Smooth operators communicating for communications test

When you’re designing complex devices and systems for communications and other sophisticated applications, you often need to make measurements with instruments that can talk to each other. You might, for instance, need to synchronize a signal source and an analyzer to evaluate a prototype, or you may need to make automated measurements to collect voluminous characterization data.
Instruments have long been able to communicate with controllers and with each other to serve such applications. That capability dates back to the late ’60s, to the invention of the HP-IB (Hewlett-Packard interface bus), codified as the IEEE-488 standard and taking the vendor-neutral name of GPIB (general-purpose interface bus). And instruments have long been available with general-purpose computer-centric interfaces ranging from RS-232 to USB. But those interfaces have limitations. GPIB cables are bulky and expensive, and data rates are limited. USB cables are ubiquitous and cheap, but the interface has no instrument-specific features and limits you to communications with a few instruments near a single computer.

The LXI (local-area-network-extensions-for-instrumentation) and PXI (PCI-extensions-for-instrumentation) standards are overcoming these limitations. The standards’ respective proponents, which the LXI Consortium and PXI Systems Alliance represent, touted each standard’s features and benefits at industry events during the summer and fall. You can use instruments conforming to either standard alone or in hybrid systems to bring automated test capability to your laboratory. Indicating that there is no single correct instrument-system architecture for every application, several vendors are supporting or are at least investigating both standards (highlighted in Table 1).

If you are working in the microwave range, you’ll need an LXI system or a hybrid system with LXI-microwave instruments. In the PXI format, Phase Matrix Inc offers the 26.5-GHz PXI-1420 downconverter, and Pickering Interfaces offers PXI microwave switches. However, general-purpose PXI RF signal sources and receivers, from Aeroflex and National Instruments, top out at 6 and 6.6 GHz, respectively. Rather than looking to boost bandwidth, makers of PXI RF modules have been targeting support for emerging technologies. Aeroflex, for example, recently announced new LTE (long-term-evolution) measurement capabilities for its PXI systems. In contrast, just about any type of instrument you can buy for the bench top is probably available in an LXI-compliant version. For example, Rohde & Schwarz just introduced its R&S ZVA67, a 10-MHz to 67-GHz vector network analyzer. The instrument complies with LXI Class C.

In addition, LXI can prove to be the technology of choice if you require some form of remote access or if you must cover long distances. For example, if you need to conduct tests on a radar range in which your source and receiver are hundreds of meters apart from each other, an LXI system can easily accomplish measurements that would otherwise be impractical (Reference 1).

On the other hand, PXI affords a way to easily configure an instrument system without dealing with LAN issues and without involving your IT department. PXI inherently provides clock synchronization among instruments in a backplane—an ability available on LXI Class B and C instruments but not on the more common—particularly in the RF and microwave range—Class C versions. Further, PXI systems can stream data at high rates for off-instrument storage and analysis.

Making the case for LXI
“Today, 1211 products are certified as LXI-compliant, approximately a 50% increase from 12 months ago,” said Agilent Technologies’ Von Campbell, who serves as the LXI Consortium’s president, speaking Sept 16 at Autotestcon. Over the same period, he said, instrument families available with LXI connectivity increased from 64 to 140, with 24 member companies having LXI-compliant products. He noted that LXI products cover every major product category for building a high-performance test system, with no holes or gaps in instrument capability.

Joining Campbell at the Autotestcon LXI presentation were Rob Purser, manager for test-and-measurement products for Matlab and Simulink at The MathWorks; Bob Stasonis, sales and marketing manager at Pickering Interfaces; Chris Van Woerkom, senior marketing engineer at Agilent Technologies; and Tom Sarfi, vice president of business development at VTI Instruments.

Purser noted that LXI “leverages the telecom wave” to reduce interconnect costs and ensure long-term stability for your test system. Ethernet, he said, has a 30-year history of evolution and maintains compatibility. Most LXI implementations don’t require special hardware, although Purser cautioned against buying the cheapest Ethernet cables for your instrument system. He said LXI complements current technologies, allowing you to use LXI with your GPIB, PXI, or VXI (Versa Module Eurocard-bus-extensions-for-instrumentation) system.

Despite the capabilities of Ethernet, Purser said, Ethernet alone is not enough. If you use Ethernet alone—and many instruments do have Ethernet interfaces—you’ll need a way to configure hundreds of LAN options, to discover instruments, to connect test-and-measurement software to instruments, and to coordinate measurement activities. LXI, he added, provides a standard default-LAN configuration to handle the details you would otherwise have to deal with when connecting your instruments to a LAN. To show how easy it can be to set up an LXI system, Purser at The MathWorks’ Autotestcon booth demonstrated a communications-channel test setup comprising Agilent and Tektronix instruments operating in conjunction with a PC running MathWorks’ Matlab software.

