Measurement starts from trigger

A major requirement for instruments like oscilloscopes is to quickly and reliably detect and trigger on events of interest in a known signal stream. The sooner a specific event can be detected, the faster problems that arise during the electronic design process can be corrected, saving development and production test time.

This is also important for oscilloscope vendors. Many vendors provide more than 100 predefined triggers to help users quickly separate common and uncommon signal conditions. On the one hand, this increases flexibility, but on the other hand, choosing the right trigger itself is more difficult than actually capturing the signal, because triggers come in a variety of types, speeds, bandwidths, delays, and software, and each trigger requires a trade-off between flexibility and dead time. Understanding each type of trigger and the corresponding trade-offs can help users find the ideal trigger method to increase the chance of successfully triggering an event.

 

Two factors determine the trigger performance of an oscilloscope:

1. Trigger flexibility describes the ease with which trigger thresholds or conditions can be defined to accommodate a variety of signal conditions under test to improve efficiency. Most oscilloscopes offer vendor-defined trigger functions that include minimum parameter settings, such as level or width, but do not provide a way to customize these parameters.
2. Trigger dead time refers to the length of time that the oscilloscope cannot detect a trigger between two consecutive acquisitions. This means that if the event of interest occurs within the dead time, it will miss the trigger condition. Trigger dead time is an inherent characteristic of all trigger architectures, but there are methods and techniques that can be used to minimize this time. Many oscilloscope vendors provide software-based triggers for increased flexibility, but due to the need for post-processing, this type of trigger requires a long dead time and is therefore not suitable for the detection of rare and infrequent events.

 

1. Traditional trigger

Edge triggering (data acquisition starts when a logic signal changes from high to low or from low to high) is the most common oscilloscope trigger mode. Most simple debugging and test functions are handled by edge triggering, but sometimes more complex triggering is needed to isolate a signal of a specific shape or to isolate multiple signals of a certain shape in succession. Oscilloscopes also include some more advanced triggering options that provide greater flexibility to capture serial protocols such as I2C or SPI as well as advanced events and signal characteristics such as glitches, runt pulses, widths, conversion rates, timeouts, etc.

 

Figure 1. This is a block diagram of a digital signal processing-based oscilloscope. The acquisition memory and signal processing unit determine the acquisition update rate and dead time of the oscilloscope.

 

Many trigger conditions are implemented in hardware, but more complex trigger options and signal authentication are often implemented in software similar to the one in Figure 1. Software triggering provides the best flexibility, but also increases the necessary data transfer and processing time, during which the oscilloscope cannot detect new triggers, as shown in Figure 2. The period of time when the system cannot detect a trigger is called dead time, which is often longer than the actual acquisition data record time—in other words, the oscilloscope trigger system may have a dead time of more than 95%. This makes it more difficult to detect rare or low-frequency events and results in longer test times. Even worse, the user may incorrectly assume that the expected event did not occur because the probability of the expected event occurring was too low to be detected during the measurement.

 

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Figure 2. This diagram shows the data acquisition and analysis process of a traditional oscilloscope, where there is dead time between waveform capture (top) and continuous processing (bottom).

 

If the available trigger or signal analysis capabilities of the oscilloscope cannot meet the needs of the task, the user's only option is to acquire longer waveform segments and download the raw data of these segments to a computer for post-processing to find specific events. However, this increases the complexity of the entire system design process, and at the same time, due to data transmission delays and the time required for data processing, the entire test time becomes longer.

 

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2. Generate triggers without trade-offs

Although most software-based or intelligent trigger options can meet the design and test needs of electronic circuits, if rare events are not quickly isolated or corrected, these events often prolong product development time. Due to the limitations of oscilloscope trigger functions, users can only use the functions provided by the supplier.

If users can develop their own algorithms within the oscilloscope, they can customize the instrument's functionality for specific tasks, rather than being limited to the functionality provided by the vendor. For example, users can define their own trigger conditions to specifically capture a signal condition for a specific application, eliminating the need for subsequent processing on the PC, greatly reducing test time, as shown in Figure 3.

