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.