How to select the trigger source of the oscilloscope and what is the role of the trigger

In "A Brief Discussion on Trigger Modes and Functions of Analog Oscilloscopes", we learned a lot about the trigger modes of analog oscilloscopes. In this article, we will learn about the trigger source of the oscilloscope and the role of the trigger.

What is a trigger

Triggering means that when using an oscilloscope, in order to synchronize the scanning signal with the measured signal, we can set some conditions and continuously compare the measured signal with these conditions. The scan 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. These conditions are trigger conditions. For example, ZHIYUAN Electronics ZDS2024Plus oscilloscope comes standard with 22 protocol triggers, and the trigger mode can be set according to my needs.

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 triggers, video triggers, and mains triggers. Digital oscilloscopes have more trigger conditions called advanced triggers, such as logic triggers, glitch triggers, and pulse width triggers.

The role of trigger

When a signal is introduced into an oscilloscope, if the display of the signal is not controlled accordingly, the display will be chaotic and different on each screen. When the oscilloscope is refreshed quickly, the signal we see is aliased, there is no stable image, and it is impossible to observe and measure. In order to solve this problem, we need to specify the trigger conditions of the oscilloscope to achieve stable synchronization and display the signal clearly.

About the origin of trigger mode

It is almost always necessary to trigger the oscilloscope, but not necessarily on the displayed signal. Common sources used to trigger a sweep include:

Signals entering any input channel

External sources other than the signal applied to the input channel

"Power frequency" power signal indication

A signal calculated internally by a filter based on the evaluation of one or more inputs.

Most of the time, the oscilloscope is set to trigger on the displayed channel. However, the instrument can trigger on any channel input, whether it is displayed or not, or from a source connected to a dedicated trigger input. Most Tektronix oscilloscopes also provide a discrete output to provide a trigger signal to another instrument, such as a counter, signal source, etc.

Trigger source selection

If a stable waveform is displayed on the screen, the measured signal itself or a trigger signal with a certain time relationship with the measured signal needs to be added to the trigger circuit. The trigger source selection determines where the signal is supplied. There are usually three trigger sources: internal trigger (INT), power trigger (LINT), and external trigger (EXT). Internal trigger uses the measured signal as the trigger signal and is a frequently used triggering method. Since the trigger signal itself is part of the measured signal, a very stable waveform can be displayed on the screen. In a dual-trace oscilloscope, either channel 1 or channel 2 can be selected as the trigger signal.

Power triggering uses the AC power frequency signal as the trigger signal. This method is effective when measuring signals related to the AC power frequency. It is especially effective in measuring low-level AC noise of audio circuits and thyristors. External triggering uses an external signal as the trigger signal, and the external signal is input from the trigger input terminal. There should be a periodic relationship between the external trigger signal and the measured signal. Since the measured signal is not used as a trigger signal, when to start scanning has nothing to do with the measured signal. Correctly selecting the trigger signal has a great relationship with the stability and clarity of the waveform display. For example, in the measurement of digital circuits, for a simple periodic signal, it may be better to choose an internal trigger, while for a signal with a certain period and a signal with a periodic relationship with it, it may be better to choose an external trigger.