To figure this out, easily capture the waveform you want to see

[Guide]”Trigger” is definitely the soul-level concept of a digital oscilloscope. If there is no suitable trigger condition, there is no way to talk about waveform observation. Although many engineers are familiar with the trigger function, most of them only know the details. How to understand the trigger in depth? This ZDS oscilloscope research and development note is shared with everyone.

When the oscilloscope is in use, it must first obtain a stable trigger waveform, so as to ensure the reliability of the subsequent measurement, decoding and other advanced functions. Now the trigger function of digital oscilloscopes is becoming more and more powerful, from conventional triggering to protocol triggering to template triggering, becoming more and more powerful. However, in the basic trigger settings, the role of some small details cannot be ignored. After flexible mastery, it is also of great benefit to using the oscilloscope. The following will analyze and exchange the trigger function, the trigger filter in the settings, the trigger sensitivity, and the holdoff time.

Principle of Oscilloscope Triggering

The trigger system and sampling system of the oscilloscope are important components of the oscilloscope. The sampling system is responsible for digitizing the analog signal, but the signal comes continuously, which part should be displayed on the interface of the oscilloscope?

If the oscilloscope does not have a trigger system, the sampled waveforms are superimposed at regular intervals or at a random time. Due to the uncertainty and irregularity of the sampling position, a very confusing waveform Display in Figure 1 will appear, which can be seen on the screen. It looks like a wave rolling back and forth.

Figure 1 Waveform sampling without trigger system

This chaotic phenomenon is consistent with the unstable trigger phenomenon on the oscilloscope. As shown in the dynamic picture below.

This is achieved by the trigger system. The principle of triggering is to constantly monitor the signal flow. If the signal meets the set trigger condition, the trigger records the signal that meets the condition and starts sampling; after the data collection is completed, the controller processes and Displays the signal. The details are shown in Figure 2.

Figure 2 Trigger process

A very key factor in the trigger condition of an oscilloscope is the trigger level. In most cases, the trigger level uses a DC level as the reference, and the moment when the signal voltage exceeds the DC level is used as the starting point of the sampling waveform. Since the position of the initial sampling is regular, the waveforms of multiple samples still look a stable waveform after being superimposed. As shown in Figure 3:

Figure 3 Stable trigger waveform sampling

The trigger function of the oscilloscope, on the one hand, can make the waveform stable without shaking the waveform from side to side; on the other hand, it can shorten the user’s debugging time, and only the signal that meets the trigger condition will be captured and displayed.

By dynamically adjusting the trigger level of the oscilloscope, you can observe the dynamic changes of the position where the waveform is stably triggered, as shown in the dynamic diagram below.

Trigger filter

In the commonly used settings, the trigger type and trigger voltage are generally set, and the waveform can be displayed stably. But for a signal with relatively large noise, the trigger will be unstable and the upper and lower edges can be triggered. This is because the existence of signal glitches interferes with the trigger system’s judgment of the trigger conditions and causes false triggers. At this time, you can select the operation of[Trigger Coupling]in[Trigger Settings]. Common couplings are as follows:

● DC coupling: that is, without processing, allowing DC and AC signals to enter the trigger path;

● AC coupling: it is a high-pass filter, the cut-off frequency is about 7Hz;

● Low frequency suppression: it is a high-pass filter, the cut-off frequency is about 50KHz;

● High frequency suppression: It is a low-pass filter, the cut-off frequency is about 50KHz.

The details are shown in Figure 4:

Figure 4 Various filter performance

Trigger coupling is actually a kind of low-pass or high-pass filtering of the trigger signal. Therefore, the “high frequency suppression” coupling can be added to the noisy signal to filter out the high frequency part, so that the waveform trigger is stable as shown in Figure 5.

Figure 5 CH1 does not turn on high frequency suppression and the trigger is unstable, CH2 turns on high frequency suppression

Trigger holdoff

In the trigger setting, the function of triggering holdoff is generally ignored. By definition, holdoff is to define the minimum time interval between two triggers.

When the oscilloscope triggers once, it will enter the trigger holdoff time count, during which the trigger function will be inhibited, even if the signal meets the trigger conditions, the system will not mark the trigger point. The holdoff setting is very useful for capturing occasional polygonal edge signals, so that the original unstable waveform of the image is immediately clear. If the trigger holdoff time is set incorrectly, the oscilloscope will overlap the signals of different edges as the trigger points, causing abnormal waveform display, as shown in Figure 6. The setting of the holdoff time is shown in Figure 7.

Figure 6 Improper setting of holdoff time causes abnormal waveform display

Figure 7 The holdoff time should be between Tmax and Tmin

The role of trigger sensitivity

The trigger level is just a reference voltage, and the actual waveform has jitter at the edge. As shown in Figure 8, the waveform has very little interference, but the rising edge is still jagged. When the noise is large, the jitter will be More intense. If you want to stabilize the rising edge of the trigger waveform, you need to use hysteresis comparison within the upper and lower ranges of the trigger level to filter the waveform jitter and glitch near the trigger level. This hysteresis range is the trigger sensitivity.

Figure 8 Jitter and glitch of the trigger edge

The sensitivity of trigger signal recognition is shown in Figure 9. When measuring small signals, a higher trigger sensitivity is needed to make the signal trigger stably. This can be done by adjusting the value of trigger sensitivity to a small value or zero; in the case of waveform noise When it is larger, the trigger sensitivity needs to be adjusted appropriately, which can effectively filter out the noise that may be superimposed on the trigger signal, thereby preventing false triggering, as shown in Figure 10.

Figure 9 Hysteresis comparator

Figure 10 The effect of sensitivity on signal triggering

When using 2000/3000/4000/5000 series oscilloscopes to measure signals with smaller amplitudes, the oscilloscope defaults to 0.3div. When the oscilloscope is in the range of 2mv/div~5mv/div, the trigger sensitivity defaults to 1.0div because of small signals. The interference is large, which can play a role in anti-interference, and in the range of 10mv/div~10v/div, the default trigger sensitivity is 0.3div, so manual adjustment is required when measuring small signals using the hour-based gear, which will trigger The sensitivity value is adjusted down to between 0~0.3div.

Summarize

Triggering is a very purposeful operation, that is to say, it is necessary to know that the signal is abnormal before setting the corresponding trigger condition. How to find abnormalities quickly, this should be a prerequisite for setting a reasonable trigger.

ZDS5000 series oscilloscopes are based on 512Mpts storage depth, support 24 kinds of measurement parameters on the same screen, combined with template trigger, abnormal search, annotation and double ZOOM analysis plug-ins can quickly locate the waveform we are interested in, and can easily be in the continuous data flow Find out abnormal conditions such as runt, glitch, waveform distortion and so on. In addition to 13 conventional types of triggers, ZDS5000 also supports 30 types of protocol triggering and decoding, so that debugging no longer stays in the “counting pulse” stage, which greatly improves work efficiency.

Source: ZLG Zhiyuan Instruments