Throughout its steady climb into the mainstream, software-defined instrumentation has represented a paradigm shift from traditional box-style instruments, which are defined largely by their hardware and the inaccessible software they run, to one in which instruments are defined less by their hardware and much more by software that is very much under user control.
The traditional box instrument, designed and purpose-built for specific measurement tasks, includes highly specialized hardware that’s optimized for the task at hand. In the software-defined instrument, the hardware is fairly generic, performing some class of I/O. But the measurement functionality is a function of the algorithm that processes the raw data streaming from that I/O.
“Essentially, the function of the instrument is defined in the firmware,” says Richard McDonell, automated test senior group manager at National Instruments. Further, with the advent of FPGAs, users can embed their own firmware in their software-defined instrument. Thus, they can define the instrument at the hardware level rather than at the software level.
In this article, we’ll look into how software-defined instruments are being used today and examine some of the growth areas. But let’s consider what they cannot do. While they’re becoming more valuable, software-defined instruments will never displace the traditional benchtop test instrument. In general, when you need a measurement quickly, the traditional bench instrument is still the best way to go.
“We don’t say software-defined instruments are the perfect solution for everything. For design and validation, the traditional instrument is still a very valuable asset on the bench,” says McDonell.
When you must meet the fast-changing needs of an automated test scenario, though, the flexibility of software-defined instrumentation is a tremendous benefit. Overall, software-defined instrumentation is aimed at the automated test market.
THE BIG PICTURE
Looking at software-defined instruments from a broad perspective, one would immediately note that they’re not a new idea. Major players, such as National Instruments, have promoted the concept since the late 1970s. Early on, the products were typically associated with general-purpose data-acqusition tasks.
Yet by the late 1990s, the PXI standard brought a new focus on automated test to the software-defined instrumentation market. In going beyond data acquisition, software-defined instrumentation spread across the spectrum of digital multimeters, digitizers, oscilloscopes, arbitrary generators, and other instruments. It eventually encompassed all of the elements one would find in a complete automated test system.
More than 1500 PXI instruments are on the market, and PXI remains the predominant platform for software-defined instrumentation. However, systems can be built around any of the various flavors of the PXI standard.
BREAKOUT AREAS
Some of the biggest breakthroughs in software-defined instruments of late are in the RF arena. In the early days of software-defined instruments, the size and signaling requirements of an automated system mitigated against their use in RF testing scenarios. But today PXI is having a major impact and a fundamental transformation is taking place in automated RF test.
Software-defined systems are being used in production test for standards such as WiMAX, GSM/EDGE, WCDMA, Long-Term Evolution (LTE), multiple-input multiple-output (MIMO), and 802.11n. It’s also finding use for many other RF applications that are not necessarily standards-based but also important. These include applications such as spectral monitoring. Others include particularly challenging test scenarios involving MIMO systems (see “Addressing Multi-Channel RF Test Challenges”).
The sweet spot for RF applications is at frequencies up to the 6-GHz range, which encompasses most common applications. A good deal of work is going on now in the higher frequencies for radar and other similar applications. For example, National Instruments is collaborating with Phase Matrix on a 26.5-GHz instrument in a 3U PXI form factor that’s set to to be released later this year.
RF in production test is seeing a major transformation toward an automated approach. Test engineers are moving away from call processing and other time-intensive tests for the physical layer, which, they are finding, often does not require an expensive call-processing box.
Rather than using such equipment to make an actual physical call with the final-production version of a handset, for instance, they can use what some term “just-enough” testing using automated equipment to ensure that the physical layer works properly. This not only saves time but also a good deal of money compared to the cost of the call-processing box.
Another big growth area for software-defined instruments is in semiconductor test, where PXI has had huge wins. Some of these have been in production, but most have been in the validation and verification and chip characterization phase.
“Where we’re seeing PXI and software-defined instrumentation in semiconductor production test is in mixed-signal parts, such as Analog Devices and its MEMS devices,” says McDonell. These, and other devices, are not purely digital. In addition, systems-on-a-chip (SoCs) have increasing amounts of RF circuitry onboard that cannot be tested by traditional digital test methods.