One important requirement for instruments like oscilloscopes is the ability to detect and trigger on an event of interest within a stream of unsuspicious signals fast and reliably. The quicker a specific event can be detected, the faster a problem in an electronic design can be debugged, reducing development and manufacturing test times.

Figure 1. This block diagram of an oscilloscope is based on digital signal processing. The acquisition memory and signal-processing unit are determining the oscilloscope’s acquisition update rate and dead time.
This importance is not lost on oscilloscope vendors. Many vendors offer 100 or more predefined triggers to help users quickly isolate very common and uncommon signal conditions. This adds flexibility, but choosing the correct trigger can be more difficult than actually capturing the signal, given the large variation of triggers by type, speed, bandwidth, hold-off, software, and so on, but each comes with tradeoffs between flexibility and dead time. Understanding the types of triggers and their respective tradeoffs can help identify the ideal trigger approach to optimize the chances of a successful triggered event.

Two aspects determine an oscilloscope’s trigger performance. Trigger flexibility describes how easily a trigger threshold or condition can be defined to improve the efficiency by adapting to meet the signal conditions under test. Most oscilloscopes provide a variety of vendor-defined trigger functions with minimal settings, such as level or width, but don’t provide a way to customize them. Trigger dead time indicates how long the oscilloscope cannot detect triggers between acquisitions. This leads to missing trigger conditions if an event of interest falls into this dead time. Trigger dead time is an inherent quality in all trigger architectures, but there are methods and techniques to minimize the dead time. Many oscilloscope vendors offer software-based triggers to add more flexibility, but this requires a significant amount of dead time because of the need for post-processing, and is not ideal for very rare and infrequent events.

Conventional Triggering

Edge triggering (starting an acquisition on a rising or falling signal transition) is one of the most common trigger modes on oscilloscopes today. The majority of simple debug and test functions are handled with an edge trigger, but oftentimes a more complex trigger scenario is required to isolate a particular signal shape or multiple shapes in succession. More advanced triggering options are also common on oscilloscopes, and offer additional flexibility to capture serial protocols like I2C or SPI, as well as advanced events and signal characteristics such as glitches, runt, width, slew rate, and timeout.

Figure 2. This shows the acquiring and analyzing of waveforms in traditional oscilloscopes with dead times in between the waveform snapshots (above) and continuous processing (below).
Many trigger conditions are implemented in hardware, but more sophisticated trigger options and signal qualifications are often performed in software, as shown in Figure 1. Software triggers offer the most flexibility, but add to the necessary data transfer and processing time during which the oscilloscopes cannot detect new triggers, as shown in Figure 2. This period when the trigger system is blind is called dead time, which can easily be a magnitude longer than the actual acquired data record — in other words, oscilloscope trigger systems can be blind over 95 percent of the time. This makes it harder to detect rare or infrequent events, leading to longer test times. And worse, users could get the false assumption that there are none of the expected events because they were not detected during measurement.

If the available trigger or signal analysis capability of an oscilloscope is not sufficient for a task, the only remaining option for users is to acquire long waveform segments and download this acquired raw data to a PC for post-processing to find a specific event. This adds an additional layer of complexity to the overall system design, and also causes longer test times because of the data transfer latency and necessary processing time.

Create Triggers Without Tradeoffs

Figure 3. A specific signal transition is captured using a user-defined trigger; the function is implemented inside the FPGA of a reconfigurable oscilloscope.
Although most software-based or smart trigger options can meet the needs in the design and test of electrical circuits, there are often rare events that can slow down product development significantly if not isolated and corrected quickly. Because of the limitation of most oscilloscope trigger functions, the user can access only what is available from the vendor.

With the ability to implement their own algorithms inside an oscilloscope, users can customize the instrument for specific tasks and not be limited to the functionality defined by the instrument vendor. For example, users can define their own application-specific trigger conditions to specifically capture a signal condition, which can help to significantly reduce test time by eliminating the need for post-processing of data on the PC, as shown in Figure 3.