Trigger an oscilloscope, get a stable display

Article By : Arthur Pini

Triggering an oscilloscope gives you many options for viewing and processing signal events. Learn how to use triggers to your advantage.

The first oscilloscope I ever used professionally was an old synchronized sweep instrument. Obtaining a stable display on that old oscilloscope was an art. Now I appreciate all the triggering tools available on even the most basic oscilloscopes.

An oscilloscope trigger synchronizes the oscilloscope’s timebase to the input signal, producing a stable display. In analog oscilloscopes, the trigger initiates the sweep generator so that the horizontal sweep is synchronous with the vertical signal. Digital storage oscilloscopes (DSOs) produce the same effect through a different method. In DSOs, the digitizer runs continuously and the trigger event marks the associated data in the acquisition memory, locking the signal data for display, measurement, and further processing.

Today’s DSOs include relatively simple edge triggers, more sophisticated “smart” triggers, and still more complex “augmented” triggers. Let’s look into these multiple trigger implementations.

Edge triggers

Edge triggers are the most commonly used trigger method. With edge triggering, the oscilloscope triggers when the source trace crosses the trigger threshold level with the user specified slope (positive, negative, or either). Trigger hysteresis provides noise immunity by requiring the signal to transition through the hysteresis interval before the oscilloscope will trigger. Most oscilloscopes use a hysteresis level of 0.3 to 0.5 vertical divisions.

Trigger sources include the input channels, external trigger input, line power, and, in some cases, the built-in fast edge signal. The slope, polarity, and trigger threshold for each source can be set independently of the other sources. Figure 1 shows the edge trigger setup for a typical mid-range oscilloscope.

textFigure 1 The elements of an edge trigger setup include trigger source, level, slope, and coupling.

In addition to setting the source, trigger level, and slope, you have a choice of coupling selections: AC or DC coupling along with low or high frequency reject. The frequency selective coupling paths are used to attenuate extraneous signals. The low frequency reject inserts a 50 kHz high-pass filter in the trigger signal path while the high frequency reject uses a 50 kHz low-pass filter. These frequency selective coupling modes find use in applications such as troubleshooting switch mode power supplies. High-frequency reject makes it easier to trigger on mains related signals while low-frequency reject simplifies triggering on switching regulator signals.

Some oscilloscopes have a Find Level button that when pressed, will automatically find the trigger level for the current trigger source. Another convenience is the trigger icon (right side of the Figure 1), which summarizes the trigger setup.

Trigger hold off

Hold off is a trigger function used when there are multiple trigger events per acquisition. It lets the oscilloscope ignore extra trigger events and stabilize the display as if there was only a single trigger event per acquisition. Hold off can be based on time or a trigger event. Figure 2 shows an example of hold off by time. The waveform being acquired is a burst of eight pulses with a duration of 7 µs. There are eight possible trigger events. A trigger event is a signal condition that would normally result in the oscilloscope triggering. Think of hold off as a command to ignore the trigger for a specified time or event count.

hold off oscilloscope trigger
Figure 2 The hold off by time trigger can obtain a stable trigger on a pulse burst waveform with a 7µs duration from the first to the last positive going edge.

In Figure 2, hold off was set to ignore triggers for the burst’s 7 µs duration. When the oscilloscope trigger is enabled, any trigger will be followed by a 7 µs interval. Because 7 µs is the duration of the burst, the next trigger will occur at the beginning of the next burst.

You can do the same thing with hold off by events. In this case, you hold off by eight trigger events. This is the entire duration of the pulse burst. Again, the acquisition will synchronize itself with each pulse burst. Note that hold off does not guarantee triggering on a specific point in the burst, just synchronizing with the burst. Synchronization will be maintained until there is an interruption of the signal and then will resynchronize, perhaps at a different point, when the connection is resumed.

The starts hold off counter determines if the hold-off counter is reset at the beginning of each acquisition (the acquisition start selection) or if it accumulates continuously (the last trigger time selection). Keep in mind that the trigger input is active at all times. If trigger pulses arrive, even if not during an acquisition, they may be counted in the hold off condition unless the hold off counter is restarted at the beginning of the acquisition. Similarly, if the process you’re synchronizing with is continuous you can count all trigger events by selecting to start the count at the last trigger time.

Hold off is a useful tool, but it does require some experience. Reviewing the oscilloscope manual and any application notes or tutorials supplied by the manufacturer can be helpful.

Smart triggers

Mid-range oscilloscopes usually include a powerful set of smart triggers that are based on timing and amplitude parameters of the trigger signal. Smart triggers may include glitch, width, window, interval (period), drop out, logic pattern, runt, TV, and slew rate.

As an example of a smart trigger we’ll focus on the width trigger. The width trigger is sensitive to the width of a signal and is generally applied to a rectangular pulse. Provisions to trigger on pulses wider than, narrower than, within a range, and outside of a range make this a powerful tool for triggering on complex signals.

Figure 3 shows the waveform that will be used to demonstrate the use of the width trigger. This is a pulse width modulated waveform with eight distinct widths from 500 ns to 4 µs.

poulse width trigger oscilloscopeFigure 3 Using the width trigger to trigger the oscilloscope on a pulse width of 2.5 µs.

As previously mentioned, there are four width conditions that you can use to define the trigger. Figure 3 shows a width trigger based on the width of the pulse being between 2.3 µs and 2.7 µs. The width trigger dialog box in Figure 3 shows the setup for triggering on the width “within a range” condition. As a result, a pulse with a width of 2.5 µs is the trigger event. Other smart triggers behave in a similar manner offering a wide range of trigger events based on signal characteristics.

[Continue reading on EDN US: Exclusion and other triggers]

Arthur Pini has over 50 years’ experience in electronics test and measurement.

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