Van Woerkom of Agilent elaborated on LXI’s capabilities, noting that it provides a consistent set of LAN-communication services for test systems, supports LAN discovery for instruments, defines a standard Web page for instruments, specifies IVI (Interchangeable Virtual Instrument) drivers, requires interoperability testing for compliant products, and offers extensions for triggering and synchronization.

Dealing with your IT department
If you’ve ever dealt with your IT department, you might believe that LAN connectivity is a bug and not a feature. Speaking during the Autotestcon presentation, Stasonis of Pickering Interfaces commented on how to avoid pitfalls when connecting instruments to a corporate network and offered advice on dealing with firewalls and your IT department, which, he said, is understandably concerned with uptime and security issues.

If your automated-test-system requirements are modest, you might get by with a configuration that won’t upset your IT department, said Van Woerkom. He described a system with a PC, a router, and LXI-compatible instruments configured in an isolated subnet (Figure 1). You can let your router configure your instruments using the DHCP (Dynamic Host Configuration Protocol), although manually assigning IP (Internet Protocol) addresses and aliases can avoid problems associated with address reassignment and simplify programming. “Programs like a fixed address,” he said.

If you must communicate with instruments across a campus or around the world or if you want to provide remote access to the instruments in your lab, IT involvement is essential. According to Stasonis, you’ll need to identify the number of IP addresses you need, define a network topology that minimizes latency and simplifies discovery, and describe your anticipated network traffic with respect to bandwidth and the services and protocols you’ll be using. In addition, he said, you will want to determine where you archive test results and how to handle system updates—of virus protection, for example.

To conclude the Autotestcon presentation, Sarfi of VTI Instruments outlined the instrument characteristics of classes C, B, and A. Class C is the basic configuration, ensuring interoperability with other LXI Class C instruments. Most LXI-compliant RF and microwave instruments fall into Class C, although VTI makes Class A-compliant microwave switches. Class B instruments implement the IEEE 1588 standard to provide synchronization among instruments, each of which includes its own clock operating at a slightly different rate from its neighbors. Class B systems include one timekeeping master and several slaves, using time stamps to keep clock deltas in the 10s of nanoseconds, Sarfi said. He concluded by describing Class A instruments, which include an eight-lane M-LVDS (multipoint-low-level-differential-signaling) bus to support precise asynchronous handshaking at hardware speeds, with errors limited to propagation delays.

(Note that EDNA sibling publication Test & Measurement World presented a Webcast version of the Autotestcon presentation on Oct 13. Click here to view an archived version of the Webcast.)

From bench top to PXI
If you plug all your instruments into one backplane, you have no need for IEEE 1588 synchronization or a separate hardware bus. “Suppose you want to synchronize two digitizers,” said David Hall, National Instruments’ RF-and communications-product-marketing engineer. “In the old oscilloscope approach, you connect some cables on the back. With the PXI approach, you just say 'use PXI trigger line 1,’ and that’s all there is to it. From a timing and synchronization point of view, there are some inherent advantages to all the modules having access to a common digital bus.” Hall elaborated on PXI’s benefits for communications system design, particularly for measurements involving MIMO (multiple-input/multiple-output) radios. With traditional instrumentation, he said, “The way you would make those measurements is you would buy two vector signal analyzers and connect cables on the back end and hope that the LOs [local oscillators] are synchronized. In the PXI approach, the modules are inherently synchronized simply because you put them in the same chassis.”



Hall cited another communications example. Attendees at ION GNSS 2009, the Institute of Navigation’s Global Navigation Satellite System meeting, which took place Sept 22 through 25 in Savannah, GA, were attempting to build better GPS (global-positioning-system) receivers. As part of their efforts, Hall said, they take PXI systems into the field and acquire raw satellite signals from the air. The PXI backplane is both a command bus and a data bus, and PXI Express can stream acquired I/Q (in-phase/quadrature) samples in real time “until you fill up a hard drive,” he explained. You can then use the stored samples to exercise and optimize prototype receivers.



To emphasize NI’s commitment to ensuring that PXI hardware, along with NI’s LabView, can serve communications applications, NI fellow Mike Santori said at an Aug 5 NIWeek presentation that NI had created an internal R&D team of communications experts. To put the NI hardware and software through its paces, team members developed an LTE base-station emulator.