The key technology that provides the oscilloscope with online data processing and the flexibility to reprogram algorithms is the FPGA, which is essentially a programmable chip that can execute custom signal processing and control algorithms at high speed through true parallelism. The flexibility of the FPGA allows users to modify or add specific trigger algorithms, while high-throughput data processing can analyze data samples in real time during the acquisition process instead of processing them afterwards. This can avoid dead time, prevent trigger misses, and help users detect rare events faster.

An example of a user-defined trigger is to detect a signal waveform or level transition that does not conform to the standard trigger definition, such as the signal shown in Figure 3. This digital signal exhibits a non-monotonic edge, which may be caused by signal reflection or power failure in the circuit under test. Standard edge or width triggers cannot detect this unexpected signal, and it is basically impossible to detect it using conventional means. We need to study a new trigger to accurately and continuously capture this event. To solve this problem, we need to develop a software trigger; however, this method has a large trigger dead time and cannot quickly detect low-probability events. Alternatively, a user-programmable FPGA can be used to provide multiple window triggers. When all window triggers detect valid trigger conditions at the same time, the acquired samples are compared with the mask, thereby generating a combined trigger to acquire the signal.

Because the FPGA continuously evaluates the signal in real time, the oscilloscope can capture both a single signal and a continuous signal with no dead time between acquisitions.

 

Figure 3. Use a user-defined trigger to capture a specific signal transition; this functionality is implemented within the reconfigurable oscilloscope’s FPGA.

 

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3. Reconfigurable Oscilloscope

For many years, test engineers have used software tools such as LabVIEW to replace the fixed software in traditional benchtop instruments, automating system testing and analysis and display of measurement results, thereby saving test costs. This approach provides flexibility and takes advantage of the latest PC and CPU technology. However, users' needs are often more than this, and they often need to modify the instrument's measurement method to better meet the needs of the application.

Traditional off-the-shelf instruments are defined by the vendor and provide only fixed functions; NI is the first to use FPGA technology to provide more open and flexible instruments. As a result, hardware with both characteristics is obtained: fixed, high-quality measurement technology; the latest digital bus integration; user-customizable highly parallel logic that provides low latency and is directly associated with I/O for online processing.

Figure 4. Functional block diagram of the NI PXIe-5171R reconfigurable oscilloscope.

 

With open software provided by vendors in FPGA, users can expand the functionality of the instrument, such as custom triggers or additional timing or control signals. Users can also implement their own algorithms in the FPGA of the instrument designed by software, redefining the hardware functions to complete completely different tasks. For example, an oscilloscope can become a real-time spectrum analyzer, transient recorder, protocol analyzer, RF receiver or other instruments.

Equipment cost is a major cost of test systems, and reconfigurable devices can help users save equipment costs because fewer instruments need to be purchased and maintained. This is especially useful for test and instrument functions that need to be used for a long time (more than 10 years), such as military or aerospace test systems, which often need to reproduce the functions of old instruments that have been eliminated (end of life cycle).

Reconfigurable instruments are ideal for this application because the functionality of older instruments can be emulated by reprogramming. This helps users save costs by requiring only minimal reprogramming and recertification of the test system software to work with the new instrument.

An example of such an instrument is  the NI PXIe-5171R reconfigurable oscilloscope , which uses a Xilinx Kintex-7 FPGA to process samples from eight input channels in real time. Figure 4 shows how the user-programmable FPGA is integrated into the data path and provides access to the instrument’s control and timing signals.

 

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4 Conclusion

Due to the lack of flexibility and real-time analysis capabilities, traditional oscilloscope triggering methods face challenges in capturing low-probability and complex events. The new method uses FPGA technology to customize the trigger function to meet complex trigger conditions and real-time signal processing and analysis requirements.

Watch the demo video to learn how to avoid dead time by customizing the trigger.