“Compared with analog oscilloscopes, digital oscilloscopes have a very rich trigger function, and digital oscilloscopes have become a powerful tool for circuit debugging by virtue of its right-hand assistant-trigger function.
Abstract: Compared with analog oscilloscopes, digital oscilloscopes have very rich trigger functions. Digital oscilloscopes have become a powerful tool for circuit debugging by virtue of its right-hand helper-trigger function.
The meaning of trigger for oscilloscope:
• Capture the signal of interest;
• Determine the time zero point of the waveform.
1. Oscilloscope trigger principle
As shown in Figure 1, the signal passes through the probe and enters the oscilloscope. After passing through the attenuator amplifier, it will be divided into two channels. The function of the trigger time base module is to monitor the input signal and determine whether the trigger condition is met. The trigger time base module can control key actions such as the start and stop of the oscilloscope acquisition.
Figure 1 Trigger of 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, the waveform seen on the screen is unstable. Figure 2 shows several waveforms in the sine wave captured by the oscilloscope when the trigger conditions are not met. The acquisition of each waveform is random, so the waveform seen on the oscilloscope is unstable in rolling back and forth.
Figure 2 Unstable trigger
As shown in the following dynamic picture:
Figure 3 Unstable trigger dynamic diagram
Figure 4 is the waveform seen on the oscilloscope when there is a stable trigger. At this time, since each trigger point occurs at the position where the signal passes the trigger level, in most cases the trigger level uses a DC level as the reference. When the voltage of the signal exceeds the DC level, it is used as the starting point of the sampling waveform. . Therefore, multiple waveforms captured on the screen have the same time reference point, and the waveform appears to be stable.
Figure 4 Stable trigger
The dynamic changes are shown in Figure 5 below:
Figure 5 Stable trigger dynamic diagram
If an engineer who has used an analog oscilloscope before knows that its trigger mechanism is to scan and Display the signal after the trigger, so the waveforms observed on the screen are all after the trigger point. However, the trigger mechanism of a digital oscilloscope is that if the trigger conditions are not met, the oscilloscope will not wait for acquisition. It actually continuously acquires data in the full-speed buffer. As shown in Figure 6, because the storage depth of the oscilloscope is limited, the buffer will fill up quickly and recycle the old data with new data. This will continue until the trigger event occurs. Therefore, the internal storage of the oscilloscope is divided into two parts of data, one is the data before the trigger point (Pre-Trigger), and the other is the data after the trigger point (Post-Trigger).
Figure 6 Data before and after the trigger
Therefore, the waveform observed by the digital oscilloscope we use now can see the data before the trigger point and the data after the trigger point, and the trigger point can be set to adjust the ratio of the waveform before and after the trigger. This is the advantage of the digital oscilloscope. One, as shown in the figure, is a waveform acquired by a digital oscilloscope sampling edge trigger. The time of the trigger point is the zero time of the waveform collected on the oscilloscope. The time point before the trigger point is the negative time point, and the time point after the trigger point is the positive time point.
Figure 7 Waveforms before and after triggering
2. Oscilloscope trigger type
According to different applications, different trigger conditions are used to set the desired signal characteristics. Therefore, the ZDS5000/4000 oscilloscope is equipped with a very rich trigger condition setting as shown in Figure 8, which can adapt to different application requirements. Not only includes 13 basic triggers (edge, pulse width, slope, video, runt, over-amplitude, pattern, Nth edge, delay, timeout, setup hold, A->Bn, alternate trigger), but also supports 30 free More than one protocol trigger.
Digital oscilloscopes have a relatively large dead time, but ZDS series oscilloscopes are equipped with more than 40 trigger conditions. As long as the user can roughly estimate the signal characteristics that may be captured, they can set the corresponding trigger conditions to capture according to the signal characteristics. Greatly improve work efficiency.