LXI speeds gigahertz measurements

Ethernet isn’t generally considered a conduit for RF and microwave signals, but the appearance of the LXI version of Ethernet on a variety of instruments is simplifying and speeding RF and microwave measurement tasks.

When considering LXI for RF applications, said Chris Van Woerkom, senior marketing engineer in the Electronic Measurements Group of Agilent Technologies, “You first look at the alternative. GPIB is bandwidth-limited, and while being able to send a lot of data down the pipe is not always important for RF and microwave applications, there are certainly cases where you want to dump a fair amount of data, and extra bandwidth is useful.”

But even if you don’t need the bandwidth, said Mark Elo, marketing director of RF products at Keithley Instruments, “GPIB is old and slow and expensive. It adds a lot of cost to an instrument.” As for LXI vs. plain Ethernet, which has been a standard feature in instruments for years, Elo said, “LXI provides better remote-control standards and requires that each instrument have a Web server, which enables engineers in different countries to share measurement information fairly easily. In addition, LXI’s triggering capability emulates the GPIB triggering capability.”

Commented Bob Rennard, president of the LXI Consortium and a program manager at Agilent Technologies, “LXI Class C is basically GPIB over Cat-5 LAN cables with a few extra cool features thrown in.” (To learn how the three classes of LXI instruments differ, see “LXI triggering.”)

Figure 1.  The LXI-compliant SMU 200A allows users to internally configure up to two I/Q sources, simulate up to four fading channels, add noise, and output two RF signals for demanding applications such as 2x2 MIMO receiver testing. Courtesy of Rohde & Schwarz.

Stallings added that his firm now offers for its FSL spectrum analyzers an extension card that implements the LXI wired trigger bus and that can be configured via the FSL’s Web interface. Currently, he said, the company is working on IEEE 1588 Class B instruments: “Using the clock synchronization provided by 1588 allows triggering on absolute time,” and he added that it permits instruments to make measurements automatically—that is, without a controller.

Such capability, he said, would be useful in a distributed setup in which, for example, a signal generator and spectrum analyzer might be located at different ends of a large airplane. “There’s no easy way to connect both instruments with a hardware trigger signal,” but IEEE 1588 time stamping enables sequences of measurements to occur based on absolute time.

The LXI Consortium’s Rennard said he has seen several applications where LXI brings significant benefits to RF and microwave measurement applications. “The first that comes to mind are stimulus-response measurements, where you’ve got a source and analyzer that are tied together and using some network features of peer-to-peer communication. With LXI connecting the source and analyzer, we’ve seen a significant decrease—two orders of magnitude—in communication overhead, since you don’t have to go back and forth to a controller.”

He continued, “Further, a controller will likely be a PC environment with a Microsoft operating system, which may be off clearing a cache somewhere. You’ve got no idea exactly when things will execute. When you go peer to peer, traffic between an instrument and a controller gets reduced to almost nothing. At the same time, determinism improves significantly. In demos we’ve done, we’ve seen measurement times shrink from a couple hundred microseconds down to one microsecond or less—it’s a huge improvement.”

And if you use Class B instruments with IEEE 1588, he said, “Latency goes down to zero, because you can start scheduling things and basically overlap some of your stimulus and response.”

For high-throughput applications, the instruments themselves have to be fast, said Keithley’s Elo, adding that his firm addresses measurement speed by incorporating a digital signal processor (DSP) into its instruments, avoiding constraints related to a microprocessor executing instrument firmware. But instrument speed is only part of the solution.

Figure 2.  The MXA signal analyzer and MXG signal generator bring LXI connectivity to RF source-measurement tasks. Courtesy of Agilent Technologies.

Curran also explained that while measurement throughput is ultimately key for commercial applications, compatibility is a key factor in military and aerospace. Test systems can span decades, he said, and LXI is emerging as the standard that will mesh easily with multiple generations of interconnect technology.

But intergenerational compatibility is only one issue. Another is compatibility between lab and deployed systems. Agilent addresses this issue, Curran said, by ensuring program compatibility among the LXI Class C instruments common in lab environments and the Class A and B instruments that are likely to make up large systems deployed outside the lab.

