Mianyang Shengshi Health Technology Co.,Ltd , https://www.shengshiaminoacid.com
Oscilloscope triggering basics
The trigger event marks a specific moment in time when a "window" of waveform data stabilizes for analysis. Imagine you're driving to a destination and want to capture a few scenic photos along the way. You know you can reach your destination quickly, but how do you ensure you take the right pictures? One approach is to randomly take photos while driving, hoping to catch the scene—but that relies too much on luck. A smarter strategy would be to tell the driver exactly where to stop to get a clear shot of the point of interest.
In many oscilloscope applications, most of the waveform data isn’t relevant. In high-speed debugging, a circuit might function correctly 99.999% of the time, with only 0.001% causing a failure or containing the signal you need to analyze. Oscilloscopes have various key specifications (like bandwidth, sample rate, and record length) to help find these rare events, but without proper triggering, even the best tools are limited.
The oscilloscope’s trigger function synchronizes the horizontal sweep with a specific point on the signal, making it easier to observe and understand. It stabilizes repeating waveforms and captures single-shot events. By repeatedly displaying the same part of the input signal, the trigger makes the waveform appear static on the screen. Without triggering, each scan could start at a different point, leading to a chaotic display, as shown in Figure 1. Before trigger-based oscilloscopes, users had to manually adjust settings to view waveforms clearly.
Edge triggering is the most common and basic type of trigger found in modern oscilloscopes. It helps users see the general shape and timing of a waveform. For example, the Pinpoint trigger system in Tektronix DPO7000 and MSO/DPO/DSA70000 series allows precise edge trigger setup, as shown in Figure 2.
The trigger source can be any input channel, an external signal, power frequency, or even a calculated signal based on input data. Most oscilloscopes allow triggering on the displayed channel, but they can also trigger on any input, regardless of whether it's shown on the screen. Some models also provide a discrete output for triggering other instruments.
Each logic family has unique voltage requirements, so independent trigger level settings are essential. The Pinpoint system lets users set a unique threshold for each input or apply global settings across all channels, improving flexibility.
Trigger level and slope define the exact point where the oscilloscope starts capturing data. For edge triggering, users can choose between positive or negative slopes and set the trigger level. The arrow on the display indicates the current trigger level, which is typically around 50% of the peak-to-peak voltage, though this can vary. This setting determines when the oscilloscope initiates a capture.
The horizontal position knob adjusts where the trigger event appears on the screen. Moving this allows pre-trigger viewing, showing what happened before the trigger event. This is especially useful for intermittent issues, as it lets users review the data leading up to a problem.
Digital oscilloscopes always process incoming signals, storing data in memory until a trigger occurs. Pre-trigger viewing is a powerful tool for debugging, as it shows the state of the signal before the issue arises.
In Figure 4, the trigger position is set to 40%, meaning the trigger event occurs 40% into the waveform. The trigger level is set at 880 mV, indicated by a yellow arrow. The trigger can be placed anywhere from 0% to 100% of the record, allowing full control over what is viewed before or after the event.
Oscilloscopes support both rising and falling edge triggers, enabling analysis of jitter on high-speed signals. The Pinpoint system offers precise control over these settings, as seen in Figures 4a, 4b, and 4c. Changing the slope from a rising edge to a falling edge allows for detailed observation of signal transitions.