What's the Difference Between Modular and Rack-Mount Instruments?

Oct. 29, 2015
A growing number of PXI/PXIe modular RF/microwave test instruments provide the measurement capabilities and performance levels once possible only with benchtop instruments.
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1. This model FSUP signal source analyzer from Rohde

Assembling a test system at one time involved the acquisition of a 19-in.-wide rack-mount enclosure and many different rack-mount, benchtop instruments. Separate benchtop instruments, such as signal generators, spectrum analyzers, vector network analyzers (VNAs), frequency counters, and power meters, would be installed in the rack to form a test system that might then require specialized software to make everything work together. Today, an entire test system can be assembled by sliding compact instrument modules into a single chassis, most often based on the PCI eXtensions for Instrumentation (PXI) or PXI Express (PXIe) modular instrument formats.

While traditional benchtop RF and microwave test instruments are not going away anytime soon (Fig. 1), modular test instruments (Fig. 2) continue to gain ground in many research and production test applications. A number of leading instrument suppliers now support modular instrumentation with instruments in PXI/PXIe as well as AdvancedTCA Extensions for Instrumentation and Test (AXIe) formats. But how different are these instrument modules from traditional benchtop versions?

Modular Evolution

Modular instruments are not new. Approximately 30 years ago, instruments were designed and manufactured using the VME eXtensions for Instrumentation (VXI) modular format. This was largely driven by military requirements for a flexible and compact modular instrument technology that could fit into tighter places than possible with conventional benchtop instruments. In fact, VXI test instruments, which communicate to a controller such as a personal computer (PC) via the format’s VXIbus control bus, are still in use throughout the industry and supported by firms like VTI Instruments and Teradyne.

PXI/PXIe and AXIe represent the latest modular instrument formats, capable of impressive performance while providing the flexibility of changing test functions by changing slide-in modules in a chassis. How different are these modular instruments than traditional benchtop instruments? The most obvious difference is in size and the space required on a factory floor for standard rack-mount-based instruments compared to PXI Express (PXIe) or AXIe instrument chassis and modules. As one difference, each standalone instrument has its own power supply, versus instrument modules that draw their power from a modular instrument chassis. Of course, if problems exist in the power-supply rail of a modular instrument chassis, all of the modules in that chassis will be without power, whereas any problems in the power supplies of standalone instruments will occur on an instrument-by-instrument basis.

In terms of functionality, modern modular test solutions offer all of the measurements possible with benchtop instruments, including oscilloscopes, but at a fraction of the size of a benchtop instrument. The modular format of PXI/PXIe instruments makes it possible to assemble a full system, with test signal sources and analysis equipment, within the same chassis.

The modules communicate to each other and to a chassis-mounted controller or external PC by means of the format’s PCI/PCIe communications bus.  To test the interconnectivity of PXI/PXIe modules in a chassis and PCI/PCIe bus, one need only refer to Microsoft Windows Device Manager (WDM) software on a PC for the current status of the modules (whether they appear on the computer screen).

Modular or Benchtop?

At the system level, one key advantage that modular instrument formats have over benchtop instruments is the ability to change measurement limits. For example, a test system with a microwave VNA PXI/PXIe module that, say, operates from 10 MHz to 6 GHz could be readily extended to 26.5 GHz by substituting a VNA instrument module with the expanded bandwidth. Of course, if the modular test signal source was initially limited to 6 GHz, another signal-generator instrument module would be needed for testing to that higher frequency limit.

In a similar scenario with a benchtop-instrument-based test system, two complete test instruments must be replaced with two new benchtop instruments. Rather than replace just essential measurement functions while maintaining common functions, such as the power supply and chassis enclosure, entire instruments, with their enclosures, power supplies, and display screens, must be replaced, with significant differences in overall replacement costs.

The value of benchtop RF/microwave test instruments for both research and production testing is without question, especially for applications with well-established limits in terms of frequency range, bandwidth, power levels, and other key operating parameters. But for applications such as fifth-generation (5G) wireless devices and circuits, which will use multiple channels and relatively exotic modulation format (in addition to other performance parameters being explored), it may be possible to exceed the limits of a benchtop-based test system with a solution requiring replacement of complete test instruments. Upgrading a modular test system is simply a matter of replacing one or more slide-in test instrument modules.

No Speed Limit

The importance of measurement speed has never been greater as wireless technology extends into new applications like the wireless automobile and wireless communications for the Internet of Things (IoT). The switching and tuning speeds possible in modular test instruments makes them likely candidates for production testing, especially where measurement throughput is a concern.

2. The compact format of PXI/PXIe instruments, such as these signal generators, makes it easier to meet requirements for multichannel communications testing in a compact chassis. (Photo courtesy of Keysight Technologies)

Achieving high measurement speeds in high-volume production testing can create considerable differences in the cost of an end-product, when the cost of test time must be added to the costs of hardware and manufacturing. Even seconds difference in test time can substantially change the price for an electronic product. Moreover, in certain high-volume wireless markets, longer or shorter test times can make the difference between a final product being priced competitively or not.

For test applications where fewer devices or even one device is tested at one time, benchtop instruments can provide a greater degree of familiarity for users accustomed to reaching for front-panel instrument controls versus the PC-based software that controls modular instruments. In an automatic-test-equipment (ATE) system, a rack of benchtop instruments may be controlled by a PC with software. However, for simple applications, such as testing the response of a single RF/microwave amplifier or filter, a simple test setup with a small number of manually operated benchtop instruments can provide all of the functionality and ease of use required for that particular application.

Modular Formats

The PXI/PXIe modular instrument format, the most popular of the current modular instrument formats, is quickly gaining in popularity compared to other modular instrument formats and even compared to benchtop instruments. The AXIe standard, often referred to as “the big brother of PXI/PXIe,” is an advanced modular instrument format that’s compatible with PXI/PXIe.

AXIe offers some additional features in support of high-performance measurement applications, though. For example, AXIe incorporates a single-rail power-management approach that allows for efficient cooling of its modules. An AXIe chassis can be equipped with from 1 to 14 slots for ease of scalability; its modules can be mounted vertically or horizontally in a chassis. The modular format uses the Intelligent Platform Management Interface (IPMI) for control of single- and multiple-chassis systems, to enable the assembly and control of large, complex multiple-chassis test systems. Two versions of the standard are currently available: AXIe 1.0 for general-purpose testing and AXIe 1.1 for semiconductor testing.

Two of the leading suppliers of PXI/PXIe instrument modules are National Instruments and Keysight Technologies. Both now offer hundreds of different instrument modules and chassis. One difference between the two firms is Keysight’s long legacy as a benchtop instrument supplier, under the guises of Agilent Technologies and Hewlett-Packard Co. prior to that.

Those firms are not alone, though. With the growing popularity of PXI/PXIe modular instruments, many companies known for their benchtop instruments, such as Anritsu, Rohde & Schwarz, and Tektronix, now offer modular versions—and a real choice in test formats.

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About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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