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Synthetic instrumentation (SI) offers a different approach to test and measurement. It uses a collection of basic hardware and software building blocks in a flexible, open, and modular architecture to synthesize the stimulus and measurement functions required by a given test application. So, why do we need it?

As electronic products become increasingly integrated and dependent on software, testing procedures get more complex and time-consuming. Instruments have kept pace with these testing needs, but costs have soared since test systems are built for singular, special needs.

Automatic test systems (ATSs) speed up and simplify testing, as well as automate the process and squeeze out some test costs, especially in high-volume devices like cell phones. To some extent, modular test systems using the PXI bus have helped along these lines. However, many test systems capable of producing the desired cost and throughput goals become dead-end investments that can’t be used for other current products or even next-generation products.

This problem of investing in new test systems for just one product has become particularly acute in the military and aerospace sector—so much so that the government issued guidelines to create a more modular, generic, and flexible test approach that can be quickly reconfigured for different products and systems. Say hello to synthetic instrumentation.

While the early systems adopted by the aerospace and military proved successful, the techniques have yet to significantly impact the consumer and commercial sectors. Nonetheless, SI shows lots of promise. Its usage continues to grow, costs are on the decline, and more companies are beginning to build test products. As a result, it’s likely that SI will eventually impact your own world.

CLARIFYING THE DEFINITIONS
You know what traditional instruments are—multimeters, oscilloscopes, signal generators, and vector signal analyzers. These benchtop units are devoted to a specific purpose, such as measuring voltage or power, observing voltage waveforms versus time, generating test signals, looking at power versus frequency, or performing a complex modulation analysis.

In many cases, these same instruments or modified versions thereof also become part of production test systems. Rackand- stack traditional instruments do their jobs well, but they must be organized, sequenced, and programmed to carry out their task. GPIB, LXI, and other interconnections ensure communications between instruments, and specialized software organizes the measurement process and data collection.

Once the test system is successfully built, its performance is superior. But on the downside, as mentioned, the system typically targets just one job. Also, it often contains redundant elements (multiple displays, keyboards, digitizers, frequency converters, etc.), which results in an expensive, larger, and heavier system. Changing the applications requires new interconnections and programming to use the same equipment in a new test system.

The instruments’ high cost and the time involved in reconfiguring and preprogramming is significant. A system made to test radar sets for the Air Force can’t be used to test missile hardware for the Navy, though many of the same tests may need to be run. As the largest customer of T&M equipment in the world, the Department of Defense (DoD) wants to change that to save money and extend the life of test systems.

FROM NI’S VI TO SI
In the mid-1980s, a new approach called virtual instrumentation (VI) came along. Virtual instruments are built-in software executed on a PC or laptop. National Instruments (NI), the inventor of VI, defines it as a software-defined system in which software based on user requirements defines the functionality of generic measurement hardware.

The instrument is built around modular I/O and data-conversion hardware using PXI modules, while software does all of the processing and measuring. Even the front panel of the specialized instrument with its display and controls is softwaredefined. Software programs the system.

NI also created the software known as LabVIEW. It makes programming a snap by using a graphical user interface with icons and interconnections in a dataflow format. Because the software actually carries out the measurement, the instrument is easily customized and rapidly changed.

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VI has revolutionized the test and measurement business. Still, not all applications are a good fit, which is why traditional bench instruments continue to be very popular in design and development.

When the software approach is used in traditional instruments, though, they become more flexible for a wider range of tests. For instance, many traditional instruments work well in a maintenance and service environment, but tend to be too expensive for some specialized instrumentation applications.

For data acquisition, VI is ideal due to its modularity and flexibility. What’s needed is a version of VI that’s more generic, modular, and reconfigurable that can be reprogrammed and used in a wider range of applications, from production test to service and repair. This is where SI steps in.

The Department of Defense conceived the SI concept in the mid-1990s to reduce test instrument costs, extend the life of test systems, and make them more flexible and useful. The department defined its Next Generation Automatic Test System (NxTest) in its report to Congress in 2002. It set these goals for ATS to be used in the military:

• Reduce total cost of ownership.
• Reduce the logistics involved in supporting test systems with fewer spares and training.
• Reduce the time needed to develop and deploy new test systems.
• Increase flexibility by creating systems that are useful in multiple applications and are interoperable with one another.
• Improve the quality of the testing process.

The Synthetic Instruments Working Group (SIWG), an organization that comprises representatives of the DoD, equipment vendors, and contractors, was formed in 2004. It defined a solution and set of standards that could meet these goals. Also, it defined SI as a reconfigurable system that interconnects a group of elemental hardware and software components via standard interfaces to generate signals or make measurements using numeric processing techniques.

