Detailed explanation of the trigger function of the oscilloscope

Preface: When using an oscilloscope for measurement, the first thing you need to do is to choose the trigger mode to capture a specific event. Therefore, fully understanding the concept, principle, and setting method of triggering is a prerequisite for effectively using the oscilloscope.

This article uses the TO1000 series flat-panel oscilloscope as an example to build an understanding of oscilloscope triggering from scratch with the simplest explanation.

1. Trigger Definition

At the beginning of the article, we give a clear definition of oscilloscope triggering:

The oscilloscope will capture a waveform only when a preset condition is met. This action of capturing a waveform based on the condition is called triggering.

2. Triggering Principle

How does triggering work? Let's simulate the process to see the difference between triggering and not triggering:

A. When the oscilloscope is not triggered, it will randomly capture signals for a period of time and generate images. Since the signal is continuous, the randomly captured positions are irregular. These static images are displayed one by one, just like playing a film, and combined together to form a dynamic display. The final effect on the screen is to see a waveform rolling back and forth.

(When not triggered) the oscilloscope displays the following screen

B. We set a condition and use a DC level as a reference. The moment when the voltage of the signal is greater than the DC level is used as the starting point for capturing the signal. As shown in the figure below, the red thin line is the reference DC level. Since the position of each captured image is regular and is captured at the moment when the signal passes the DC level, the phase of the captured signal is the same each time. When displayed continuously, they completely overlap and look like a stable waveform.

(When triggered) The oscilloscope displays the following screen

3. The role of trigger

The role of triggering can be summarized in two points:

1. Stable display of a periodic signal, which can also be said to be a synchronous waveform;

2. Capture the fragments you want to observe from fast and complex signals.

The stable display of a periodic signal has been demonstrated in the second section. There is an example at the end of the article on how to capture a specific fragment, which can help us quickly master the use of triggers.

4. Understand several trigger modes

What are the trigger modes of an oscilloscope?

We know that the oscilloscope needs to be triggered in order to synchronize the scanning of the oscilloscope with the observed signal, so as to display a stable waveform. The so-called trigger mode refers to some methods selected to generate triggers to meet different observation effects. There are three basic trigger modes of the oscilloscope:

Automatic mode and normal mode

In automatic mode, the oscilloscope will first trigger according to the trigger conditions. When there is no trigger condition after the set time, the oscilloscope will force trigger and display the signal. When we don’t know the characteristics of a signal, we should use the "automatic mode". This mode can ensure that the oscilloscope will display the waveform when other trigger settings are incorrect. Although the waveform may not be stable, it can provide us with intuitive judgment for further adjustment of the oscilloscope.

Normal mode is different from automatic mode. In normal mode, the oscilloscope will scan only when the trigger condition is met. If there is no trigger, no scanning will be performed and there will be nothing on the screen. The function of normal mode is to observe the details of the waveform, especially for more complex signals. When we set a specific trigger condition for a specific signal, especially when the time interval to meet the trigger condition is relatively long, we should use normal mode.

The following figure shows what the oscilloscope screen displays in two modes when the square wave signal is not triggered:

Single mode

Single mode is also called single sequence trigger, abbreviated as "SEQ" in English. This mode is somewhat similar to the normal mode, that is, scanning is only generated when the trigger condition is met, otherwise no scanning. The difference is that once this scan is completed, the scanning system of the oscilloscope will enter a dormant state, that is, no scanning will be performed even if a signal that meets the trigger condition appears later. The next trigger action must be performed manually. The single trigger mode is often used to capture signals that appear once or multiple times but are not periodic. For example, the power-on signal generated when a circuit is powered on will only appear once. If a single trigger is not used, it is difficult to capture this signal.

In actual use, the selection of different trigger modes should be based on the characteristics of the observed signal and the content you want to observe. There are no fixed rules, but it is often an interactive process, that is: select different trigger modes to understand the characteristics of the signal, and then select an effective trigger method based on the characteristics of the signal and the content you want to observe.

5. Commonly used trigger adjustments

The core of triggering is how to set the conditions. This is the most important part of using an oscilloscope, and it is also the part that many users find the most difficult to master. Let's take a look at some commonly used trigger adjustments:

Trigger source

To make the screen display a stable waveform, the measured signal itself or a signal with a certain time relationship with the measured signal needs to be added to the trigger circuit as the comparison object of the trigger condition. This comparison object is the trigger source. The most common trigger source is the internal trigger (INT), that is, the measured signal is used as the trigger source, such as channel 1, channel 2, and channel 3. When using it, it is important to note that the channel where the signal is currently located is selected as the trigger source. This is a problem that most beginners ignore: using a channel without a signal as the trigger source.

In addition to the internal trigger (INT), there are two other trigger sources: external trigger (EXT or AUX IN) and power trigger (LINE). The external trigger is a trigger source independent of the signal channel. The trigger source can only be low-frequency and high-frequency signals, and there must be a periodic relationship between the measured signal; the power trigger uses the oscilloscope's mains input as the trigger signal. This method is effective when measuring signals related to the frequency of the AC power supply. Interested friends can learn more about it.

Trigger level and trigger polarity

The trigger level is a voltage value in the oscilloscope display, and the unit is "mV" and "V". In addition, there is a trigger level line on the interface to indicate its position relative to the signal waveform. The trigger level adjustment of the tablet oscilloscope is very simple. Just touch the "Level" line with your finger and move it. Trigger level adjustment is also called synchronization adjustment, which synchronizes the scan with the measured signal. The signal can only be triggered when the trigger level is within the range of the signal amplitude.

The trigger polarity switch is used to select the polarity of the trigger signal. When positive is selected, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal increase. When negative is selected, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal decrease.

The trigger polarity and trigger level jointly determine the trigger point of the trigger signal, and the trigger mode used is concerned at this point.

6. Trigger Type

There are many forms of trigger conditions. Common trigger types include: edge trigger, pulse width trigger, logic trigger, N-edge trigger, runt trigger, slope trigger, timeout trigger, video trigger, serial bus trigger, etc.

We will take edge triggering and pulse width triggering as typical examples and introduce them one by one:

Edge triggered:

Edge triggering is the most commonly used, simplest and most effective triggering method. More than 90% of applications can be performed using edge triggering alone. It triggers the signal by looking for a specific edge (rising edge or falling edge) on the waveform.

The figure below is an illustration of the principle of edge triggering: taking the trigger level as a reference, the trigger generated when the signal changes from below the trigger level to above the trigger level is a rising edge trigger, and vice versa, it is a falling edge trigger.

Now input a simple sine wave signal to the oscilloscope port, set it to rising edge trigger and falling edge trigger respectively, and observe the change of trigger position (the letter "T" at the top center indicates the trigger position)

Pulse Width Trigger

The trigger generated based on the pulse width of the signal is referred to as pulse width trigger. The range of pulse width can be less than, greater than, equal to, and not equal to. According to the polarity, it can be divided into positive pulse width and negative pulse width.

Positive pulse width: the time difference between the intersection of the rising edge and the trigger level and the intersection of the adjacent falling edge and the trigger level;

Negative pulse width: The time difference between the intersection point of the falling edge and the trigger level and the intersection point of the adjacent rising edge and the trigger level.

Now the input frequency is 1KHz, that is, a square wave signal with a period of 1ms. The setting method using pulse width trigger is as follows:

Logic trigger

Logic triggering requires setting the logic value of each channel and setting the logical relationship between channels (AND, OR, NOT, etc.). When the logical relationship is met and the set time conditions are reached, a trigger is generated when the edge of any channel changes.