For any instrument for collecting signals and digital processing, triggering is a very important feature. If you cannot trigger according to a specific waveform feature, you may never see the points of interest in the digitized waveform. Digitalizers can be collected in a variety of modes. The ring buffer or normal mode is working like a digital oscilloscope. The acquired data is loaded into the annular buffer. When triggering occurs, the data allows the post trigger delay is locked for display and processing. The digitizer also supports stream acquisition mode, digitally acquired, digitized, and continuously stores waveforms in this mode. Therefore, the trigger does not indicate the starting position of the waveform, but a time point that occurs in a particular feature. No matter which mode, you can see what happened before and after the incident.
A common trigger method is an input signal for a channel with a digitizer. The basic principle is to detect a defined point on the waveform, and the "trigger event" is labeled as a known location on the acquisition data. Figure 1 provides an example of a substantially edge trigger. The signal source is an input channel, and the triggering event occurs when the waveform rising edge crossing the 500mV trigger level. When the trigger event occurs, the position on the signal is labeled as the zero time point on the timeline, as shown in the cursor position in the figure. If the signal is repeated, the digitizer will trigger the same point when collecting new data each time, thereby achieving stable display.
Figure 1: Basic edge trigger, defines the zero time point on the timeline (with a vertical dashed line mark), and the triggering event occurs when the waveform rises over the trigger level (horizontal dashed line).
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Wide range of signal waveforms, horizontal and timing requirements require that the trigger circuit of the digitizer is very flexible. Figure 2 shows the trigger "engine" block diagram of the SPECTRUM M4i series digitizer. It provides an example of a wide range trigger condition for modern digitizers.
Figure 2: The Spectrum M4i digitizer box illustrates the "trigger engine" of these general digitors, triggering and trigger logic.
The left side of the block diagram shows the hardware trigger source of the digitizer, which includes one of the input channels and two external trigger inputs (EXT0 or EXT1). Each source can support a variety of trigger types. Because many digitizer modules have multiple-purpose I / O lines, you can use them to run / loaded with them, which can provide trigger output signals and other functions. In addition to hardware triggerings, you can also use software triggers in program control, which is useful for development automation testing.
The digitizer may contain powerful trigger and / or logical units that can help you combine input from multiple sources into a complex multi-feature trigger. Such triggers ensure that the digitizer is only triggered when the specifically defined pattern occurs. Another function is to cross-trigger with other digitizers.
Trigger mode
The main trigger source has a dual trigger level comparator that supports a variety of trigger modes. These trigger sources include unilateral edges and bilateral triggers, reload (hysteresis) flip flops, window triggers, and related trigger strip signal generators for multi-source triggers.
The edge trigger is a basic trigger type. You only need to set the trigger level and select the desired trigger edge. The digitizer will mark the time point of the selected edge of the trigger source over the trigger threshold. The edge selection can be a rising edge, a falling edge or two edges.
Reload or lag the trigger set two levels, the load level, the second trigger level. As with the trigger along the edge, the user should choose the edge type. First, the signal selected by the signal must be overloaded, and the trigger is loaded. Then, when the same edge of the signal is again, the digitizer is again triggered. Re-load trigger mode can be used to prevent the digitizer from being triggered by the error edge of the noise signal.
Window triggers define amplitude windows using two trigger thresholds of each trigger source. Window triggers have two working modes: enter the window trigger and exit the window trigger. Getting to window trigger refers to triggering only one of the only source signals over the threshold level and enters the window. Exit window trigger is that the source signal is already between two trigger thresholds, and then triggers when leaving the window. The source signal can be used to use a window trigger when changing the state in any direction.
When multi-source trigger mode with digital trigger logic is used, you typically need to use a channel to create a selective waveform for supporting trigger from another channel. You can use high level, low, window or window selection to achieve this. The internal strip signals generated by these trigger modes can be used with the second trigger source and passed with a logical strip trigger.
Figure 3 shows an example of a trigger source on another channel using a high level trigger. As long as the sine wave on the CH0 channel exceeds the trunking level, a rising strobe signal is generated for the signal at the signal over the threshold. This strobe signal is to be operated with the signal on the CH1 channel. Since the strobe signal is only rising when the low amplitude pulse occurs on the CH1, the digitizer will trigger when the pulse waveform passes through the triggerion, and the horizontal red dotted line in the figure.
