PXI’s hidden role in communications-standards testing
PXI offers flexibility in standards-based testing.
By Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 11/1/2009 1:00:00 AM
Communications equipment requires considerable testing to ensure interoperability as well as conformance to regulatory restrictions. Test instruments designed for testing to specific standards, such as WiMAX, Bluetooth, CDMA, GSM, and LTE, can greatly speed the development and production efforts of manufacturers of communications equipment. PXI can play an important role in such standards-based testing, but the way the technology is being employed may prove surprising.
The standards that wireless communications equipment must meet are both complex and continually evolving. Several times during the past decade, technology advances have triggered major changes in standards for modulation techniques, operating frequencies, and information protocols, and some advancements have led to the introduction of entirely new standards. Test equipment that communications equipment developers and manufacturers use must be flexible enough to adapt to such changes.
At the same time, developers and manufacturers can benefit greatly from test equipment that is purpose-built for testing to a specific standard. Tim Carey, product manager for automated test systems at Aeroflex, pointed out that while tests for various wireless standards may need the same basic RF measurements, the interpretation of test data can vary significantly. “The definition of output power is different for the various standards,” said Carey. “Modulation quality also has different characterizations among the different standards, and some standards have multiple modulation techniques depending on the quality of service being offered.”
Of course, the design choices developers make can lead to product-specific variations within standards-based test requirements. For example, a fixed-function tester for CDMA might need slight variations to deal with chips from two different manufacturers. According to Carey, “methodology depends as much on the choice of chipset vendor as on the standard in question. Communications [setup and control signaling] to configure devices are chip specific.”
This wide range of variations creates a significant burden on test engineers working with generic equipment, requiring them to develop complex test procedures to measure standards compliance. Purpose-built communications test equipment can relieve that burden by offering preconfigured settings and data analysis specific to the standards in question. Yet, the rapid evolution of wireless communications makes flexibility a critical requirement as well. This is where the configurability of PXI can shine.
Evolving standards need flexible test instruments
“As the industry and individual standards proliferate, engineers need to be able to customize their test equipment as quickly as possible,” said David Hall, RF product marketing manager at National Instruments. “Benchtop instruments need vendor involvement for updates even when small changes arise in standards testing. With PXI, the user has access to source code and can make modifications on the fly if needed.”
Hall also pointed out how readily PXI-based equipment can increase in performance. “With PXI, data processing happens on a Windows CPU module, which can be replaced as the technology improves. In a traditional box, the processing is built in.” The net effect is an ability for PXI-based equipment to easily keep pace with changes in standards.
PXI’s flexibility also allows it to easily adapt to the rise of variations to standards. “A lot of wireless standards are modifications to a basic theme,” said Hall. “Zigbee, for instance, is a superset of IEEE 802.15 [wireless personal area networks], and other similar variations on that standard have now arisen.”
Testing these new variations using PXI becomes simply a matter of new programming for the existing hardware. Similarly, PXI supports test of products that must conform to several standards, such as netbook computers that incorporate both WiFi and cellular communications capabilities in their designs.
For designers, PXI provides access to information that is buried in traditional bench instruments, allowing development of additional tests beyond the standard. “Tools like LabView allow you to do things you can’t with traditional instruments,” said Hall. “For instance, you can utilize I/Q data that are hard to get to in benchtop instruments.”
Traditional preconfigured instruments do offer some advantages. One is at the highest levels of RF performance. “Traditional instrument designs can reach 40 GHz to 60 GHz as their upper range,” said Hall. “Current PXI technology tops out at about 26.5 GHz.” Traditional instruments can also offer battery-powered portability that PXI cannot match. “A handheld spectrum analyzer can be very useful,” said Hall.
Shortage of PXI systems
Overall, however, PXI offers more compelling benefits than traditional instruments in addressing test for evolving standards, yet there seems to be a shortage of PXI offerings for standards-based testing. National Instruments offers several RF test reference platforms, including a WLAN toolkit package as well as a test platform for wireless sensor networks. Aeroflex has created the 3000 series of communications testers that can handle a variety of standards, including Bluetooth, WiMAX, CDMA, and GSM. But most other PXI vendors offer only building blocks, not entire packages.
One reason for this shortfall may be inertia among test engineers who resist the switch from bench to modular instrumentation, limiting the market. “There is a generation gap among test engineers so that many potential users are not comfortable in using the modular approach,” said Hall. “PXI is different from what they are familiar with.”
Hall pointed to spectrum analysis as an example, noting that traditional RF spectrum analyzers have video bandwidth filters for the analog display, but as analysis has moved from analog swept frequency to digital sampling, digital systems such as PXI have stopped offering that setting. “It only affects the display, not the data,” Hall said, “but it makes PXI seem different, leaving us with a need to educate users about such differences.”
Another reason for the shortfall may simply be that manufacturers hesitate to expend the effort required to develop software to handle the complex communications protocols, especially considering that protocol test capability is not always needed. “Protocol handling forms the most significant part of the test-development effort,” said Aeroflex’s Carey. “Those tests can require many man years to develop and maintain.”
Further, having standards-based testing capabilities in an instrument may be unnecessary. “Preconformance testing at the design phase may need protocol test capability, but manufacturing may not,” said Carey. “Customers don’t want to pay the cost of protocol test development if all they are doing is parametric testing against standards limits to evaluate the manufacturing quality of circuit boards.”
These factors seem to be limiting what PXI vendors offer the standards-based test market, but that does not mean that PXI itself has been limited. Instead, PXI is finding its role in standards-based test hidden within the products that test equipment OEMs are offering the market. Aeroflex, for instance, has developed the 7100 series of communications testers for LTE cellular handset developers. PXI lies at the core of the 7100 series, but the fact is not advertised. “We just sell it as a digital radio test set,” said Carey.
| Read more articles from our November 2009 PXI Test Report. |
PXI’s hidden role
Other manufacturers of test equipment also use PXI as a foundation without mentioning it. The SP6010 TD-SCDMA communications test set from China’s Beijing StarPoint Communication Software Co. is another example of a bench instrument hiding a PXI system inside. According to Carey, PXI has similarly seen quiet adoption by test equipment manufacturers ranging from OEMs in Korea to tier-one manufacturers in the US.
While PXI gives these manufacturers the benefits of flexibility, it is the hidden benefits of PXI that have prompted its adoption for bench instrument design. One such benefit is quicker time to market.
“For a lot of OEMs, time to market is a greater need than design flexibility,” said Carey. “PXI allows these companies to make COTS (commercial-off-the-shelf) choices and customize them rather than developing the technology themselves. There is something about PXI that makes it easy to integrate with pretty much anything else.”
PXI also allows manufacturers of test equipment to leverage their expertise at communications protocols without having to gain expertise in RF technology. “If a company can focus on its core competence and leverage COTS products to fill in the holes, that is a very attractive proposition,” said Carey. Typically, companies will develop a custom real-time baseband processing card to run their protocol test software, said Carey, and connect that to PXI for the RF segment.
Thus, the role that PXI finds itself playing in standards-based testing is not that of a configurable instrument but that of a concealed foundation. It has earned this role not through its widely touted flexibility but through its relatively hidden abilities to speed product development and to supplement OEM core expertise with off-the-shelf solutions addressing non-core technologies. Manufacturers adopting PXI when developing dedicated standards-based test equipment will still benefit from PXI’s flexibility. “These designs are only dedicated for a time,” said Carey, “they can always be remodeled at another time.”
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