What is the function of an oscilloscope trigger circuit?

In order to synchronize the scanning signal with the measured signal, some conditions can be set, and the measured signal is constantly compared with these conditions. The scanning is started only when the measured signal meets these conditions, so that the scanning frequency is the same as the measured signal or there is an integer multiple relationship, that is, synchronization. This technology is called "triggering", and these conditions are called "trigger conditions".

There are many forms of trigger conditions, the most commonly used and basic one is "edge trigger", which is to compare the change of the measured signal (i.e. the rising or falling edge of the signal) with a certain level. When the change of the signal reaches this level in a selected way, a trigger signal is generated to start a scan. The trigger level can be selected at 0V. When the measured signal crosses this level from low to high, a scan is generated, so that a scan signal synchronized with the measured signal is obtained. Other trigger conditions include "pulse width trigger", "slope trigger", "state trigger", etc. These trigger conditions usually appear in more advanced oscilloscopes.

Oscilloscope triggering principle

When a signal is input to the oscilloscope, if the display of the signal is not controlled accordingly, the display will be chaotic, as shown in the figure below. The display of each screen is different. When the oscilloscope is refreshed quickly, the signal seen is a mixed and unstable image, which cannot be observed and measured.

Waveform display when the oscilloscope is not triggered normally

In order to solve this problem, it is necessary to specify the trigger conditions of the oscilloscope to achieve stable synchronization and display the signal clearly.

Let's take the simplest and most commonly used edge trigger to illustrate the principle of triggering. First, analyze the waveform to be observed: the waveform characteristics of a sine wave in one cycle have only one rising and falling edge, so you can choose the rising edge as the trigger condition, and set a trigger level to intersect with the rising edge to get a trigger point. In different cycles of the sine wave, the position of the trigger point is the same and unique. At this time, when we define the trigger point at a specific position on the display screen, the oscilloscope will place the points that meet the trigger condition at the same position on the screen every time it refreshes a screen. Since this point is the only one in the waveform, all the trigger points on different screens are at the same position, so the display on the screen is stable and synchronized. At the same time, only the first point that meets the trigger condition is defined as the trigger point when refreshing a screen, and the others are ignored. For convenience, place the trigger point at the far left of the screen. When each screen is refreshed, the oscilloscope places the points that meet the trigger condition at the same point, and the waveform is stably displayed on the screen of the oscilloscope, as shown in the figure below.

Waveform display when the oscilloscope is triggered normally

Oscilloscope trigger circuit

The function of the trigger circuit is to provide a trigger pulse that meets the requirements to start scanning or acquisition. It is part of the horizontal channel. It includes circuits such as trigger source selection, trigger coupling mode selection, trigger mode selection, trigger polarity selection, trigger level selection, and trigger amplification and shaping. The figure below is a schematic diagram of the trigger circuit of an analog oscilloscope. The principle of the trigger circuit of a digital oscilloscope is similar to that of an analog oscilloscope.

Trigger source

That is, which channel's signal is used as the trigger object. The trigger source can be any channel of the oscilloscope or an external channel. As shown in the figure below, 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 to use 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.

Trigger Points

The trigger point is sometimes called the 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 stop, and the position corresponding to the small red triangle on the oscilloscope. As shown in the figure below, 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. A specific trigger level is required to set any trigger condition. 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 want to find 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 time interval between 50% of the rising edge and 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 position of the trigger level and the pulse width corresponding to the position of the trigger point intersects is the width of the trigger condition.

In the example, it is between 3ns and 10ns, which means that the trigger function isolates the pulse width between 3ns and 10ns that we are interested in. I remember when I was doing research and development, the oscilloscope I used had a very low memory depth. In order to capture the maximum value of the MOSFET, I did not capture a long period of VDS voltage signal at once to automatically measure the peak value, but constantly adjusted the amplitude of the trigger level, gradually increasing the trigger level to see if the signal could be triggered.

The trigger section of the oscilloscope panel button

Trigger Mode

The oscilloscope has four trigger modes: Auto, Normal, Single, and Stop. As shown in the panel in Figure 5. Many engineers do not understand Auto and Normal. Auto means that the waveform is refreshed in real time regardless of whether the trigger condition is met. At this time, the waveform on the oscilloscope screen usually looks "shaky". Normal means that the trigger condition is met before triggering, otherwise the waveform will be still, and for LeCroy oscilloscopes, there is a red prompt in the lower right corner of the screen: "Wai TI ng for Trigger". Single means that only the waveform that meets the trigger condition for the first time is captured, and it stops after capturing. Stop means that the waveform is forced to be still. The flashing speed of the green TRIG on the panel shown in Figure 5 represents the speed of the trigger rate.

How is the width defined in width trigger?

The role of oscilloscope triggering

Triggering means that when using an oscilloscope, in order to synchronize the scanning signal with the measured signal, we can set some conditions, constantly compare the measured signal with these conditions, and start scanning only when the measured signal meets these conditions, so that the scanning frequency is the same as the measured signal or there is an integer multiple relationship, that is, synchronization. These conditions are trigger conditions. For example, the ZDS2024Plus oscilloscope of Zhiyuan Electronics comes standard with 22 protocol triggers, and the trigger mode can be set according to my needs. The purpose of triggering is simply to start at the same position of the waveform every time it is displayed, and the waveform can be displayed stably.