Leveraging test platform diversity

Of the four “XI” test instrumentation platforms—VXI, PXI, LXI, and AXIe—and USB, PXI often is chosen for a number of reasons. National Instruments’ (NI) David Hall, principal product manager-RF test systems, cited PXI’s use of PC technologies. He said, “In PXI, off-the-shelf PC technologies such as multicore CPUs and PCI Express backplanes are shared with the modern computing industry .… The technical capabilities of modern PXI systems leverage decades of investment in the PC industry—and will continue to do so in the future. In contrast to PXI, technologies such as USB, LAN, and AXIe each have their merits but are less ideal for high-performance instrumentation.” For RF instrumentation, PXI offers a “small footprint, high data throughput, and the capability to execute real-time signal processing,” Hall added.

Keysight Technologies also highlighted the choice of PXIe for new RF modules. Among the advantages listed by the company’s Andrew Smail, product and solutions marketing manager, were the high-performance backplane and multichannel scalability. As Smail explained, “The existing RF power amplifier/front-end module characterization and test Reference Solution primarily was built with PXI equipment …. So, it was natural to develop the [M937xA] VNA and [M9195A] Digital Stimulus/Response [unit] using the PXI form factor. Other formats, such as LXI, did not provide the required data bandwidth, scalability, and size advantages provided by PXIe.”

Nevertheless, even though PXI and PXIe are popular formats for modular RF instruments, they are not the best solution for every application. VTI Instruments’ Tom Sarfi, director of product management and support, discussed the reasons that the company used a specific format for each of three switch modules recently developed.

“Each module was introduced to address different needs,” he said. “VXI has the largest available board space and was best suited to address a large-scale military test application that required a big 200-V matrix. We were able to fit more than 3,000 relays capable of switching 200 V and 2 A in four VXI slot spaces.” The SMP-1200 is a double-wide C-size subchassis that houses up to six SMP switch modules. Two of these subchassis housed sufficient switches to solve this application.

“Our LXI-based EX1200-3824: (8) 1×24 one-wire multiplexer was used in an electronic control unit test system,” he continued, “and incorporated onboard attenuators and filters to maximize signal integrity between the UUT and the digitizers that were connected to the switch. The switch module is designed with 100-V solid-state relays that could interface directly to the ECU I/O.”

Sarfi concluded, “Our PXI switching is designed on the PXIe platform. One of the switch modules [SMX-3276] we introduced last year is a high-density 300-V multiplexer that includes configuration relays onboard to allow it to be used as a large two-wire multiplexer or two individual banks for four-wire measurements.”

Mike Dewey, director of marketing at Marvin Test Solutions, discussed the advantages associated with the larger 6U PXI format. “One of our newest products,” he said, “is our GX7016 PXI switching subsystem which leverages the flexibility of the PXI platform. This subsystem, which consists of a 6U PXI chassis and various switching cards, features internal cross-connect buses, high bandwidth signal routing, and high-density matrix and multiplex switching as well as an integrated Scout receiver interface, eliminating potentially hundreds or even thousands of wires.”

Depending on the application, size is important. A 6U C-size VXI board measures 233-mm high x 340-mm deep. In contrast, a 6U PXI board is 230-mm high x 160-mm deep. Clearly, the 6U VXI board area is about twice as large as a 6U PXI board because of VXI’s much greater depth and about four times larger than 3U PXI. AXIe, the latest format to be adopted, was derived from the ATCA architecture with a 320-mm x 280-mm area—about 13% larger than 6U C-size VXI.

Taking a pragmatic approach, Bustec uses carrier boards to adapt a range of function cards to either LXI or VXI solutions, whichever better suits the application. Fred Blönnigen, the company’s CEO, referred to the “Lego-like” fashion in which systems are built. He explained, “… every new function card or signal conditioning unit is always [compatible] for both VXI and LXI standards.”

Expressing a similar view, VTI’s Sarfi said, “One thing we preach in our product marketing and development process is to be platform-agnostic. We look toward modular architectures to increase flexibility and integrate LAN/LXI where it makes sense, but each platform has strengths that resonate with different applications.” He concluded, “The board space available for VXI allows us to address very large-scale switching or power-hungry applications whereas PXIe is an ideal compact form factor for data acquisition applications that require high data throughput rates.”

For people already using VXI, such as VTI’s and Bustec’s military/industrial customers, the C-size board area is useful. However, a bigger consideration often is compatibility with the large VXI investment already in place. AXIe may develop into a popular large format for test applications, but the original -1 implementation is expensive, and the -0 simplified version was introduced only a year ago. Nevertheless, AXIe-0’s board area is suitable for modular switching applications that do not need the extreme speed and additional features of AXIe-1.