The focus is on modular and flexible components that can be quickly added, removed, updated, and reconnected to quickly and easily make changes and updates. Initially, the SI emphasis is on RF/microwave equipment, because that’s the largest part of the military’s arsenal of electronic gear. It includes radio communications, radar, satellites, electronic warfare systems, missile guidance, and remote reconnaissance.

The basic SI architecture has a signal input path and signal output path (Fig. 1). Input signals to be measured undergo some signal conditioning and scaling, usually to a downconverter that translates the signal to a lower intermediate frequency (IF) before it reaches the analog-to-digital converter (ADC).

The digitized signal is then stored in memory. The numeric processor employs that data file using various digital-signalprocessing techniques to make the desired measurement. The processor may be a PC, FPGA, or DSP. The software defines the measurement that would emulate an oscilloscope, spectrum analyzer, vector signal analyzer, or other instrument. The front panel is often omitted.

The other signal path handles signal generation for device-under-test (DUT) stimulus. Data to be transmitted in baseband form is processed as required and then sent to an arbitrary waveform generator (AWG), where the final signal is created. This signal at some IF is then upconverted to the final RF output. Note that all of the different modules have control lines that allow them to be switched in or out, reconfigured, and rerouted to fit the application.

With this flexible architecture, almost any test, measurement, or signal-generation need can be met. The hardware modules’ design allows for easy replacement with newer, more capable units as the technologies improve. Since no front panels or control panels are needed, the entire test system is smaller and lighter, uses less power, and is far more mobile, which is a necessity in military applications like repair depots, flight lines, and battlefields.

The hardware part of SI is probably the simpler part of making a system. The massive software development effort, on the other hand, can be difficult and expensive. That’s why the progress of SI has been rather slow.

While the military and government aerospace companies can afford the effort, that software burden doesn’t fit the budgets of most commercial equipment vendors or their customers. Eventually that burden will lessen, and you’ll likely see SI emerge as a desirable and viable ATS option. Cell phones are a potential target.

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REAL SYNTHETIC TEST PRODUCTS
Not all test and measurement companies are involved with SI, but some of the major players do have products. Aeroflex, Agilent, National Instruments, and Phase Matrix offer a wide range of different configurations and interfaces that still comply with the NxTest initiative standards.

Aeroflex is one of the largest and most heavily invested firms delivering SI. Late last year, it announced its fifth-generation system, known as the Synthetic Multifunction Adaptable Reconfigurable Test Environment, or SMART^E (Fig. 2). This is a version of Aeroflex’s original proprietary chassis, which has evolved to commercial off-the-shelf (COTS) LXI modules.

SMART^E now supports multiple vendors and multiple industry-standard platforms, including LXI, PXI, Compact PCI, and GPIB. A wide range of up/downconverters, digitizers, digital-to-analog converters (DACs), and accessory modules (local oscillator, amplifiers, etc.) is available. The SMART^E system is a complete, highly integrated solution, unlike conventional bench instruments simply packaged into a system that requires vendor support and integration.

The SMART^E 5000 system provides a modular approach for implementing multifunction configurable test systems. It includes all hardware and software needed for calibration, test, execution, test reporting, and test analysis. The system focuses on RF and microwave testing, which includes electronic warfare (EW), radar, satellite, communications, navigation and identification, military automatic test equipment (ATE), and general-purpose microwave test.

Though the system comes standard with one stimulus channel and one measurement channel, parallel channels may be added to each. The stimulus subsystem can operate to 8, 12, 20, 26.5, or 40 GHz in a CW, pulsed, or AWG mode. Modulation options are provided, and this system includes a noise generator. Power amplifiers are available.

The measurement subsystem can be configured to operate to 8, 26.5, or 40 GHz with an RF bandwidth of up to 400 MHz. A signal calibration and routing subsystem contains a local calibration unit for calibrating RF/microwave signals to NIST traceable standards, an RF switch matrix for multiplexing I/O signals to multi- I/O port units under test (UUTs), and an S-parameter test set for microwave vector measurements.

As for software, the SMART^E system is based on Microsoft Windows, C/C++/C#, and National Instruments’ TestStand. There’s also an extensive general measurement test library with built-in test personality customization via user-settable parameters and a Transmit/Receive module test library.

Agilent Technologies offers a series of modules that support the DoD’s ATS NxTest vision by delivering a flexible, modular, and highly morphable system. These modules are based on the increasingly popular LAN eXtension for Instruments (LXI) interface standard. LXI, which is based on Ethernet connectivity, allows users to monitor and control instruments via the browser on a PC.

