Make the most of oscilloscope triggering: part 1

Learning the basics of oscilloscope triggers can help you leverage advanced features.

In an earlier series, we discussed how the oscilloscope has evolved over the past three-quarters of a century. We noted that the basic horizontal and vertical controls remain functionally similar, but today’s digital oscilloscopes offer many features, including waveform math functions, that go well beyond what an oscilloscope could do in the 1950s. We touched on triggering in our earlier series,^[1] now we will take a comprehensive look at the topic, addressing some topics that university-level electrical-engineering courses might not cover. ^[2]

Can we begin with a look at triggering basics?
Sure. First, keep in mind what we’re trying to do with triggering. If you apply a fixed-frequency sinusoidal input to an oscilloscope channel without triggering, you’ll see what appears to be multiple waveforms that dance across the screen. What you are actually seeing are successive snapshots of random segments of the same waveform with no particular phase relationship. Triggering stabilizes the display, so the segments overlap and appear static, letting you measure parameters such as amplitude, frequency, and, for multichannel displays, phase.

Analog oscilloscope triggering — Figure 1. An analog oscilloscope offers trigger slope and level controls. (Detail of image by Drumcliff from Pixabay)

Figure 1 shows a close-up of a vintage analog oscilloscope we discussed in our earlier series, highlighting the oscilloscope trigger controls. As you can see, you can select a trigger level as well as the slope of the waveform on which the trigger occurs. In addition, you can select from multiple modes, including AUTO and NORM.

Now let’s see how that compares to the trigger controls of a modern digital scope, as shown in Figure 2. For this demonstration, I am using a USB oscilloscope (a PicoScope 2204A) from Pico Technology, which replaces the physical knobs in Figure 1 with on-screen icons. I have clicked the trigger function from the top row, and the trigger menu appears to the left of the trace display. As with the analog scope, I can still select the AUTO trigger mode, as shown at the top of the trigger menu. Moving down, I can choose the trigger type (simple edge), the source (channel A in this case), the trigger threshold level (0 V in this case), and the edge direction at the time of trigger (falling, or negative slope), just as I could have with the analog oscilloscope.

USB oscilloscope triggering — Figure 2. A USB oscilloscope replaces physical knobs and buttons with a virtual interface on a laptop.

One thing the PicoScope does that the analog oscilloscope couldn’t is allow me to select the pre-trigger range, which I have set to 50%. Consequently, the oscilloscope triggers at the center of the screen — at the position of the yellow diamond — when a negative-going portion of the displayed 1.6 V_PP 1 kHz sinusoid passes through 0.

Could we see another example?
Yes. In Figure 3, I’ve chosen to trigger on a rising edge at a level of 200 mV, and I’ve changed the pre-trigger setting to 20%. The instrument then triggers when the waveform rises to the 0.4 V level at a point one-fifth of the way across the screen.

What’s the EXT input in Figure 1 used for?
Generally, you’ll want to trigger on an internal signal applied to a measurement channel — channel A in Figure 2 and Figure 3. In some cases, however, you may want to trigger on an external signal that you don’t need to view on the scope screen. For example, your system under test may generate a fault signal in the event of an overcurrent or overvoltage condition. You can connect that fault signal to the external (EXT) trigger input and use your measurement channels to display what happens elsewhere, leading up to or in the immediate aftermath of the fault. There are several other times when the use of an external trigger may be useful[3]: when the signal you want to measure lacks sufficient regular features (such as zero crossings) to ensure stable internal triggering, for example.

What else should we know about oscilloscope triggering?
In this post, we’ve looked at the basic functions that could be performed on an analog oscilloscope. Often, you’ll want to trigger on a condition that would be difficult or impossible to detect using analog techniques — when a signal enters or departs from a specified voltage range, for example, or when an anomaly occurs on the transmission or reception of a specific digital code. We’ll take a closer look next time.

References
[1] Nelson, Rick, “Get the most from your oscilloscope: part 1,” Test & Measurement Tips, September 29, 2025.
[2] Etheridge, Ian, “Oscilloscope Triggers: What They Didn’t Teach Me in School ,” Digilent, August 18, 2025.
[3] Reed, Callum, “What is an External Trigger in Oscilloscopes? ” Keysight Technologies, September 17, 2025.