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.

The key enabling technology that provides both inline data processing in oscilloscopes and the flexibility to reprogram the algorithms are field-programmable gate arrays (FPGAs), which are essentially programmable chips that can perform custom signal processing and control algorithms at high through - put rates in true parallel fashion. The flexibility of FPGAs allows modifying or adding user-specific trigger algorithms, while high-throughput processing enables analyzing of data samples in real time during acquisition, instead of in post-processing. This eliminates dead time, prevents missing triggers, and helps to detect rare events much faster.

An example of a user-defined trigger is one that detects signal shapes or transitions that do not fit into the standard trigger definitions, such as the signal shown in Figure 3. This digital signal shows a non-monotonic edge like one that can be caused by signal reflections or a faulty power supply of the tested circuit. A standard edge or width trigger would not detect this undesired signal, and detection within normal means is almost impossible. To accurately and consistently capture this event, a new trigger needs to be created. To address scenarios like this, a software trigger can be developed; however, because of the large trigger dead time associated with this method, a rare event is not quickly detected. Alternatively, a user-programmable FPGA can be employed to provide a number of window triggers that compare acquired sample points with a mask such that whenever all window triggers simultaneously detect a valid trigger condition, a combined trigger is issued and the signal is acquired.

Because the FPGA evaluates the signal continuously and in real time, the oscilloscope can capture single events as well as successive events without dead time in between acquisitions.

Reconfigurable Oscilloscopes

Figure 4. This is the block diagram of a reconfigurable oscilloscope (NI PXIe-5171R).

For many years, test engineers have used software tools such as National Instruments’ LabVIEW — instead of the fixed software in traditional boxed instruments — to automate test systems, analyze and present measurements, and reduce cost of test. This approach provides flexibility and takes advantage of the latest PC and CPU technologies. Very often, however, users demand to go one step further and also modify the way the instruments take measurements to better meet the needs of an application.

Off-the-shelf instruments traditionally are vendor-defined and provide only fixed capability; however, open, flexible instruments based on FPGA technology are available. The result is off-the-shelf hardware that has the best of both worlds: fixed, high-quality measurement technology, the latest digital bus integration, and user-customizable logic that is highly parallel, provides low latency, and is tied directly to I/O for inline processing.

With the open, vendor-provided software inside the FPGA, users can extend the instrument capabilities, for example, with custom triggers or additional timing or control signals. Users can also implement their own algorithms in the FPGA of software-designed instruments to re-purpose the hardware for a completely different task. An oscilloscope, for instance, can be turned into a real-time spectrum analyzer, transient recorder, protocol analyzer, RF receiver, and much more.

This helps to reduce equipment cost because fewer instruments need to be purchased and maintained, which is a significant cost aspect in test systems. This can be especially helpful where there is a need to provide test and instrument capabilities over a very long time (than 10 years) — for example, in military or aerospace test systems. These applications often require recreating the behavior of old instruments that are no longer available (end of life).

Reconfigurable instruments are useful for this application because they can be reprogrammed to mimic the behavior of an old instrument. This helps save cost, because test system software requires much less rework and re-certification to work with this new instrument.

Conclusion

Conventional triggering methods in oscilloscopes are challenged in capturing very rare or complex events because of the lack of flexibility and real-time analysis. New approaches can take advantage of FPGA technology to define custom triggering functions to meet the most complex triggering conditions, as well as process and analyze the signal in real time.

This article was contributed by National Instruments, Austin, TX. For more information, Click Here .

NASA Tech Briefs Magazine

This article first appeared in the August, 2015 issue of NASA Tech Briefs Magazine.

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