Team member Ian Wong, a senior RF-communications-software engineer at NI, brings to the team academic and industry experience in meeting communications-design challenges. For NIWeek attendees, Wong described the challenges that LTE presents (Reference 2). LTE, he said, when it rolls out next year, will support 300-Mbps data rates, versus 500 kbps for the EDGE (enhanced-data-for-global-system-for-mobile-communication-evolution) technology in common use today. An LTE base station will execute 200,000 2048-point FFTs (fast Fourier transforms) per second and will include a 300-Mbps turbo decoder, all adding up to performance of trillions of operations per second. Wong’s team built the base-station emulator using a PXI Express system that includes a real-time dual-core controller, which communicates with a PXI IF (intermediate-frequency)-transceiver FPGA board that in turn performs physical-layer processing for the LTE base-station transmitter. A PXI RF upconverter delivers the output to a device under test. Signals from that device are routed to an RF downconverter and on to the IF-transceiver FPGA board.

A matter of taste
If you are a bench-top-instrument user in an application that both LXI and PXI can serve, your choice of platform when moving to automation may center on your experience, your comfort level, and your age. Hall at NI said that he learned to use an oscilloscope in college, and the point was for the instrument to provide him with diagnostic information that would enable him to make decisions. With the proliferation of PCs into test applications, it becomes important to get measurement information into a PC and use the PC to control the instruments and assist in decision-making. Although LXI and GPIB permit automation of bench-top instruments, “that wasn’t the primary-use case” for which the instruments were originally developed, he said.

Hall acknowledged that users like the feel of knobs and buttons and that users approaching instrument automation for the first time can find the idea of programming scary. He studied computer engineering in college and still finds the task of programming an instrument in C somewhat difficult. The graphical LabView language, he said, simplifies the task, and the company provides many sample programs to help new users get started.

Christopher Ziomek, president of ZTec Instruments, which makes digitizers in formats including PXI and LXI, said his inclination as an engineer would be to grab a Tektronix scope if he wanted to make a measurement in a lab. He noted some practical considerations. For example, if you want to make measurements while power-cycling a PXI scope under test, you’d need two PXI chassis: one for the scope under test and one for the test equipment. “I’m more comfortable with something that has knobs,” he said, even though ZTec has developed its Zscope software, which mimics a traditional oscilloscope’s front panel on a computer screen.

ZTec’s younger engineers are more comfortable with the modular format, Ziomek added, and are happy running Zscope on tablet PCs. Modular instruments can also enhance productivity. Given PXI or LXI instruments and a computer interface, engineers are more likely to automate a measurement that they would otherwise manually repeat many times if restricted to a traditional bench-top instrument.

Ultimately, there will be room for a variety of instrument formats, a fact that vendors will take advantage of. Tektronix, for example, maker of the traditional engineering scope that Ziomek favors for lab work, said at NIWeek that it was teaming up with National Instruments to develop a 10G-sample/sec, 3-GHz PXI digitizer. Craig Overhage, chief technology officer of Tektronix, said that the company had been looking at how to bring its high-bandwidth, high-sampling-rate oscilloscope technology to customers who prefer a modular-instrument architecture. Overhage said he sees different use cases for PXI scopes and traditional scopes, and he doesn’t expect the introduction of the new PXI scope, which NI will market, to affect sales of the company’s popular traditional instruments.Author Information
You can reach Editor-in-Chief Rick Nelson at 1-781-734-8418 and rnelson@reedbusiness.com.

References
1. Nelson, Rick, “LXI speeds gigahertz measurements,” Test & Measurement World, November 2007, pg 49.
2. “Prototyping Complex Communications Systems,” National Instruments, NIWeek video, Aug 12, 2009.
Caption

Figure 1: You can buy a router to connect your PC and your LXI instruments to your corporate network in a way that won’t upset your IT department, according to Chris Van Woerkom of Agilent Technologies.

Table 1

LXI instruments extend well into the microwave region. A recently introduced example is the Rohde & Schwarz R&S ZVA67, a 10MHz to 67GHz vector network analyzer that is LXI Class C-compliant. The instrument features a 110dB dynamic range at 67GHz and can serve in R&D as well as production applications.

At Autotestcon, The MathWorks demonstrated the test of cable impedance characteristics using Matlab software and LXI-compliant instruments from Agilent and Tektronix.

National Instruments offers PXI hardware and software-defined instrumentation capabilities to enable the test of multistandard communications devices, including support for GPS, Bluetooth, Wi-Fi, and WiMAX.