How to use an oscilloscope for trigger debugging

The oscilloscope is a commonly used electronic measuring instrument, which is widely used in the fields of industry, chemical industry, electronics, electricity, medicine, military, etc. When we use the oscilloscope, have we learned about the trigger debugging of the oscilloscope? What is the trigger debugging of the oscilloscope? What is the method of trigger debugging of the oscilloscope? Let me introduce it to you in detail.

The oscilloscope is a basic instrument for electrical engineers, but I often find that some engineers cannot use its triggering function effectively. Triggering is often considered to be very complicated, and there is a tendency to go directly to the lab and ask the experts to help set up the trigger if there are any problems. The purpose of this article is to help engineers understand the basic principles of triggering and strategies for using triggering effectively.

What is a trigger?

Any oscilloscope has limited memory, so all oscilloscopes must use triggers. A trigger is an event of interest to the user that the oscilloscope should detect. In other words, it is something the user wants to find in the waveform. A trigger can be an event (i.e., a topic in the waveform), but not all triggers are events. Examples of triggers include edge triggers, glitch signal triggers, and digital pattern triggers.

The reason why oscilloscopes must use triggers is that their memory is limited. For example, the Agilent 90000 Series oscilloscopes have a memory depth of 2 billion samples. However, even with such a large memory, the oscilloscope still needs some events to distinguish which of the 2 billion samples need to be displayed to the user. Although 2 billion samples may sound like a lot, it is still not enough to ensure that the oscilloscope memory can capture the event of interest.

The memory of an oscilloscope can be thought of as a conveyor belt. Whenever a new sample is taken, it is stored in memory. When memory is full, the oldest samples are deleted to allow the newest samples to be stored. When a trigger event occurs, the oscilloscope captures enough samples to store the trigger event in the required location in memory (usually in the middle), and then displays this data to the user.

Repeated sampling mode and single sampling mode

Historically, the most common mode in which an oscilloscope operates is in repetitive mode. This means that once the oscilloscope triggers and displays data to the user, it will immediately begin searching for the next trigger event. This is why oscilloscope waveforms update so frequently.

Any oscilloscope needs time to re-arm the trigger in order to trigger and display data to the user. This time is also called "hang time". During the hang time, the oscilloscope cannot capture any waveform. Therefore, the shorter the hang time, the fewer events will be missed. For example, if a glitch signal happens to appear during the hang time, it will not be displayed on the oscilloscope's display. If this glitch signal is a rare event, the user may think that there is no glitch signal in the waveform, but in fact it does exist. Therefore, the shorter the hang time of the oscilloscope, the lower the probability of missing important events in the waveform.

Another way to describe this concept is the “update rate”, which is the number of waveforms per second. For example, the Agilent 7000 Series oscilloscope has an update rate of 100,000 waveforms per second.

The single sampling mode is used to find a single trigger without continuing to acquire more waveforms. Therefore, when the user wants to find an event, check the cause of the event and the problems that occurred after the event, the single sampling mode can be used. This mode is especially important for analyzing waveforms that are not repeated and change with each operation.

Automatic mode and trigger mode

What happens if no trigger event occurs? This is a great topic. In this case, the waveform on the screen will not update. This is not what we want, because the user may not know how to change the trigger to get the waveform on the screen. For example, if the probe slips off, the oscilloscope may stop triggering. However, if the screen cannot be updated, the loss of signal will not be obvious.

To solve this problem, oscilloscopes have a mode called "Auto" triggering. In this mode, if a trigger cannot be found within a period of time, the oscilloscope will automatically trigger to update the screen. Usually, there is some indicator on the oscilloscope (such as an LED on the front panel) to indicate whether the last trigger was a real trigger or an automatic trigger. In this way, if the user sees the "Auto" indicator, they will know that the trigger they set did not occur. For example, if the user set the trigger to be a glitch signal, they will know that the oscilloscope did not detect the glitch signal.

However, as you will recall from the previous paragraph, when auto-triggering occurs, it means that after each trigger, there is a hang time while the oscilloscope re-arms. To avoid this time completely, the oscilloscope should be changed to "triggered" mode. (This is called "normal" mode on some oscilloscopes.) In "triggered" mode, the oscilloscope will not trigger unless a trigger event is found.