Figure 3: Creating a strobe signal using a high level trigger on CH0 for selecting a lower amplitude of the two pulses on the CH1 channel. As long as the trigger source is located above the trigger level (trigger level 0), the high level trigger creates a selection signal of a rising state. This strobe signal is coupled to the pulse waveform of the CH1, and the digitizer can be triggered when encountering a lower amplitude pulse.
Trigger logic
The example shown in Figure 3 shows a use of trigger logic when encountering a plurality of trigger sources. It is supported with (and) logic and or (or) logic. Or entries for functions include any channel, external trigger input, software trigger, and forced triggering. Logic or functions allow any one of these trigger sources to trigger the digitizer. The input to the logical function includes all channels, external trigger inputs, and enable trigger functions. With the function requires all selected trigger inputs to be effectively activated to start the trigger. Remember, the Try trigger mode (high level and low) provides logic logic to taking the reverse function, so you can create a non-NAND and or non-(NOR).
Figure 4 shows an example of a wireless positioning application using or triggered logic. Each input channel is connected to a sensor. The direction of the source depends on the arrival time of each sensor.
Figure 4: You can use or trigger logic to trigger the channel that occurs the radio frequency pulse.
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The location of the source determines which channel is first seen. Or trigger logic allows a pulse to trigger a digitizer to ensure that both sensor outputs are collected.
Other trigger related functions
There are other two kinds of triggering functions worth mentioning. One is a unit that triggers delay, that is, the trigger block diagram of Figure 2. This feature uses a counter that allows you to trigger the number of samples after the event is delayed. If the delay changes from the default (0) value, the trigger point on the horizontal axis changes from 0 to the input delay value.
The second function is that the external trigger output and trigger status line. These features are useful when synchronizing multiple instruments. Trigger output, load and running status can be obtained by multi-purpose I / O channel, as shown in Figure 2.
Synchronize
In theory, two problems occur in synchronizing multiple instruments. The first is to arrange public triggers. Second, let two instruments work based on synchronous clocks. We can easily see that there may be problems when trying to synchronize multiple digitizers.
Synchronization of the clock can be realized using an external clock having a target clock rate. The second method is to provide an external reference of 10 MHz, and then applied to the phase-locked loop (PLL), and the reference clock frequency is multiplied to the desired clock rate. Many digitors such as the SPECTRUM M4i series used herein have two external clocks by using public external clock inputs. The external clock input is connected to the internal PLL, you can set it to a multiplier basis, or phase the digitizer to the external clock, and do not change the frequency directly. This ensures that the clock frequency is correct, but the clock in each digitizer cannot be guaranteed to have the same phase.
On the trigger side of the synchronous process, you must take into account the external trigger input of each digitizer to use a separate comparator to detect triggers. A little different from the reference level and the difference in establishment and holding time may result in discrete changes in the trigger point position on the timeline, thereby forming some kind of trigger jitter. Methods to ensure that multiple digitizer synchronization is to assign clocks to each module and then synchronize the trigger event to the system clock.
Synchronize multiple digitizers
The SPECTRUM M4I series digitors used in the above examples have an optional synchronization accessory called a star-shaped concentrator. The STAR HUB. This STAR-HUB module supports more than 8 synchronization of the same series of cards.
This module is used as a star-shaped clock and trigger signal concentrator. The digitizer with this module is used as a clock main device. This card or any other card can be a primary flip-flop. If you use the Star-Hub module, all trigger modes available on the primary card are still available. It can also extend and / or trigger logic to accommodate input from any of the connected digitors. By synchronizing ARM signal from the digitizer, Star-Hub can also synchronize different pre-transumers, memory segments, and post trigger settings in multiple digitizers.
Summary
The digitizer requires the trigger to associate signal acquisition to a known point. A variety of trigger sources and patterns make it easy to select the trigger point you want. In addition, the capabilities of the STAR-HUB synchronous time base are supported by multiple instruments to provide a large number of acquisition channels. A digitizer with a smart trigger engine can help you trigger and collect a wide range of complex signals.
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Source: Wiku Electronic Market Network
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