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Oscilloscope triggering basics
The trigger event marks a specific moment in time when a "window" of waveform data becomes stable for analysis. Imagine you're driving to a destination and want to capture scenic photos along the way. You want to get there quickly, but you also need to take pictures at key points. One approach is to take random photos while driving, but this relies heavily on luck. A more practical strategy is to tell the driver where to stop to capture the best shot. Similarly, in oscilloscope applications, most of the waveform data may not be relevant. In high-speed debugging, a system might function correctly 99.999% of the time, but only 0.001% of the time does it fail — that’s the part you need to analyze. Even with high-performance oscilloscopes, if you can’t capture the relevant data, your debugging efforts will be limited.
The oscilloscope's triggering function synchronizes the horizontal sweep with the signal, making it easier to observe and understand the waveform. Triggering stabilizes repetitive signals and captures single events. By repeatedly displaying the same portion of the input signal, the trigger makes the waveform appear static on the screen. Without triggering, each scan would start at a different point, causing chaotic results, as shown in Figure 1. Before the introduction of triggered oscilloscopes, users had to visually inspect waveforms without any stabilization.
Edge triggering is the most common and fundamental type of trigger available on modern oscilloscopes. It allows users to view general amplitude and timing characteristics of a waveform. The Pinpoint trigger system, found in Tektronix DPO7000 and MSO/DPO/DSA70000 series, provides an intuitive edge trigger setup interface, as shown in Figure 2.
Trigger sources are not always limited to the displayed signal. Common sources include signals from any input channel, external signals, power frequency, or calculated signals based on multiple inputs. Oscilloscopes can be configured to trigger on any channel, regardless of whether it is being displayed. Some models also provide a discrete output for triggering other instruments like counters or signal generators.
Each logic family has unique voltage requirements, so independent trigger level settings are essential. Previously, all channels shared the same trigger level, requiring manual changes when switching sources. The Pinpoint trigger system now offers both global and per-source trigger levels, offering greater flexibility.
The trigger level and slope determine the exact point at which the oscilloscope begins capturing data. For edge triggering, users can select positive or negative slopes, and set the trigger level accordingly. This defines the crossing threshold, as shown in Figure 3. The trigger level is often set at 50% of the peak-to-peak voltage, but it can be adjusted as needed.
The horizontal position knob allows users to adjust where the trigger event appears on the screen. This feature enables pre-trigger viewing, showing signal behavior before the trigger occurs. Digital oscilloscopes always process incoming signals, and the trigger simply tells the device to save data when the condition is met. Pre-trigger viewing is especially useful for intermittent issues, allowing users to analyze what happened just before a problem occurred.
In Figure 4, the trigger position is set at 40% of the horizontal scale, meaning the trigger event occurs after 40% of the waveform is displayed. At 100%, the entire record appears before the trigger, maximizing pre-trigger visibility. At 0%, the entire record appears after the trigger, providing maximum post-trigger insight. Delayed triggering can be used to capture data after the initial trigger event, as discussed later.
Rising and falling edge triggers allow users to capture signals based on their slope. The Pinpoint trigger system supports both positive and negative slopes, which is useful for analyzing jitter in high-speed clocks and data signals. Figures 4a, 4b, and 4c illustrate how the trigger slope affects the waveform display.