PXI RF development shifts focus

Much of the drive to improve PXI's RF performance came from the wireless communications industry.

By Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 11/1/2010 12:00:00 AM

At first, engineers wondered whether or not PXI could even tackle RF testing. Then, it became just a question of what noise and frequency performance PXI could achieve. Now, the only questions are more like "how many channels do you want?"

See more articles from the November 2010 PXI Test report.

"When the industry first came out with RF instruments in PXI," said David Hall, RF product manager at National Instruments, "the naysayers pointed to electronic noise as a problem."

Equally pessimistic assessments abounded about operating frequencies, bandwidth, and even the ability to fit useful functionality into the space constraints of a PXI module. But all that has changed.

"There is no doubt that the industry has proven the feasibility of RF and microwave instruments in PXI," said Larry DesJardin, GM of modular product operations at Agilent Technologies. "In some applications, such as multichannel radar analysis, modular instruments can bring tangible advantages to the user." [Editor's note: This is a corrected version of the quotation; we misquoted Mr. DesJardin in our print issue.]

Much of the drive to improve PXI's RF performance, as well as the technology that supported its implementation, came from the wireless communications industry. "The needs of wireless LAN, WiMAX, MIMO, 3G, and LTE all pushed technology to new levels," said Barry Hack, PXI product specialist at Aeroflex.

NI's Hall also noted that innovations in the communications industry have led to smaller RF components, making it easier for vendors to implement RF capability within the space constraints of PXI modules.

Thus, from simply being a rising contender in the RF arena a few years ago (Ref. 1), PXI has emerged as a dominant technology in RF test. A new emphasis on PXI from traditional box-instrument vendor Agilent Technologies highlights this elevated stature. The company released more than 40 new PXI products—many for RF—at this year's Autotestcon in Orlando, FL,  and has also upgraded its membership in the PXISA (PXI Systems Alliance) to full Sponsor status.

RF specs meet current needs
The currently available RF products in PXI would astound those original naysayers. Pickering Interfaces has microwave switch modules with bandwidths to 65 GHz. Phase Matrix and Agilent both offer downconverters capable of operating at frequencies to 26.5 GHz. Agilent also offers a 1-GHz IF digitizer, a 3-GHz-to-10-GHz local oscillator, and other modules that combine with software to form the M9392A VSA (vector signal analyzer) with frequency coverage from 50 MHz to 26.5 GHz.

Noting that the technology needs that propelled PXI to achieve higher frequency and greater bandwidth are being met by the latest generations of PXI hardware, Aeroflex's Hack commented that future developments in PXI may shift toward addressing other system attributes. "That is, of course, until another technology comes along with new demands," he added.

One of the areas likely to see increased attention in PXI RF is channel density, according to NI's Hall. "Today's wireless handsets have multiple signals—Bluetooth, GPS, WLAN, and so on—all on a single antenna," he said, "so there is an increasing need for things like eight-port VNAs [vector network analyzers]."

Such needs are shifting PXI RF development from pure performance toward a focus on increasing the number of ports and the density of channels on modules. NI's recently released PXIe-5630 6-GHz, two-port VNA, for instance, occupies only two slots. Agilent has recently demonstrated a four-channel, phase-coherent microwave downconverter in a three-slot package.

To increase PXI channel capacity at the system level, as well as to improve  configurability, Agilent has introduced an 18-slot all-hybrid chassis. "Mixed cages are constrained in what you can do," said DesJardin. "Eliminating the positioning constraints of those mixed-hybrid chassis can help simplify microwave RF, especially where cable locations are critical. In some cases, a mixed chassis doesn't have the right number and positioning of hybrid slots for the needed configuration."

Channel limits loom
There may be a fundamental limit on how dense PXI channels and ports can become, however. "Moore's Law is allowing us to get denser and denser," said DesJardin, "but the tradeoff is increased power consumption on modules. The shielding needed for RF means that circuits are essentially enclosed in aluminum blocks, so we need low-power designs and very effective cooling."

