Electronic Design
Lifting The Lid On Oscilloscope Triggering

Lifting The Lid On Oscilloscope Triggering

Triggers have been an essential feature of oscilloscopes for a very long time. But new triggers pop up all the time and are often a critical in the buying decision. In this engineering essentials article, the author explains the variety of triggers found on modern scopes as well as delving into advanced (smart) triggers. 

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A trigger is defined as “anything, as an act or event, which serves as a stimulus and initiates or precipitates a reaction or series of reactions.” The same definition applies to an oscilloscope trigger. An oscilloscope trigger involves waiting for an event to occur, triggering upon occurrence of the event, and then the oscilloscope capturing and displaying the electrical signaling (data) that follows the trigger event.  For more complex events, it’s important that an oscilloscope incorporate advanced triggering capabilities.  Overall, oscilloscope triggers have become important to the point where they’re often the deciding factor when purchasing an oscilloscope.

Table of Contents

The Importance Of Triggering

“Banner” specifications such as bandwidth, memory depth, and sample rate typically define oscilloscopes. However, advances have pushed real-time oscilloscope banner specifications far beyond most technology needs. As a result, banner specifications tend to be qualifiers when evaluating an oscilloscope, not the deciding factors. 

Oscilloscope users no longer have to settle for oscilloscopes that only trigger on an edge, because they can find scopes with advanced triggering in both hardware and software. Thanks to modern advances in triggering technology, a good triggering system will save hours of time trying to isolate unique problems in a design and, more critically, allow designers to quickly improve their designs. The potential time savings made possible with a good trigger underscores the importance of fully understanding the ins and outs of the triggering system, and the available triggers.

The Edge Trigger

At the heart of every basic, advanced, and software trigger is the edge trigger—the key building block of all triggers.  It’s conceptually the easiest trigger to understand.  The edge trigger comes in three forms: rising, falling, or rising and falling.  It resides at core of the triggering system, providing a window into the workings of a triggering system.

Oscilloscope designs include a front end and back end. The front end typically contains the preamplifier, the edge-trigger chip, attenuators, and the analog-to-digital converter (ADC) (Fig. 1).  The edge trigger is the key component of the entire system. Notice that the signal path goes through both the edge trigger and the preamplifier.  Inside the preamplifier, the data is ignored until the edge trigger confirms that the data is available and should be digitized. 

1. The edge trigger, located in the front end of an oscilloscope, is the key component of the entire system.
1. The edge trigger, located in the front end of an oscilloscope, is the key component of the entire system.

When observing the basic hardware functionality of an edge trigger, it can be categorized as a simple comparator that looks to see if the input signal crosses the entire threshold level (Fig. 2).  If the signal crosses the threshold, the preamplifier is told to process the data and the signal is digitized.

2. An event triggers when the input signal crosses the entire threshold level. The preamplifier then processes the data, and the signal is digitized.
2. An event triggers when the input signal crosses the entire threshold level. The preamplifier then processes the data, and the signal is digitized.

Edge-Trigger Limitations

Of all the specifications, perhaps edge-trigger bandwidth is the least understood. For less expensive oscilloscopes, vendors don’t even specify edge-trigger bandwidth.

The edge-trigger chip’s comparators will only work to a certain frequency. If the signal frequency exceeds the edge-trigger bandwidth, the edge trigger will not trigger, even though the signal passed both thresholds. That’s due to the timing between the comparators—if the signal passes too fast, the edge trigger will know that a signal crossed one threshold, but will not be fast enough to know if the second one was crossed. 

Thus, trigger sensitivity is specified to accelerate the edge-trigger bandwidth. Essentially, trigger sensitivity specifies how large the signal needs to be at certain frequencies in order for a chip to trigger. The larger the signal, the easier it is for the edge trigger to work and, hence, the faster the trigger can function.  Say, at 1 GHz, the amplitude must be larger than 1 MHz due to the speed of the signal (see the table).

It seems there’s always something more to the story for every specification—and the edge trigger is no exception. For instance, how can a 60-GHz oscilloscope trigger on a 60-GHz sine wave when the edge trigger is less than 60 GHz? 

A couple options for triggering (auto and triggered) exist inside the scope. The edge trigger is the key qualifier for the “triggered” system to work. For “auto,” though, there’s no need for the edge trigger.  The “auto” triggered mode ignores the edge trigger and lets all signals pass, whether or not they’re qualified. It then uses software to align the edges. 

