Oscilloscope Basics Series 3: About the Oscilloscope Trigger Function (Part 1)

I remember when I first joined LeCroy, I spent a whole day practicing the trigger function in the three-day basic knowledge training on oscilloscopes . "Triggering" seems to be the difficulty for beginners to learn oscilloscopes. We often help engineers solve many cases of trigger test problems on site. Usually, some engineers only know that after "Auto Setup", they can see the waveform on the screen and then "Stop" to expand the waveform and move it left and right to view the details. Therefore, sometimes I even receive such calls, questioning whether our oscilloscope has problems, because the waveform he sees after "Auto Setup" always "shakes" back and forth on the screen. But when I asked him whether the trigger source was set correctly, whether the trigger level was set appropriately, whether the appropriate trigger method was used, etc., I did not get an answer; even when I met the experts in my mind, I often found that they did not establish the basic concept of triggering. I like to google before writing a topic, but unfortunately I did not find a complete enlightenment article. Although there are a lot of triggers in the PPT presentations of the three oscilloscope manufacturers, there are really few Chinese articles that introduce triggers in detail. Of course, this is also fortunate, because my humble article may be an enlightenment work for many engineers.

Triggering is one of the biggest features that distinguishes digital oscilloscopes from analog oscilloscopes. The triggering function of digital oscilloscopes is very rich. Through the trigger setting, users can see the signal before the trigger and the signal after the trigger. For the analysis of high-speed signals, triggering is rarely discussed, because it is usually to capture waveforms for a long time and then do eye diagrams and jitter analysis. Triggering may be used more frequently for the measurement of low-speed signals, because low-speed signals often encounter very strange signals that need to be isolated through triggering.

If the trigger circuit of the oscilloscope is broken, the oscilloscope can still work, but the waveform you see on the screen will "shake" back and forth, or flash on the screen. This is actually equivalent to setting the trigger mode to "Auto" and setting the trigger level to exceed the maximum or minimum amplitude of the signal. The acquisition memory of the oscilloscope is a circular buffer, and new data will continue to overwrite old data until the acquisition process ends.

As shown in Figure 1. Without a trigger circuit, these collected data are constantly alternating between old and new, and the waveform on the screen appears to be "shaking" back and forth. Auto Setup is an automatic trigger setting. The oscilloscope automatically sets the horizontal time base, vertical sensitivity, offset and trigger conditions of the oscilloscope according to the characteristics of the measured signal, so that the waveform can be displayed on the oscilloscope. Its main purpose is to ensure that the waveform can be displayed, which is useful for novices who don't know how to make the waveform "come out" when they get an oscilloscope. However, if you don't understand the concept of triggering, it is wrong to start observing, measuring, or even drawing conclusions through the Auto Setup settings. After all, the oscilloscope is the eyes of the engineer, and the engineer needs to thoroughly master this tool and use this pair of eyes well.

The so-called trigger, according to the professional explanation, is: set certain trigger conditions according to the needs. When a waveform in the waveform stream meets this condition, the oscilloscope captures the waveform and its adjacent parts in real time and displays them on the screen. The uniqueness of the trigger condition is the primary condition for accurate capture. In order to observe more events that occurred before a specific waveform, move the trigger point to the right of the display window for a period of time, which is a delayed trigger; in order to understand more events that occurred after a specific waveform, move the trigger point to the left of the display window for a period of time, which is an advanced trigger. As shown in Figure 2. In a digital oscilloscope, the trigger point can be located at any position of the acquired and stored records. As shown in the figure on the right of Figure 1, the trigger point stays at the middle moment of the acquisition and storage.

In order to understand triggering more vividly, I often use a very sour phrase to describe it. The so-called triggering means "staying at this moment", or "waiting for that moment". The trigger circuit can be understood as a pair of innocent eyes watching everyone who passes by her (signal flow). When she sees her ideal person (trigger condition), her eyes stare at this person and let the ideal person stay at the position she is looking at (trigger point). But she will continue to look for her next ideal person. Every time she finds her ideal person, she will let the ideal person stay at the position she is looking at (trigger point). Therefore, the position of her eyes' gaze point (trigger point) only stays on those ideal people (waveforms that meet the conditions).

