Make the most of oscilloscope triggering: part 2

Advanced triggering techniques can help you track down elusive problems.

In part 1 of this series, we investigated the fundamentals of the oscilloscope trigger function, including the simple edge trigger that originated with analog oscilloscopes. In this part, we look at additional triggers that help with troubleshooting.

What are some other types of triggering?
Advanced triggering techniques for digital oscilloscopes can trigger on waveform characteristics such as pulse width and rise time as well as logic relationships between multiple waveforms—including protocol-aware capabilities that can trigger on the content of standard digital-communications protocols, such CAN, SENT, I²C, and I3C.

How do you select these various different triggers on a oscilloscope?
I’ve been using a PicoScope USB oscilloscope from Pico Technology for this series on oscilloscopes. For that oscilloscope, I click the “Trigger” button in the top row in Figure 1 and see the “Trigger” menu to the left of the trace. From the menu, I choose “Change type” and see a selection of trigger options with icons that suggest their functions. Other oscilloscope manufacturers offer similar capabilities, although their terminology and implementation details may differ.

How do some of these trigger types work?
PicoScope supports trigger functions including “Advanced edge,” which enables dual-edge triggering that can help display eye diagrams. It also adds adjustable hysteresis, which can reduce false triggering on noisy signals. Another option is “Interval,” which helps find mistimed edges, missing pulses, or sudden changes in frequency. Yet another is “Level dropout,” which helps detect the end of a pulse train. “Rise/fall time” captures signals whose transition times are greater than, less than, or within a user-specified range.

Several trigger types are related. “Window,” for example, lets you see when a signal enters or leaves a specific voltage range, while “Pulse width” lets you trigger on pulses with a specified range of widths. Combining the two, “Window pulse width” initiates trigger when a window condition lasts for a specified period of time, ignoring under-voltage and overvoltage excursions that last less than the specified period.
Finally, “Logic” enables a mixed-signal oscilloscope, which combines the functionality of a logic analyzer and oscilloscope, that can trigger on the results of the logical operators NAND, OR, NOR, XOR, or XNOR acting on as many as four digital inputs.

Could you show a specific example of one of these trigger types?

Sure. Figure 2 shows a 1 kHz sine wave with a peak magnitude just under 400 mV. For this advanced triggering example, consider it as an AC component riding on a DC power-supply rail. Our hypothetical design spec calls for a 600 mV maximum peak ripple, so everything looks good, and we go work on something else.

But then something goes wrong…
What if you come back in an hour and see the trace in Figure 3, which shows a nearly 800 mV peak signal?

We don’t know how this happened. Was the change gradual, or sudden? We would like to focus on the time when the signal first exceeded our 600 mV maximum peak. So, we can set up a “Window” trigger, as shown in Figure 4, with the vertical bar in the center of the screen representing the window. We select Threshold 1 and Threshold 2 voltages of +600 mV and -600 mV, respectively. Then under “Direction,” we choose “Exiting,” because we want to know when our signal exits the ±600 mV window. We also select the “Single” mode because once we capture the signal where it exceeds the 600 mV level, we don’t want future traces to overwrite our captured waveform.

Figure 5 shows the trace that our “Window” trigger captures. We note that some noise appears on our signal shortly before the trigger point, after which, the signal appears to stabilize at the higher 800-mV peak level. We can then use any additional available oscilloscope channels (only one in this case) to investigate what else is happening in our circuit under test until we find the cause.

I noticed in part 1 of this series that the analog oscilloscope we looked at had an X-Y mode. What is that for?
Typically, an oscilloscope plots a voltage level (y-axis) against its internal time base, with the x-axis representing time. X-Y mode plots one input channel against another — channel A vs. channel B, for example, instead of channel A vs. time. This mode can plot a device’s current-voltage (I-V) characteristic, or it can illuminate phase differences by displaying Lissajous patterns. Perhaps we can take a closer look in a future article.

Related EE World content
The trigger function of an oscilloscope
Oscilloscope’s horizontal-axis measurement fundamentals
Logic analyzer basics: The difference between an oscilloscope and a logic analyzer
Getting the most from your oscilloscope: part 3
Understanding oscilloscope display modes
Using an oscilloscope to display Lissajous patterns