For example, in the case of a 60-GHz sine wave, the edge trigger would fail and not trigger. Therefore, the oscilloscope would never enter “triggered” mode. But, if the user understood this limitation and changed the oscilloscope to “auto,” the oscilloscope would digitize the 60-GHz sine wave through software triggering.

Advanced (Smart) Triggers

Basically, the edge trigger’s main function is to look for an edge. However, it also keys more advanced triggers. When the edge trigger sees the edge, it “triggers” another block of the oscilloscope—known as the “smart” trigger—to begin its work. The smart triggering system can consist of a number of different triggers, including runt, pulse width, window, setup and hold, etc. 

In many cases, the smart trigger holds the key to simplifying the debugging of a designer’s board-level problems. It can be used to find events that occur very infrequently. One misconception is that because the smart trigger is done in hardware, it provides zero dead time.  Rather, the smart trigger looks at a data set in hardware, scanning for the anomalous event. If the event occurs in the data that it scans, then it triggers. If it doesn’t trigger, then it waits for the edge trigger to indicate that more data is ready to be scanned.

Runt Triggering

Runt pulses are pulses that don’t reach a valid high or low level. They cause designs to fail and will appear for a variety of reasons.With the handy runt trigger, designers can quickly find runt pulses and evaluate what caused them.

Oscilloscopes are able to trigger on these pulses by setting two threshold levels and looking for signals that only cross one of them.  Once there’s a successful trigger on the runt pulse, the scopes can pan and zoom to locate the problem area.

Setup-And-Hold Triggering

Setup and hold is a critical specification for many of today’s applications. Setup-and-hold violations can destabilize a system. This instability can cause an IC to oscillate between a high and low state, which introduces inaccuracies into the system.

Setup-and-hold triggering allows users to quickly scan a system for violations. Users simply need to set the instability-inducing specification and then look for the trigger. If the oscilloscope finds a violation of the specified setup or hold time, it will then trigger.  

Subsequently, the oscilloscope can be used to find what preceded the failure by looking at typical and atypical failure mechanisms, such as slew rate or asynchronous behavior.  If the scope is unable to trigger, then it becomes apparent that the system isn’t failing due to setup or hold violations. This debug analysis capability can be invaluable for both analysis and debugging system problems.

Pulse-Width Trigger

The pulse-width trigger, another smart trigger, looks for pulses that are smaller or larger than a certain width. As a result, it’s sometimes referred to as the time-qualifying trigger.  The pulse-width trigger can work on positive or negative pulses. Typically, when users trigger with the pulse-width trigger, they’re looking for pulses that are faster than the reference clock. If this event occurs, it could mean a significant design problem. Pulse-width triggering offers fast and simple debugging in this case. 

Still, among all of the smart triggers, the pulse-width trigger seems to lag the furthest behind technology. Currently, the fastest pulse-width trigger operates at approximately 200 ps, which is much slower than today’s fastest data-rate unit intervals (ranging well below 50 ps).  Thus, it’s important to know the oscilloscope’s pulse-width limit.  Enhancements through software can help accelerate the pulse-width trigger—in fact, recent advances allow software triggers to trigger on pulse widths as fast as 40 ps.

Software Trigger

In the last decade, software triggering has advanced to the point where it’s now considered a key part of an oscilloscope trigger.  In 2006, Agilent Technologies introduced the InfiniiScan software-based trigger system. Using software trigger, InfiniiScan can look for narrow pulse widths (half the pulse width of hardware triggers), monotonic edges, pattern trigger, and zone qualify.

Software-based triggers actually work much like hardware-based smart triggers. The hardware edge trigger finds an edge and the signal is digitized. Then the software scans the acquisition to see if the event has occurred. If so, the software triggers on the event.

The big upside of software-based triggering is its extreme flexibility.  For instance, software packages make it possible to draw multiple shapes on screen and then specify whether or not the signal can go through the shape or zone (Fig. 3). The downside is that software runs significantly slower than hardware triggers. If an event occurs relatively infrequently, the software trigger will likely miss it. The hardware trigger is much more likely to catch the event, assuming that what can be done in hardware is also possible in software. 

3. With software-based triggering, one can, for instance, draw multiple shapes on screen and specify if a signal can go through that shape or zone.
3. With software-based triggering, one can, for instance, draw multiple shapes on screen and specify if a signal can go through that shape or zone.

Brig Asay, product manager for high performance oscilloscopes, received a BSEE from the University of Wyoming, Laramie, and an MBA from Northwest Nazarene Univ., Idaho. He can be reached at [email protected].

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