Figure 1 The memory of a digital oscilloscope is a circular buffer

Figure 2 Schematic diagram of triggering principle

Let 's return to the explanation of professional terms from the above pure and beautiful imagination. The so-called trigger, as I understand it, has two functions: first, to isolate the event of interest. Second, to synchronize the waveform, or to stabilize the displayed waveform. To explain these two functions clearly, let's first review some aspects that need to be paid attention to when setting the trigger: trigger source, trigger point, trigger level, trigger mode, and trigger method.

Trigger source: refers to the signal of which channel is used as the trigger object. The trigger source can be any channel of the oscilloscope or an external channel. As shown in Figure 3, the selected trigger source is C2, that is, channel 2. When measuring four signals at the same time, there are sometimes some tricks to choose which signal as the trigger source, which is related to the problem you want to debug. For example, you need to view the power-on timing of six signals at the same time, but the oscilloscope has only four channels. At this time, you can use two single trigger captures to capture four signals first, and save these four signals as data files so that you can call the oscilloscope back, and then capture three signals. In these two captures, the same power-on reset signal is used as the trigger source to synchronize the waveforms. (If you have any good test cases in the selection of trigger sources, please share them with everyone.)

Figure 3 Trigger setting interface - trigger source

Trigger point: The trigger point is sometimes called trigger delay, but I think it is more intuitive to call it the trigger point. Its meaning has just been explained. It is the point where the eyes are looking at, the moment when the oscilloscope allows the waveform to stay, and the position corresponding to the small red triangle on the oscilloscope. As shown in Figure 4, the small red triangle in the red circle is the trigger point. After setting the trigger conditions, the waveforms corresponding to the trigger point should all meet the trigger conditions. In other words, the oscilloscope isolates the waveforms that meet the trigger conditions at the position of this trigger point. Regarding the setting of the trigger point, I remember that the first trick my first boss taught me when I was working was to observe the oscilloscope settings during the soft start process of the power supply. He emphasized that the trigger point must be moved to the left side of the oscilloscope and then use a single trigger after setting the trigger conditions. The purpose of moving the trigger point to the left is to make full use of the storage space of the oscilloscope. During my training at LeCroy, the boss told me that every time I set up the oscilloscope, I should first check where the trigger point and trigger level are. It is best to set the trigger point in the middle to facilitate observation and adjustment, because the waveform of the oscilloscope expands with the trigger point as the symmetrical point. On the panel of the LeCroy oscilloscope, you can simply press the Delay button to automatically return the trigger point to the center of the screen.

Trigger level: The trigger level refers to the level that the signal needs to reach in order to be triggered. In Figure 4, the trigger level is the amplitude of the position of the small red triangle on the right relative to the zero level, that is, the amplitude between the two white lines. In this example, the value is the 1.00V value marked by the red box in the lower right corner of the figure. Setting any trigger condition requires a specific trigger level.

The trigger level defines whether the signal is an "event" that meets the trigger condition. The signal in Figure 4 has a rising edge, but the rising edge is not necessarily the event that the trigger circuit is interested in. Perhaps the pure eyes are looking for a taller (higher amplitude of the trigger level) person (signal that meets the trigger condition). When the rising edge is triggered, only the position where the rising edge reaches the trigger level during the rising process is considered an "event" and is "isolated" at the trigger point.

The trigger level can be set in the Trigger menu or adjusted using the knob on the panel. The conditions of many trigger modes are relative to the trigger level. For example, in the case of width trigger , the width (time interval) recognized by the trigger circuit is not the 50% of the rising edge to the 50% of the next rising edge, but the time interval between the intersection of the two rising edges where the trigger level crosses. As shown in Figure 6, the time interval between the two blue points where the blue line crosses the waveform from the trigger level position and intersects with the pulse width corresponding to the trigger point position is the width size that satisfies the trigger condition.