The trigger event defines a point in time at which a duplicate "window" consisting of waveform information will stabilize for viewing. Imagine you are traveling by car. You must arrive at your destination in the least amount of time. However, you must also photograph certain landscape photos along the way. You know that you can get to your destination quickly, so your speed is very fast, but what strategies do you take to take pictures at the point of concern? One option is to take random pictures while driving and hope to capture the landscape image. Obviously, this depends too much on luck. The more logical approach is to tell the driver where to park to get a clear picture of the point of interest.
The waveform data in many oscilloscope applications is similar to what you don't care about at all. In high-speed debugging applications, the circuit may operate normally at 99.999% or (usually) higher time, and only 0.001% of the time causes the system to fail, or this part is the waveform you need to analyze in more detail. Oscilloscopes may have many key indicators (bandwidth, sample rate, record length) to complete this process quickly, but if you can't capture the data you care about, then the debugging and analysis tools will be very limited.
The triggering function of the oscilloscope synchronizes the horizontal scanning at the corresponding point of the signal, which is very important for clearly identifying the signal. The trigger control function stabilizes repeated waveforms and captures a single waveform. By repeatedly displaying the same part of the input signal, the trigger causes the repetitive waveform to appear statically on the oscilloscope display. Imagine if every scan starts from a different point in the signal, and the results on the screen will show up in chaos, as shown in Figure 1. Before the trigger sweep oscilloscope appears, the user must view this display before visually inspecting the waveform.
Figure 1. Oscilloscope display without trigger. Figure 2. Edge trigger menu.
Edge trigger
All modern oscilloscopes provide edge triggering, which is the original, most basic, and most common type of trigger. Edge triggering is usually sufficient for the user to see the general amplitude and timing characteristics of the waveform. Figure 2 shows the Pinpoint trigger system edge trigger setup window in the Tektronix DPO7000 and MSO/DPO/DSA70000 oscilloscope series.
Trigger source
It is almost always necessary to trigger an oscilloscope but it is not necessary to trigger on the displayed signal. Common sources for triggering the scan include:
â—†Signal into any input channel
â—† External sources other than the signal applied to the input channel
â—† "Power frequency" power signal
â—† Signals calculated within the wave device based on one or more input evaluation results
The oscilloscope can be set to trigger on the displayed channel most of the time. However, the instrument can trigger on any channel input, regardless of whether it is displayed; it can also be triggered from the source of the connection-specific trigger input. Most Tektronix oscilloscopes also provide a discrete output that provides a trigger signal for another instrument, such as a counter, a signal source, and so on.
Independent trigger level setting
Many electronic devices include various logic families that have different input voltage requirements, which in turn require a separate trigger threshold voltage for each logic family. In the past, the oscilloscope shared trigger level settings on all source channels. Each time a different channel is selected as the trigger source, the user has to change the threshold. The Pinpoint trigger system offers a choice: you can use a unique trigger level setting for each input source, or you can apply global settings to all channels.
Trigger level and slope
The trigger level and slope control function provides the definition of the basic trigger point and determines the waveform display mode, as shown in Figure 3. For edge triggering, the slope (positive or negative) and level can be selected and the oscilloscope will trigger acquisition when the signal meets these conditions, which is called the crossing threshold. The small arrow on the right side of the display indicates the Trigger Level (Figure 4a-4c). The arrow color corresponds to the selected trigger source channel color. The trigger level is typically set at 50% of the peak-to-peak voltage offset, but this is not a requirement.
Figure 3. Trigger Level and Slope.
Trigger position
The Horizontal Position knob on the front panel of the oscilloscope is used to locate where the trigger event is displayed on the screen. Changing the horizontal position can capture the signal behavior before the trigger event, called pre-trigger view. In this way, it can determine the length of the signal that can be viewed before and after the trigger point.
Digital oscilloscopes provide pre-trigger viewing because they always process the input signal, regardless of whether a trigger is received. A steady stream of data flows into the oscilloscope's memory; the trigger simply tells the oscilloscope to save data in memory when the trigger occurs. Pre-trigger viewing is an important debugging aid. If the problem occurs intermittently, you can trigger the system when a problem occurs, scroll through the records, and analyze the events that caused the problem. You can usually find the cause of the problem in the information before the trigger.
In FIG. 4, the trigger position is set to the fourth main horizontal ruled line, which corresponds to 40% of horizontal scanning. The trigger point can be anywhere from 0% to 100% of the record. At the 100% position, the entire record occurs before the trigger point, which maximizes the trigger preview capability. At 0%, the entire record occurs after the trigger, allowing maximum post-trigger viewing. If you need to get a full record after the triggering event, you can use delayed triggering. Delayed triggering is discussed later in this article.
Rising edge trigger and falling edge trigger
For many years, the triggering system has been providing both positive and negative slope settings (Figures 4a and 4b). The Pinpoint trigger system also allows triggering on positive and negative slopes (Figure 4c), which is typically used to see jitter on high-speed clocks and data signals. Figures 4a, 4b, and 4c respectively show the results when the trigger slope changes from a rising edge to a falling edge and then to two edges.
Figure 4c. Positive and negative trigger slopes.
Figure 4. The trigger position is 40%, represented by orange 'position'. The trigger level is 880 MV, indicated by a yellow arrow.
Rain Poncho Co., Ltd. , http://www.zjraincoat.com