Consortium president Rennard cited a number of RF/microwave application areas in which he’s seen LXI provide benefits: “One we’ve seen over and over again in military and communications applications is what I’ll call cross-domain analysis. Think of a mixed-signal oscilloscope [MSO] with logic and oscilloscope functions—analog and digital—on the same product. The reason they are so popular is there is a lot of utility, a lot of value in that. Now, if you take that paradigm of an MSO and—instead of just a scope and a logic analyzer with a common time base—if you extrapolate that into any instrument pair, it becomes super powerful.”

Figure 3.  The LXI Class C-compliant 60-100 chassis allows 3U PXI RF and other switching modules to be supported in an LXI environment. The chassis comes with a generic IVI driver to control the PXI modules in accordance with the LXI specification. Courtesy of Pickering Interfaces.

“Until now, it’s been almost impossible to make that measurement—it’s been difficult to get adequate correlation between the baseband and the RF. But LXI with Class B gives you a common time frame, or a common reference, and that makes the measurement trivial.”

Similarly, he said, consider another application, “on a radar range where you have a source and receiver that are separated by tens of meters, hundreds of meters, or kilometers.” Making good measurements “has been just impractical over such long distances,” he said, “whereas using LAN makes them trivial.”

Finally, Rennard cited applications where integrators are trying to control a system shutdown. Consider, he said, an aircraft radar system where you have power regulators, an RF front end, and a number of different sensitive components on a DUT, and the DUT’s pretty expensive—$50,000 to $100,000. “The DUT comes off an airplane, and you know it doesn’t work—otherwise it would still be on the airplane. You are trying to find what’s wrong with it, you are running it through a routine, you find a fault—say, an overcurrent situation—and you want to shut down in a hurry before you blow up sensitive components and make a problem even worse.”

He explained that with “garden-variety” LXI or with Class B LXI, you can employ peer-to-peer communications to, for example, respond to an overcurrent situation sensed by a power supply. The power supply, he explained, can fire a trigger that shuts down the power supply itself as well as initiating a cascading sequence that shuts off, for instance, RF receivers and generators as well as other power supplies in a way that doesn’t further damage the DUT in the process. “This used to take hundreds of milliseconds or even seconds to complete. It can now be done in a very orderly fashion with timing standard deviations down in the microsecond range.”

Companies like Agilent, Rohde & Schwartz, Keithley, and Pickering Interfaces make LXI-compliant RF/microwave instruments, but, as Rennard’s aircraft radar example illustrates, other, low-frequency instruments and power supplies can also form part of an RF/microwave test system. The ability of RF and microwave as well as other instruments to work together was amply demonstrated at the Autotestcon show in September, when products from companies including Agilent, C&H Technologies, Keithley, Kepco, The MathWorks, Rohde & Schwarz, VXI Technology, and Xantrex all worked together over a wireless network.

Figure 4.  The 2800 and 2900 family of 2.5- and 6-GHz LXI-compliant generators and analyzers include onboard DSPs to enable, for example, a complete suite of GSM tests in 16 ms. Courtesy of Keithley Instruments.

Of course, LXI systems can include instruments featuring connection schemes other than LXI. Bob Stasonis, North American sales and marketing manager for Pickering Interfaces, a firm that makes PXI and LXI products, noted that his firm’s LXI Class C-compliant 60-100 chassis allows Pickering’s 3U PXI RF and other switching modules to be supported in an LXI environment. The chassis comes with a generic IVI driver to control the PXI modules in accordance with the LXI specification, he added.

It seems likely that PXI and LXI will coexist, each serving specific applications areas while complementing each other in many cases. But it seems equally likely that, although GPIB won’t disappear overnight, LXI will gradually supplant it, much as in the PC world where USB, for instance, has supplanted RS-232 and the parallel printer port.

Concluded Elo of Keithley, “Most of the leading companies developing wireless products have a roadmap to utilize LXI. Now, we are in the transition period. There is a lot of legacy equipment around and legacy test stands and test executives that depend on GPIB. But GPIB is old and slow and expensive. LXI will be the dominant connectivity standard for instrumentation as we pass though this century.”