2014-2015 Activity

During the last year, a wide range of “XI” and USB products was introduced. Data Translation’s president, Fred Molinari, said the USB VIBbox was one of the more important introductions for the company. He explained, “It is an outgrowth of our modular products for applications that require noise, vibration, and acoustic measurements. It can directly connect up to 64 channels in a rugged, portable instrument.”

Figure 1. VIBbox for noise, acoustic, and vibration measurement
Courtesy of Data Translation

The VIBbox (Figure 1) houses up to four 16-channel DT9857E modules, each with a separate 24-bit delta-sigma ADC per channel, a sampling rate up to 105.4 kHz/channel, a 32-bit counter, eight input and eight output digital lines, and two 32-bit analog outputs with two-pole Butterworth filtering. A separate external power supply is included. Molinari concluded, “Customers needed the ability to measure many points on a large structure, such as aircraft wings, large industrial machines, and turbines. Thus, the VIBbox modular instrument was developed to handle these 64-channel applications. Additionally, we added the capability to expand to an even larger number of channels beyond 64 by being able to synchronize four VIBbox instruments to achieve 256 channels of vibration measurement … all operating in parallel and synchronized to the master.”


The company’s LXI MEASURpoint range of temperature measuring instruments now includes a model rated for up to 5,000-V common-mode spikes with continuous working at 3,500 V. Small thermocouple or RTD signals can be accurately measured in the presence of large common-mode voltages because of the unit’s true galvanic isolation. Models based on USB instead of LAN also are available.

Figure 2. DN2.445-4 digitizerNETBOX
Courtesy of Spectrum Instrumentation

Spectrum Instrumentation has developed nine new models of the DN2.44x digitizerNETBOX series (Figure 2), also based on LXI. As described by the company’s technical director Oliver Rovini, “The new digitizerNETBOX products offer sampling rates from 1.25 GS/s up to 5 GS/s and bandwidths from 500 MHz to more than 1.5 GHz. Available with two, four, or eight channels, the units come with large acquisition memories up to 8 GS, which make them ideal for capturing long, complex high-frequency signals and characterizing fast timing events that go down to the ns and sub-ns ranges.”

He continued, “The digitizerNETBOX products facilitate remote control and data transfer over fast Gb Ethernet. They include Spectrum’s SBench 6 software that allows full instrument control, graphical display, data storage, analysis, and report generation. The program offers both oscilloscope and transient recording modes, including continuous data streaming. A special feature of SBench 6 is the segmented acquisition view, which is ideal for capturing burst-type signals together with all the associated trigger timestamping information. SBench 6 can be used to measure parameters, perform FFTs, and run a variety of different math and processing functions such as filtering, averaging, and histograms. Data can be exported into a number of formats such as ASCII, Wave, and MATLAB.”

Several vendors mentioned LXI in conjunction with the need for a remote solution—for example, Pickering Interface’s LXI-based Model 65-110 wideband matrix used to monitor signals at CERN.1 Because LXI uses Ethernet for communication and control, long distances between the instrument and the PC can be accommodated. VTI’s Sarfi commented, “Adding Ethernet to the mix allows us to distribute measurements closer to test articles, which is becoming a more prevalent requirement in test-cell applications to reduce analog signal cable length.”

In another example, Bustec’s Blönnigen described use of the company’s ProDAQ 3424 ADC cards and ProDAQ 6100 carrier in a wind-tunnel application. He said, “The reason for this selection was twofold and resulted from the main requirement, which was measurement precision. The user wanted to place the equipment as close as possible to the test object to make the connection cables between the sensors and the data-acquisition units as short as possible, thus preventing electromagnetic noise pickup. Therefore, a classical test setup in one rack was excluded.” Regarding the second reason, Blönnigen said that the combination of the two ProDAQ units provided 0.005% accuracy.

The company also developed the ProDAQ 5716 16-channel bridge signal conditioning unit with 350-kHz bandwidth and less than 0.05% excitation error. The 5716 and all other Bustec ADCs and signal-conditioning function cards feature differential inputs with >10-MΩ input impedance on both sides. In addition, as Blönnigen explained, “all 16 channels can be individually configured by software to act either in bridge mode, as a voltage input, or as an IEPE/ICP input. For this unit, both TEDS class 1 or class 2 are supported.” Although this product is included under the LXI heading, a VXI version also is available.