On the measurement side, the N8201A downconverter boasts a range of 3 Hz to 26.5 GHz that can be extended to 110 GHz with external mixing. It also offers three IF outputs at 7.5, 21.4, and 321.4 MHz. The N8221A is an IF digitizer with a 30-Msample/s rate that’s used with the 7.5-MHz IF output. It has an 80-dB dynamic range, 14 bits of vertical resolution, and 10-MHz modulation bandwidth.

Agilent’s Acqiris high-speed digitizers feature sampling rates of 500 Msamples/s and 1, 2, 4, and 8 Gsamples/s. Resolutions of 8, 10, and 12 bits are available, and they provide large acquisition memories.

As for stimulus modules, the N8211A analog upconverter upconverts a baseband signal to final RF and has a frequency range from 250 kHz to 20 or 40 GHz (Fig. 3). It also includes AM, FM, and pulse modulation or external modulation input.

The N8212A is a 20-GHz vector microwave source that generates a stimulus signal by upconverting from baseband. It additionally offers AM, FM, and pulse modulation, 2-GHz I/Q vector modulation, and external modulation inputs. Two AWGs are supplied as well.

The N8242A is an AWG that provides 10 bits of resolution and comes with a sampling rate of either 625 Msamples/s or 1.25 Gsamples/s. Bandwidth can be 500 or 250 MHz. The unit is dual-channel with single and differential outputs.

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The N8241A is another AWG with 15-bit resolution. Its sampling rates and bandwidth options are the same as the N8242A. All of these units use the LXI interconnect system and are typically mounted in a compact enclosure like the N8200 (Fig. 3, again). For software, Agilent offers the Vector Signal Analysis package and Signal Studio suite.

Of course, National Instruments is a natural to offer synthetic instrumentation, since its basic products are close to the standard definition of SI. NI’s PXIe-1075 chassis, which consists of modules for each of the major blocks in an SI system tem, has 18 slots for both PXI and PXIe modules. Figure 4 shows the PXIe-5663 6.6-GHz vector signal analyzer (VSA) and the PXIe-5673 6.6-GHz vector signal generator (VSG).

The 5663 VSA is an instrument unto itself, consisting of three NI PXIe modules— the 5601 RF downconverter; the PXI-5652 CW voltage-controlled oscillator (VCO), which is used as a local oscillator; and the PXIe-5622 digitizer, a 16-bit 150-Msample/s ADC. These modules can be used individually for SI configurations.

The PXIe-5673 VSG is also an instrument, but it comprises three separate modules— the PXIe-5450 dual-channel AWG with DAC outputs, the PXI-5652 CW local oscillator, and the PXIe-5611 RF upconverter. Again, the individual modules may be configured into the desired SI format. These modules can be connected to form a basic SI (Fig. 5). The local oscillators aren’t shown.

NI’s RF test systems are designed to work with the company’s well-known LabVIEW software. Using this softwaredefined architecture provides great measurement flexibility. Users can develop their own wireless protocols or utilize standard specific LabVIEW toolkits that generate and measure most wireless standard signals. The latest version of LabVIEW 8.6 implements parallel measurement algorithms on multicore CPUs. This can mean significantly faster measurements than those made on more traditional instruments.

Phase Matrix makes a line of VXI bus and PXI bus modules suitable for use in SI systems. These include signal generators, downconverters, and local oscillators. The company’s newest products targeting SI revolve around a family of RF/microwave downconverter modules for the PXIe bus. These modules can be configured into any one of six primary modes operating over the frequency ranges of 100 kHz to 2.9 GHz, 2.7 to 26.5 GHz, and 100 kHz to 26.4 GHz.

The modules include the RF input conditioner module, the microwave band input module, the low band input module, the local-oscillator module, and the IF output conditioner module. They’re designed to support small, portable, and transportable synthetic instruments that can be programmed to perform signal analysis or to emulate older, obsolete instruments.

Also, the modules use programmable input signal conditioning in the form of pre-selection filtering in the 2.7- to 26.5-GHz range with bandwidths of 40 MHz minimum to 120 MHz maximum. The input attenuator can be programmed from 0 to 70 dB in 10-dB steps. Local-oscillator switching speed is less than 1 ms.

These downconverters operate in both a narrow-band IF of 21.4 MHz and a wide-band IF of 250 MHz. The units are designed to work with National Instruments’ digitizers and other PXIe modules.

Phase Matrix has joined with BAE Systems and National Instruments to produce a next-generation 26.5-GHz synthetic instrument based on the PXI platform for military and aerospace applications. BAE Systems is a defense and aerospace company offering products and services for air, land, and naval forces.

REFERENCES
Lupinetti, Francesco, “New Synthetic Instrumentation Methods Solve Tough System-Level Test Problems,” Electronic Design, January 31, 2008.
Nadovich, C.T., Synthetic Instruments: Concepts and Applications, Newnes/Elsevier, 2005.