Discuss several trigger methods of oscilloscopes

Thread Content

Let’s first briefly review what oscilloscope triggering is. Since the signals are changing constantly, if they are all displayed on the oscilloscope at once, it will become very chaotic and it will be impossible to see clearly, thus making it impossible to observe the signals in order to solve the problem. Given that signals usually appear periodically according to a certain pattern, we only need to identify this repeating pattern and display each repetition on the oscilloscope; in this way, the signal can be observed steadily. Stabilizing the display of the signal is what triggers it, and this is also known as synchronous scanning. And finding the pattern of signal repetition is the process of selecting the triggering method. Next, let’s take a look at the common trigger methods used in oscilloscopes, and how they help us identify the patterns of signal repetition. The most common and widely used trigger method for oscilloscopes is edge triggering. Because most signals change periodically in an upward and downward manner. Edge triggering means that when the edge of the signal reaches a certain set trigger level and continues to rise or fall, the oscilloscope is triggered to display the signal at that moment. It can be seen that for edge triggering, the trigger point can be selected as the rising edge, falling edge, or both edges. Generally, the rising or falling edge is chosen, because with a dual-edge condition, both the rising and falling of the signal will trigger events, which often results in unstable fluctuations in the signal. Next, let’s take a look at pulse width triggering. As the name suggests, pulse width triggering means that when the pulse width of the signal reaches a certain condition, the oscilloscope is triggered. When triggered by a positive-polarity pulse, if the constraint is true, the trigger will occur at the transition of the pulse from high to low ； When triggered by a negative-polarity pulse, if the constraint is true, the trigger will occur at the transition of the pulse from low to high. As shown in the square wave signal in the figure above, the pulse width is approximately 500 μs based on the time base size; setting the condition that the pulse width be less than 515 μs will meet the requirements for a stable waveform. The set polarity is positive, so the trigger point is at the moment the pulse goes from high to low. Then let’s look at logical triggering. Logical triggering occurs when the levels between analog channels satisfy certain logical operations (AND, OR, NAND, NOR), and the signal voltage reaches the set trigger level along with the required trigger logic width. In the first diagram in the above image, the trigger occurs when CH1 is at a trigger level below 2.92V, and (AND) CH2 is at a trigger level above -320mV, regardless of the signal pulse width. The signal indicates that this condition is met, therefore the signal is stable. The second graph is the exact opposite: it is triggered when CH1 reaches a trigger level of above 2.92V, and (AND) CH2 reaches a trigger level of below -320mV, regardless of the signal pulse width. Since CHI and CH2 are clearly the same signal, it is also impossible for there to be a voltage value that is both greater than 2.92 V and less than -320 mV. It can be seen that at this point the signal does not meet the triggering conditions, and therefore it is unstable. Next, let’s look at N-edge triggering. This triggering method is easy to understand: it means that the trigger occurs at the Nth edge, after a specified idle time has passed, which is what is referred to as Nth-edge triggering. As shown in the figure above, the signal is triggered at the fifth rising edge. The next one is under-amplitude trigger. By setting high and low level thresholds, those pulses that exceed one threshold but not the other are triggered. Let’s look at the above two signal diagrams to help with understanding. The first pulse in the first graph crosses below the lower trigger level, but not above the upper trigger level; therefore, the condition is met, and triggering begins with the first pulse. In the second diagram, the first pulse not only exceeds the lower threshold of the trigger level but also exceeds the upper threshold of that level; therefore, it does not meet the conditions. The second pulse, on the other hand, meets the conditions, so triggering begins with this second pulse. In undershoot triggering, the greater than, less than, and not equal to conditions refer to pulse width; we did not set these in the diagram above. Then let’s look at slope triggering. Slope triggering refers to the occurrence of a trigger when the slope time for the signal to go from one level to another meets the specified time conditions. In the signal shown above, the slope time set for the rising edge is such that triggering occurs between 250 μs and 5 ms, with the starting point of triggering at the upper limit of the trigger level. The slope time of this signal occupies approximately one grid, which is around 4 ms; this meets the triggering conditions, allowing the waveform to remain stable. Timeout triggering is somewhat similar to slope triggering; it means that a trigger is generated when, starting from the point where the signal meets the trigger level, the duration of being above (or below) that trigger level reaches the set time. A positive polarity indicates that timing starts when the rising edge of the input signal reaches the trigger level, while a negative polarity indicates that timing starts when the falling edge of the input signal reaches the trigger level. In the signal shown in the figure above, it is set to trigger 9 ms after the signal’s rising edge passes the trigger level; it can be seen that the distance the signal shifts covers a little over 2 units, with a time base of 4 ms, which corresponds exactly to around 9 ms. Video triggering is a triggering method specifically designed for video signals, and it varies depending on the video standard; common standards include PAL/625, SECAM, NTSC/525, 720P, 1080I, and 1080P. Video triggering can be initiated at different voltage levels; the appropriate voltage level can be adjusted as needed to observe the waveform. To better observe the waveform details in video signals, the storage depth can be set to a higher value first. During the trigger debugging of video signals, since digital oscilloscopes have a multi-level grayscale display function, different brightness levels can reflect the frequencies of various parts of the signal. Experienced users can quickly assess the quality of the signal during debugging and detect any abnormalities.