Figure 3. M9195A PXIe digital stimulus/response with PMU unit
Courtesy of Keysight Technologies

Within the last year, Keysight launched five separate PXIe instruments and the M937xA series of single-slot PXIe VNAs, which has six models with upper frequency limits of 4 GHz to 26.5 GHz. The company’s Smail said, “The M937xA PXIe Vector Network Analyzers, introduced in September 2014, are full two-port vector network analyzers that fit in just one slot. … Each module is a completely independent analyzer that also can be cascaded to measure multiport devices. Because all ports are fully synchronous, multiple ports can be simultaneously measured and multiport error correction applied. As an example, a single chassis containing 16 M937xAs can be configured as eight four-port VNAs, four eight-port VNAs, or one 32-port VNA.”

He continued, explaining the benefits of the M9195A PXIe digital stimulus/response with PMU unit (PXI DSR) shown in Figure 3 and introduced last February. He said, “[The DSR] provides industry-leading technology for powerful on-the-fly pattern creation, flexible edge placement, and advanced per-vector timing capabilities. The PXI DSR was specifically designed to provide digital stimulus/response testing of power-amplifier and front-end module semiconductors during device characterization or production. It typically is used to emulate serial buses including RFFE and SPI.”

Figure 4. PXIe-5668R signal analyzer
Courtesy of National Instruments

NI also was active, introducing eight instruments, the PXIe-1085 24-GB/s chassis, and a couple of multicore controllers. Hall specifically commented on the large 765-MHz instantaneous bandwidth of the PXIe-5668R signal analyzer shown in Figure 4. He said, “A range of applications requires extremely wideband signal analysis. For example, in wireless communications, newer standards such as LTE Advanced and 802.11ac use up to 100 MHz and 160 MHz of instantaneous bandwidth, respectively. In addition, when testing power analyzers designed for LTE Advanced and 802.11ac, the use of digital predistortion technology requires signal generators and analyzers with up to 3x the signal bandwidth. Of course, both of these applications often involve harmonics measurements up to 26.5 GHz.

“In addition to RFIC testing, the new signal analyzer’s wide bandwidth also is ideal for radar device testing,” he continued. “Modern radar pulses frequently use pulse compression techniques to improve range resolution and/or range accuracy. However, sophisticated pulse compression techniques also increase the bandwidth of the radar signal. With up to 765 MHz of instantaneous bandwidth, the new signal analyzer is capable of testing increasingly sophisticated ultra-wideband radar systems.”

Format considerations

If space is important in your application, 3U PXI/PXIe must be the form factor to consider first. Years ago, the small board area available in a 3U module severely restricted PXI RF/microwave solutions. Today, CAD supports very complex module designs that leverage high-performance semiconductors. Other than coaxial switches, which remain relatively bulky and may occupy multiple slots, many 3U single-slot RF/microwave modules are available.

For test systems that need the highest speeds and many channels, PXI’s size and 30-W/slot power can be limitations. AXIe’s large board area suits switching very well, and perhaps the lower cost AXIe-0 modules will be used for that purpose. However, the original AXIe-1 specification supports multi-GB/s module-to-module local bus communication as well as very high power and facilitates extremely fast ADCs and memory subsystems often needed for high-energy physics investigations.

Because the standard is relatively new and the modules address specialized applications, the number of products being developed is much less than for PXI/PXIe. As an example, Keysight introduced three AXIe instruments during the last year: the four-channel M8195A arbitrary waveform generator with up to a 65-GS/s sampling rate and 20-GHz bandwidth; the M8020A J-BERT for single- or multilane devices with data rates up to 16 Gb/s or 32 Gb/s; and the 32-channel, 1-GS/s M9709A 8-bit digitizer.

Electronic test systems more often turn to LXI to accommodate larger board areas as well as higher power requirements because neither physical size nor power is specified by the stand­ard. Further, as commented upon by several vendors, Ethernet allows LXI modules to be positioned close to the signal source, even if it is miles from the controlling PC.

USB is a useful protocol, perhaps best suited to low-power solutions that can be directly powered via the USB cable. Nevertheless, several USB ports are provided on most PCs, so it’s generally easy to use and has found its way into many consumer applications as well as several PC-based instruments.

Finally, VXI is not entirely going away any time soon, but neither are vendors making significant investments in it. Gradually, PXI- and LXI-based technology insertions are both reducing VXI’s dominance in MIL/aero test systems as well as enhancing the capabilities of those systems.

Lecklider, T., “Difficult signal selection,” EE-Evaluation Engineering, July 2015, pp. 8-11.

For more information
Data Translation
Keysight Technologies
Marvin Test Solutions
National Instruments
Spectrum Instrumentation
VTI Instruments

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