Explanation of several triggering modes of oscilloscope

What is oscilloscope triggering? Since signals are changing all the time, if we display them all on the oscilloscope, it will be very messy and we can't see clearly at all, so we can't observe the signal to solve the problem. Considering that signals appear periodically in a certain pattern most of the time, we only need to find the repetitive pattern and display each repetition on the oscilloscope, so that the signal can be observed stably.

This kind of stable display of the signal is triggering, also called synchronous scanning. And finding the pattern of signal repetition is the process of selecting the trigger mode. Let's take a look at the common trigger modes of oscilloscopes and how they help us find the pattern of signal repetition.

The most common and most frequently used triggering mode of an oscilloscope is edge triggering. Because most signals change periodically with rising and falling. Edge triggering means that when the edge of a signal reaches a certain set trigger level and continues to rise or fall, the oscilloscope triggers and displays the signal at this time.

It can be seen that the edge trigger can choose the trigger point as rising edge, falling edge or dual edge. In general, the rising or falling edge is selected, because in the case of dual edge, both rising and falling signals will be triggered, which often causes the signal to shake left and right and unstable.

Next, let's look at pulse width triggering. As the name implies, pulse width triggering means that the oscilloscope will trigger when the pulse width of the set signal reaches a certain condition. When triggering on a positive pulse, if the restriction condition is true, the trigger will occur when the pulse flips from high to low; when triggering on a negative pulse, if the restriction condition is true, the trigger will occur when the pulse flips from low to high.

As shown in the square wave signal in the figure above, according to the time base size, the pulse width is about 500μs. The setting condition is that the pulse width is less than 515μs to meet the restriction condition of stable waveform. The polarity is set to positive, so the trigger position is when the pulse goes from high to bottom.

Then let's look at logic triggering. Logic triggering is when the level between analog channels meets certain logic operation (AND, OR, NAND, NOR) results and the signal voltage reaches the set trigger level and trigger logic width.

In the first figure above, when CH1 is below the trigger level of 2.92V and (AND) CH2 is above the trigger level of -320mV, it is triggered, regardless of the signal pulse width. It can be seen from the signal that this condition is met, so the signal is stable.

The second figure is just the opposite. It triggers when CH1 is higher than 2.92V and (AND) CH2 is lower than -320mV, regardless of the signal pulse width. Since CHI and CH2 are obviously the same signal, it is impossible for a voltage value to be both greater than 2.92V and less than -320mV. It can be seen that the signal does not meet the trigger condition at this time and is unstable.

Next, let's look at N edge triggering. This triggering mode is relatively easy to understand. It means that when the trigger signal is triggered on the Nth edge after the specified idle time, it is called Nth edge triggering. As shown in the figure above, the signal is triggered on the fifth rising edge.

The next one is the runt trigger. By setting the high and low level thresholds, trigger on pulses that cross one level threshold but not the other. Let's look at the two signal diagrams above to help understand.

The first pulse in the first figure crosses the lower limit of the trigger level, but does not cross the upper limit of the trigger level, so the condition is met and the trigger starts from the first pulse. In the second figure, the first pulse not only crosses the lower limit of the trigger level, but also crosses the upper limit of the trigger level, so the condition is not met, while the second pulse meets the condition, so the trigger starts from the second pulse.

The greater than, less than, and not equal to conditions in the runt trigger refer to the pulse width, which we did not set in the above figure.

Then let's look at slope triggering. Slope triggering means that when the slope time of a signal from one level to another meets the set time condition, a trigger is generated. As shown in the signal in the figure above, the slope time set to the rising edge meets the trigger time of 250μs to 5ms, and the trigger start point is on the upper limit of the trigger level. The slope time of this signal occupies about one grid, which is about 4ms, which meets the trigger condition, so the waveform can be stable.

Timeout trigger is similar to slope trigger, which means that the trigger is generated when the signal crosses the trigger level and the duration above (or below) the trigger level reaches the set time. Positive polarity means that the rising edge of the input signal passes through the trigger level to start timing, and negative polarity means that the falling edge of the input signal passes through the trigger level to start timing.

As shown in the signal in the figure above, it is set to trigger 9ms after the rising edge of the signal passes through the trigger level. It can be seen that the signal displacement distance occupies a little more than 2 grids, and the time base is 4ms, which is exactly about 9ms.

Video trigger is a trigger mode specifically for video signals, which varies according to different video formats, generally including PAL/625, SECAM, NTSC/525, 720P, 1080I and 1080P, etc. Video trigger can be triggered at different voltage levels, and the appropriate voltage level can be adjusted as needed to observe the waveform.

In order to better observe the waveform details in the video signal, you can first set the storage depth to a larger value.

During the trigger debugging of the video signal, since the digital oscilloscope has a multi-level grayscale display function, different brightness can reflect the frequency of different parts of the signal. Experienced users can quickly judge the quality of the signal and find abnormal conditions during the debugging process.