Hack noted that the need for shielding and isolation may also place physical limits on how dense PXI RF modules become. "There comes a point where you cannot compromise," he said, and he added that connectors place constraints on what a PXI RF module can contain. "There is a limit on how many connectors can be usefully employed on the front panel. Too many, and it becomes difficult to torque up the mating connector."

But increasing channel density and capacity are not the only directions PXI RF development can follow. NI's Hall, for instance, sees lower costs as a key goal. "As chips for WiFi and other wireless networks became less expensive,  they have found applications in other technologies, such as smart utility meters," he said. "That means there are a lot more engineers who need to perform RF testing, and they are going to want lower-cost instruments."

The cost reductions will arise in part from the modularity inherent in PXI, the benefits of which are one of the fundamental messages of PXI, according to Hack. "Pay for what you need, not for what you get in the box." As an example, Hack pointed to the test of dynamic frequency selection in wireless routers. "With PXI, it [dynamic-frequency-selection test] becomes a single eight-slot hand-carried test solution. The equivalent in GPIB or LXI would  need at least six different pieces of equipment commanding a price around three times that of the PXI solution."

Processing power rises
Another trend in PXI RF, especially with growing availability of FPGA boards for PXI (Ref. 2), is increasing processing power. "Signal processing is of great importance in RF," said Hall, "and PXI is bringing more and more processing to test that was not available to the traditional RF engineer. The peer-to-peer streaming available with PXI Express, for instance, lets you use an RF instrument as though it had its own onboard FPGA, allowing such functions as real-time spectrum analyzers and on-the-fly demodulation." Hack pointed out that, unlike with box instruments, this processing power is under the user's control, not the test equipment manufacturer's.

Enhanced processing power is an area where PXI RF is likely to consistently stay a step ahead of box instruments. "Box instruments tend to have processors inside with compute power that is typically two or more years old," said Hack. "They launch with the same processing power they had when they were first being developed. PXI instruments can launch with whatever processing power is available today."

PXI RF is also staying ahead of box instruments in addressing new application needs. Streaming digitized RF signals to storage, for instance, has been available for many years according to Hack. "What's happened with PXI," he said, "is that those type systems have been miniaturized further than benchtop equipment could have ever taken them, with field portable systems replacing vehicle-mounted systems of the past." Such size reduction, he added, can be a driving factor for adoption of RF streaming in military and intelligence applications.

Boxes retain some advantages
Box instruments do retain an advantage over PXI for RF, according to Agilent's DesJardin, in their ease-of-use on the bench, especially when engineers need to make pure parametric measurements. Here, the flexibility that PXI offers creates an obstacle, as it requires users to configure the system and load a program before running a test. Box instruments, DesJardin pointed out, are ready to be used as soon as they power up.

While PXI is continually making advances in the simplification of such configuration and programming, these needs can still be a barrier for many test engineers. "Customers quite often need significant support to understand and grasp the concepts behind this advanced technology," said Hack. "I still get asked for the GPIB address of PXI instruments."

Vendors are, however, increasingly providing such support in terms of development tools and software packages for RF test capability. Agilent's new M9392A VSA, for instance, has the support of the company's 89601A VSA software package, providing users with immediate access to both general-purpose and standards-based demodulation and signal-analysis functions. When Aeroflex released the 3061 multiport RF combiner earlier this year, the company also updated its WLAN Measurement Suite software to take advantage of the combiner's MIMO capabilities.

The increasing software support as well as the achievement of performance levels that satisfy the needs of communications test has placed PXI RF on an equal footing with box instruments for most applications. And where box instruments still offer a technical edge, PXI is almost certain to quickly follow. "There is nothing in traditional instrumentation that cannot be done in PXI," said NI's Hall, "and the transfer of technology from box to PXI is accelerating."

With performance no longer a major issue, then, PXI RF developers are free to focus their development efforts on characteristics such as cost, channel density, and system capacity.

REFERENCES
1. Quinnell, Richard A., "PXI makes inroads into RF test." PXI Test Report, Test & Measurement World,
November 2007.
2. Quinnell, Richard A., "User-configurable FPGA modules boost PXI system versatility," PXI Test Report, Test & Measurement World, May 2010..