Make sure you get a stable trigger on the signal to make confident measurements.

Triggering is usually done on one of the normal input channels however, in some instances, it may be beneficial to utilize the external trigger in channel if/when the trigger signal is coming in from another piece of equipment.

You’ll want to adjust the trigger level for the best display.

Conclusion

Proper oscilloscope triggering is one of the most important controls to understand when using an oscilloscope. Setting up proper oscilloscope triggering isn’t too difficult once you understand the fundamentals as conveyed in this short article.

References and Further Reading

1. Oscilloscope Probe Considerations, In Compliance Magazine, June 2021.
2. Let’s Talk About the Low Noise Measurement Probe, In Compliance Magazine, June 2020.
3. Let’s Talk O’Scopes and Their Capabilities Beyond Basic Measurements, In Compliance Magazine. (2019, June 10). .
4. What Every Electronics Engineer Needs to Know About: Oscilloscopes, In Compliance Magazine. (2018, June 12). .
5. Oscilloscope Triggering Techniques:- how to trigger a scope – YouTube
6. Oscilloscope Basics How to use the Trigger System – YouTube.