Modular measurement instruments provide plug-in test power, along with the capability to add functions as needed. In the past, modular instruments were too limited in terms of available functions to entice RF/microwave engineers from abandoning their standalone instruments. But the industry has adopted several "universal" interface formats, and the test capabilities of the available modules are getting better. This makes modular test options quite attractive for both research laboratories and production lines.

A modular instrument is usually based on a mainframe unit and standard-sized modules that slide in or plug into the mainframe and provide the test personalities for the combination. In contrast to standalone instruments, which each have dedicated power supplies and control software, the modules derive power and control from the mainframe. This sharing of resources allows the modules to be made smaller and less expensive than standalone instruments for comparable capabilities and performance.

Early modular instruments were plagued by compatibility issues. An automatic test system built from standalone instruments requires synchronization and control over an instrument bus, such as the long-popular General Purpose Interface Bus (GPIB), for a controlling computer and its software to recognize each instrument and for commands from the computer to be acknowledged by each module. Similarly, a modular instrument system requires the same synchronization and communication, but all within a single mainframe and its modules.

One of the earliest ventures into modular microwave instrumentation was by Hewlett-Packard Co. (now Agilent Technologies) in the 1980s with a line of products that comprised their Modular Measurement System (MMS). The initial MMS mainframe and modules formed a powerful microwave spectrum analyzer, and the format was effective in terms of module compatibility and high RF/microwave performance levels. But it was a proprietary interface format that was only supported by a few companies. In the end, it became a one-company effort at a modular measurement solution.

It was only when the electronics test industry in general realized that an industry-wide interface format was needed before modular instrumentation would ever gain widespread acceptance. That first modular instrument standard was the VME eXtensions for Instrumentation (VXI), which was based on the VMEbus computer bus standard. This would eventually be followed by other modular instrument standards, including one based on a local-area-network (LAN), the LAN eXtensions for Instrumentation (LXI), and several based on the Peripheral Component Interconnect (PCI) computer bus standard, including PCI Express and the PCI eXtensions for Instrumentation (PXI) open standard. Several years ago, the AdvancedTCA Extensions for Instrumentation and Test (AXIe) was started by Agilent Technologies, Aeroflex, and Test Evolution Corp. as a high-performance open standard based on the AdvancedTCA (ATCA) standard. With more than 70 member companies, and the support of the PXI Systems Alliance, PXI has in recent years become the most popular of the modular instrument interface standards.

Perhaps the reason for PXI's popularity is its combination of performance and flexibility. A host computer for a PXI test system can be contained within the mainframe, or can be in a separate personal computer (PC). The internal buses in a PXI system are the same as a PC, such as a PCI or PCI Express bus, and a standard PXI mainframe occupying a 4U height in a 19-in. rack-mount chassis can provide as many as 18 PXI card slots. It can accommodate a large number of instrument functions in a housing about the size of a typical single-function stand-alone instrument. With its shared reference clocks and triggers, a PXI test system can achieve tight synchronization among its many modules, with less than 200-ps timing skew.

According to market research firm Frost & Sullivan, PXI is currently the dominant standard for modular instrumentation, supplanting predecessor VXI in terms of popularity. But a newer, emerging standardAXIemay be the top modular test instrument bus standard in the years to come. The company notes that the worldwide market for modular instrumentation saw revenue of just over $524 million in 2010, at a growth rate of 17.1% compared to revenues in 2009. Frost & Sullivan projects continued growth of modular instrumentation, at a compound annual growth rate (CAGR) of 8.8% between 2010 and 2014.

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AXIe offers some advantages over PXI in terms of power per slot (200 W to 30 W) and circuit-board space per slot (900 cm2 to 160 cm2). Supported by the AXIe Consortium, Inc., it is extremely flexible, since it includes PCIe and LAN interfaces, allowing AXIe instruments to act like virtual PXI or LXI instruments. And some noteworthy manufacturers, such as Keithley Instruments, have publicly supported the standard with promises of future product developments. But available commercial products based on the AXIe open standard are still limited, outside of Agilent Technologies.

Still, the products that are available based on AXIe, like the model M8190A arbitrary waveform generator from Agilent, are impressive. The M8190A can produce a 5-GHz analog bandwidth, and offers 12 b resolution at rates to 12 GSamples/s and 14 b resolution at rates to 8 GSamples/s. It achieves a spurious-free dynamic range (SFDR) of −80 dBc and provides signals at output levels from −10 to +10 dBm. Its pulsed (time-domain) performance is exceptional, with 20%/80% rise-time transitions of 50 ps. The arbitrary waveform generator can operate at sample rates from 125 MSamples/s to 8 or 12 GSamples/s.

Yet, compared to the sheer number of test instruments now available in the PXI format, AXIe has a long way to go. Companies well established in the PXI market, such as Aeroflex and National Instruments, offer complete suites of instruments, allowing engineers to assemble a virtual test system within a single mainframe. As an example, the model 3036 PXI RF digitizer module (Fig. 1) operates from 250 kHz to 13 GHz with a 1-dB capture bandwidth as wide as 90 MHz. It features 13-b analog-to-digital-converter (ADC) resolution and occupies just four PXI slots in a mainframe. The company provides its PXI Studio software application with the 3036, for performing vector signal analysis of complex modulated signals. By running other software programs, the measurement "personality" of the module can be easily modified.

One of the better known of National Instruments' PXI modules is the model PXIe-5630 two-port vector network analyzer (VNA). The compact module (Fig. 2) delivers measurement capability once associated with a large rack of instruments. It operates from 10 MHz to 6 GHz with 10-Hz resolution and a −123-dBm/Hz noise floor. The VNA can gather test data from 1 to 3201 points, using intermediate-frequency (IF) bandwidths from 10 Hz to 30 kHz. The full-span measurement speed with a 30-kHz IF bandwidth and 801 measurement points is only 283 ms.

National Instruments has worked with Tektronix to develop its model PXIe-5186 3U PXI Express digitizer module. It features a 5-GHz bandwidth and 12.5-GSamples/s sampling speed. The module leverages Tektronix's application-specific integrated circuits (ASICs) for its lines of high-performance oscilloscopes, and fabricated with an advanced silicon-germanium (SiGe) semiconductor process from IBM.

Some companies "hedge their bets" on the direction of future markets by supporting several standards. Phase Matrix, for example, offers instruments in both VXI and PXI module form, with frequency downconverters in PXI form, a 20-GHz signal generator in VXI format, and a 9-GHz local oscillator (LO) in a choice of either format.

In addition to these test instruments based on a mainframe/module configuration, a growing number of RF/microwave test instruments and even components are incorporating USB ports for ease of connection to a PC, including the CS8 wireless device test system developed by Spirent Communications and reviewed elsewhere in this issue. And to assist companies in adding that capability, LeCroy now offers test suites for performing serial data testing per the latest USB 3.0 and PCI Express 3.0 high-speed interface standards.