What is an oscilloscope’s trigger mode? How to use an oscilloscope well and effectively?

What is an oscilloscope’s trigger mode?   

The "trigger" of the oscilloscope is to synchronize the oscilloscope's scan with the signal being observed, thereby displaying a stable waveform. To meet different observation needs, different "trigger modes" are required. There are three basic triggering modes of the oscilloscope: The first is "automatic mode (AUTO)": In this mode, when the trigger does not occur, the oscilloscope's scanning system will automatically scan according to the set scan rate; and when there is 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 conditions are met or not, the oscilloscope will scan, and you can see changing scan lines on the screen. This is Characteristics of the pattern.  

The second is "normal mode/regular mode (NORM)": This mode is different from the automatic mode. In this mode, the oscilloscope will only scan when the trigger condition is met. If there is no trigger, it will not scan. Therefore, if there is no trigger in this mode, you will not see the scan line and nothing on the screen for the analog oscilloscope, and you will not see the waveform update for the digital oscilloscope. If you don’t understand this, you will often think that the signal is not connected or What other faults. The third is "Single mode (SINGLE)": This mode is somewhat similar to "normal mode", that is, scanning will only occur when the trigger conditions are met, otherwise no scanning will occur. The difference is that once this kind of scan is generated and completed, the oscilloscope's scanning system enters a resting state. Even if a signal that meets the trigger condition appears later, it will no longer scan. That is, it will only scan once after triggering. , that is, once, 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. In most cases, this mode is of little use. The above three trigger modes are provided by most oscilloscopes.  

How to choose and use it in practice?

In actual use, the selection of different trigger modes must be judged based on the characteristics of the signal being observed and the content to be observed. There are no fixed rules, but it is often an interactive process, that is, understanding the characteristics of the signal by selecting different trigger modes. Characteristics, and select an effective trigger mode based on the characteristics of the signal and what you want to observe. The most important thing in this process is to understand the working mechanisms of different trigger modes, understand the characteristics of the observed signal, and clarify what is to be observed. 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 That's fine, rather than nothing. Once there are scan lines, you can "find" the waveform by adjusting parameters such as vertical gain, vertical position, and time base rate, and then stabilize the waveform by selecting the trigger source, trigger edge, trigger level, etc. For analog oscilloscopes, as long as the signal is periodic, its frequency is within a range suitable for observation by the corresponding oscilloscope, and it is not too complex, a general understanding of the signal can generally be achieved through such steps, and further observations can be made as needed. . For the normal mode, many friends may feel that there is no difference in the observation effect from the automatic mode. It is often the case that when the trigger mode is switched between automatic and normal, the screen waveform does not change. However, this situation is often only Occurs when the observed signal is some relatively simple periodic signal.  

The function of the normal mode is to observe the details of the waveform, especially for more complex signals, such as video synchronization signals. Why do you say this way? This is because in order to observe details, we must increase the time base scan rate to expand the waveform. When we do this, the frequency of the observed signal becomes lower relative to the oscilloscope scan rate, which means that the oscilloscope may scan many times between triggers. In this case, if we select the automatic mode at this time, the oscilloscope will actually perform all these sweeps, and the result is that the waveforms corresponding to these sweeps (which are not generated by the trigger) will be together with the waveforms corresponding to the trigger sweeps. display, resulting in aliasing of displayed waveforms, so the waveform we want to see cannot be clearly displayed. And if we choose the normal mode, the oscilloscope will not actually perform these scans between triggers. It will only perform the scans generated by the trigger, thus only displaying the waveforms associated with the trigger that we want to see, so that The waveform will be clearer, which is the function of normal trigger mode. In Figure 1a, the scan rate is low, making 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; the scan rate in Figure 1c is different from that in Figure 1c. 1b is the same, but uses the normal triggering method and scans only when there is a trigger, thus displaying a clear waveform.  

Above, we have briefly described the basic triggering modes of oscilloscopes and their considerations in practical use, hoping to help beginners master the oscilloscope. In addition to what is discussed in this article, the adjustment of other parameters of the oscilloscope is also very important. On the one hand, the user must have a clear understanding of the meaning of various parameter adjustments. On the other hand, it is also necessary to understand the characteristics of the observed signal and clarify the observation target, so that the oscilloscope can be used effectively to achieve the purpose of measurement and testing.  

 How to use an oscilloscope well and effectively?  
  For power engineers, once there is a problem with a product, they need to capture waveforms, capture timing, and test accurate values ​​to help engineers analyze and deal with it. Speak with facts and look at waveforms. How to make the test data accurate and reliable is very important. Accurate numbers can help us, but distorted waveforms and values ​​can only mislead us, make us go in the opposite direction, make us lose direction, and do a lot of useless work. Thinking about it carefully, although I am not that proficient in oscilloscope research, I have read many articles about oscilloscopes. In practice, I encountered many problems and solved many problems. I still have some experience along the way that I can share with you. Yes, I hope it can be helpful to everyone. If the writing is not good, please forgive me. 

I often see that the oscilloscopes used by many small companies are too low-end, with low bandwidth and low sampling rate. They think it is enough to capture the waveform, and there is no need to buy such a good oscilloscope. They also think that the operation of the oscilloscope is simple and there are not so many specifications. I saw that they were operating the oscilloscope without making preparations before testing, and just picked it up and used it. In fact, that is incorrect. It may often be that incorrect operation leads to distortion of the test results and affects the analysis. Even some very experienced engineers may not notice some details. Many engineers lack understanding of oscilloscopes, and how to better use oscilloscopes needs to be improved. Below I will correct the common problems that many engineers I see make and share some of the knowledge I have.  

1. Many engineers just pick up the probe and test it without checking whether the probe needs compensation or whether the oscilloscope needs calibration. Only in some large companies or trained engineers will do the preparation work before use. Before use, the oscilloscope requires self-calibration and probe compensation adjustment. This adjustment is performed to make the probe match the input channel. When operating the instrument for the first time and when displaying data from multiple input channels simultaneously, it may be necessary to calibrate the data vertically and horizontally to synchronize time base, amplitude, and position. For example, significant temperature changes (>5°) require calibration.  

Disconnect any probes or cables from the channel input connectors. Make sure the instrument is running and warmed up for a while. R File menu, select Selfalignment.  

On the Control tab, click Start Alignment.  

R alignment state field. The results of each calibration step for each input channel are displayed in the Results tab.

The steps for probe compensation adjustment are as follows: 1. Connect the oscilloscope probe to the channel and press the PRESET button on the front panel (in the setting area of ​​the left panel). Connect the probe signal end and reference ground to the reference output on the oscilloscope panel and press Autoset. If using the probe hook tip attachment, securely attach the signal pin tip to the probe to ensure proper connection. As shown in Figure 1:    

Group Picture 1 Probe Compensation Adjustment 2. Check the shape of the displayed waveform. Possible situations are shown in Figure 2.    

Figure 2 Overcompensation, undercompensation and correct compensation Over and undercompensation require adjustment of the probe. To better test the accurate value.  

3. If the waveform is incorrect, please adjust the probe. As shown in Figure 3 below, until the waveform becomes the correct compensated waveform above.