Analyzing the triggering principle of oscilloscope

We often hear that the triggering modes of oscilloscopes include level triggering and edge triggering, but what exactly is triggering? What is its use in an oscilloscope? In order to synchronize the scanning signal with the measured signal, we can set some conditions and constantly compare the measured signal 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. We call this technology "triggering", and these conditions are called "trigger conditions".

The purpose of triggering is to start at the same position of the waveform each time the display is displayed, so that the waveform can be displayed stably. Generally, analog oscilloscopes have edge triggering, video triggering, and mains triggering; while digital oscilloscopes have more triggering conditions called advanced triggering such as logic triggering, glitch triggering, and pulse width triggering.

① Edge trigger, edge trigger, can set the trigger level, rising edge or falling edge. Edge trigger is also called basic trigger.

② Advanced trigger, which includes various trigger functions. You can set the corresponding trigger conditions according to the characteristics of the measured signal and locate the waveform of interest. Advanced trigger is the key to circuit debugging. During the circuit debugging process, if you do not know the possible problems of the measured signal in advance, you can use different advanced trigger functions to locate the details of the fault, which can shorten your debugging cycle.

Basic trigger modes of an oscilloscope

What is the trigger mode of an oscilloscope? As we know, an oscilloscope needs to use a method called "triggering" to synchronize the scanning of the oscilloscope with the observed signal, thereby displaying a stable waveform. The so-called "trigger mode" refers to some methods selected to generate triggers to meet different observation needs.

There are three most commonly used and basic trigger modes for oscilloscopes: the first is the "automatic mode", which is generally marked as "AUTO" on the oscilloscope panel. In this mode, when the trigger does not occur, the scanning system of the oscilloscope will automatically scan according to the set scanning rate; and when a trigger occurs, the scanning system will try to scan according to the frequency of the signal. Therefore, in this mode, regardless of whether the trigger condition is met, the oscilloscope will generate a scan, and the changing scan line can be seen on the screen. This is the characteristic of this mode.

The second mode is "normal mode", also called "conventional mode", which is usually marked as "NORMAL" or "NORM" on the panel. This mode is different from the automatic mode. In this mode, the oscilloscope will scan only when the trigger condition is met. If there is no trigger, it will not scan. Therefore, in this mode, if there is no trigger, for an analog oscilloscope, you will not see the scan line, and there will be nothing on the screen. For a digital oscilloscope, you will not see the waveform update. If you don't understand this, you will often think that the signal is not connected or there are other faults.

The third mode is "single mode", usually marked as "SINGLE" or "SIGL". This mode is similar to "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 generated and completed, the oscilloscope's scanning system enters a dormant state, so that even if there are other conditions that meet the requirements, the oscilloscope will not scan again.

The scanning will not be performed again when the trigger condition signal appears, that is, it will only scan once after one trigger, i.e., single time. The scanning system must be restarted manually to generate the next trigger. Obviously, for ordinary analog oscilloscopes, you will often find that you can't see anything in this mode, because the waveform flashes by and the oscilloscope cannot retain it. Therefore, except for cooperating with a camera to take a picture of the flashing waveform, this mode is useless in most occasions.

The above three trigger modes are provided by most oscilloscopes. So, how should we choose and use them in practice?

Using the Oscilloscope Trigger Mode

Generally speaking, when you don't know much about the characteristics of the signal, you should choose the automatic mode, because at this time the oscilloscope will scan no matter what the signal is, and you can at least see something on the screen, even if it is just a scan line, instead of nothing. After there is a scan line, you can "find" the waveform by adjusting the vertical gain, vertical position, time base rate and other parameters, and then stabilize the waveform by selecting the trigger source, trigger edge, trigger level and so on. For an analog oscilloscope, as long as the signal is periodic, its frequency is within the range suitable for the corresponding oscilloscope to observe and it is not too complicated, through such steps, you can generally get a general understanding of the signal, and then make further observations as needed.

Many people may think that the normal mode is no different from the automatic mode in terms of observation effect. There is often such a situation that the waveform on the screen does not change when the trigger mode is switched between automatic and normal. However, this situation often only occurs when the observed signal is a relatively simple periodic signal. The role of the normal mode is to observe the details of the waveform, especially for relatively complex signals, such as video synchronization signals. Why do we say this? This is because in order to observe the details, we must increase the time base scan rate to expand the waveform. When we do this, the frequency of the observed signal will become lower relative to the oscilloscope scan rate, that is, the oscilloscope may scan many times between two triggers. In this case, if we choose the automatic mode at this time, the oscilloscope will actually perform all these scans, and the result is that the waveforms corresponding to these scans (which are not generated by triggering) are displayed together with the waveforms corresponding to the triggered scans, causing aliasing of the displayed waveforms, and thus the waveforms we want to see cannot be clearly displayed. If we choose the normal mode, the oscilloscope will not actually perform the scans between triggers, but only perform the scans generated by the triggers, so that only the waveforms we want to see related to the triggers are displayed, making the waveforms clearer. This is the function of the normal trigger mode. Figure 1 is an illustration of this situation. In Figure 1, the upper left side is the observed waveform, the lower side is the scanned waveform, and the right side is the waveform display. In Figure 1a, the scan rate is low, which makes it difficult to observe the details of the waveform; in Figure 1b, the scan rate is increased and the automatic trigger mode is used. At this time, the displayed waveform is unclear and has aliasing; in Figure 1c, the scan rate is the same as Figure 1b, but the normal trigger mode is used, and the scan is only performed when there is a trigger, so a clear waveform is displayed.

As mentioned above, the single-shot mode is basically difficult to use for ordinary analog oscilloscopes, but it is a very useful trigger mode for digital storage oscilloscopes. In digital storage oscilloscopes, the single-shot trigger mode can be used to capture signals that appear once or multiple times but are not very periodic. Although the normal mode can also capture single-shot signals, if the signal is multiple times rather than single-shot, the scan triggered by the signal that appears later in the normal mode will erase the previously captured result, so a stable waveform cannot be obtained. At this time, if the single-shot mode is used, there will be no such problem, that is, the trigger of the single-shot mode has the ability to select one from the multiple-occurring signals.