With performance claims heating up in comparisons between traditional one-box testers and modular approaches, many engineering firms are starting to wonder if they can save money by using modular equipment. After all, more top-line RF/microwave test instrument manufacturers are developing high-performance modular test products that can generate and analyze high-frequency signals through 40 GHz and beyond. At the same time, test instruments equipped with a Universal Serial Bus (USB) port allow a personal computer (PC) and even smartphones and tablets to be used for control and programming. Although there are clearly options beyond the one-box tester, there is no "one-size-fits-all" solution in terms of modular RF/microwave test instruments.

"The performance of an instrument depends on the application, the measurement need, and the use model (interactive or automated)," explains Jean Manuel Dassonville, Agilent Technologies' Modular Solution Outbound Manager. "Therefore, finding the best performance consists of finding the best compromise between speed, measurement breadth, quality, and cost. That's why different form factors will be considered as providing the best performance in a given test scenario. Modular instrumentation usually favors speed, while traditional instrumentation is focused on quality and the breadth of measurement capabilities." (For some background on modular approaches, see Test Modules Offer Measurement Versatility).

One-Box Advantages

Dassonville emphasizes that traditional instrumentation offers four main benefits. The first is higher measurement quality, which is derived from the fact that all of the building blocks in a "one-box" instrument have been designed and optimized to work together. "This is especially true for RF and microwave analyzers and sources, where the internal layout and shielding between building blocks have a significant influence on the measurement quality," states Dassonville. "Because this type of equipment is used in the early design stages of new technology life cycles, it also includes measurement algorithms that are not yet available with other form factors."

Secondly, Dassonville points to ease of use and fast time to first measurement. "Traditional instrumentation better fits with the needs of R&D engineers who need to quickly turn on and use an instrument," he notes. "Benchtop instruments include ready-to-use hardware and measurement softwarea front-panel interface with a graphical user interface (GUI) designed to provide quick access to all the features included in the instrument." (When test automation is needed, however, Dassonville notes that LXI technology enables the same equipment to be used in a controlled test environment.)

Third, the technical specifications of a single box instrument are clearly defined by the instrument vendor. When making high-quality measurements, it is crucial that the engineer know the accuracy of the instrument itself. When combining various modules from different vendors in a single modular chassis, the technical specifications of the complete measurement subsystem are not easily defined.

Finally, Dassonville points out that support and calibration services are part of the standard package offered with traditional equipment. When combining elements from different vendors in a modular subsystem, the support and calibration of modular instrumentation bring new possibilities. But they also are more complex to manage.

When Modular Makes Sense

Modular approaches also offer clear benefits for an increasing number and variety of applications. Flexibility and scalability are two obvious examples, as a modular system can be scaled up and down to meet various application needs. Because these approaches do not include redundant functions or unnecessary features, they also have a smaller footprint. Size savings also are derived from the sharing of common resources like the display, controller, power supply, and interface hardware. Of course, the lower price point also makes them very attractive.

Craig Walsh, CEO at Telemakus, adds that a signal source can be placed directly at the input and a sensor can be placed directly at the output of a modular system, avoiding the loss and calibration issues associated with long cables. "In field-test applications, part of the test system could be controlled remotely via wireless or LAN connections, allowing a single operator to complete testing of long cable systems or microwave links," Walsh adds.

PXI in particular has shown itself to be a good fit for the complexities of multiple-input multiple-output (MIMO) testing. NI's Raajit Lall, Product Manager for RF and Wireless Test, points out that all of the modules have access to the PXI backplane, which can be used to share triggers and clocks. "Applications such as MIMO need multiple analyzers and generators to be tightly synchronized, and you can get as tight as a few nanoseconds of skew (or 0.1 deg. of phase offset) between multiple instruments," states Lall. "To achieve such synchronization on one-box systems is complicated, as extra cabling is required between the instruments and some of the triggers and clocks are not accessible."

In addition to synchronization between multiple analysis and generation instruments, NI offers a technology called peer-to-peer streaming (Fig. 1). Here, the PXI backplane allows modules to share data. "For example," notes Lall, "you can acquire a large-bandwidth spectrum on a spectrum analyzer and stream all of the datawithout missing a sampleto a field-programmable gate array (FPGA) for real-time processing. Processed data can then be transferred to a generator for generation if needed. All of this transfer and processing happens in real time without the involvement of a PC, thus making the process much more reliable and faster."

Telemakus' Walsh concedes that a PC-based approach can hamper some applications: "USB devices powered directly from a PC have a DC limitation of 0.5 A per port, which can restrict the functionality. However, the addition of a powered HUB increases the current limit and, ultimately, products can be self-powered and independent of the host PC." For many applications, the true portability of modular systems based on USB will make them the best fit (Fig. 2). While USB's reliability may still be questionable in some test and measurement equipment, it is a clear winner in the power-measurement arena (see USB Reigns In Power Measurements).

In closing, gone are the days when one could just choose between competitive boxes or handheld instruments in a single performance/application class. Look into modular approaches like PXI. Talk to vendors. When shopping for a test solution, research all of the optionsmuch as someone does when specifying a component, for example. The right solution is out there. Yet first, one must determine whether a one-box system is overkill, a modular approach will provide needed accuracy, and other factors. Many choices are out thereand they continue to evolve and improve.