Synthetic Means More Than Nylon

At last! Test systems bigger on the inside than on the outside.

If you�ve heard about synthetic instruments (SIs), then you already may know that SI systems are solving many military and aerospace testing problems. SIs are not new: BAE Systems developed some of the first arbitrary function generators and sampling measurement systems more than 20 years ago for ATE use. Nevertheless, legacy ATE systems associated with military programs typically have used a mix of specially designed and COTS instruments.

When a COTS instrument becomes obsolete, it�s a major problem. Billions of dollars have been invested in developing and verifying the test program sets (TPS) that run on these ATE systems. Many types of systems exist because, until relatively recently, the separate branches of the military required ATE systems specific to their own equipment.

To bring the situation up to date, Dr. Francesco Lupinetti, chief technical officer at Aeroflex Test Solutions, provided the following information: �The Next Generation Automatic Test Systems (NxTest) multiservice initiative is aimed at modernization of existing vintage ATE systems such as the reconfigurable transportable consolidated automated support system (RT-CASS) and the third echelon test set (TETS). The Agile Rapid Global Combat Support (ARGCS) automatic test system upgrade, part of NxTest, is now in its initial integration phase.

�ARGCS is a synthetic test system specifically designed to transparently execute TPS in their current form by providing direct translation between single-instrument interface commands and the new synthetic platform. This allows the user to preserve all the TPS libraries while dramatically reducing the footprint and cost of traditional test systems. The goal is to transition to a measurement-based, synthetic, modular architecture.�

The benefits of SIs seem clear enough, but what exactly is an SI? The Synthetic Instrument Working Group (SIWG) has defined an SI as a reconfigurable system that links a series of elemental hardware and software components with standardized interfaces to generate signals or make measurements using numeric processing techniques.¹

The critical terms in this definition are reconfigurability, elemental components, standardized interfaces, and numeric processing techniques. Most stand-alone benchtop instruments are not reconfigurable because they have been designed to accomplish a specific objective. Also, although a benchtop instrument can provide a large number of distinct functions, it may not be possible to identify each function with a separate hardware module. In contrast, modern software design philosophies all stress modularity.

RF instrumentation provides good examples of SIs. Typically, digitizing occurs at baseband following down-conversion. Both the digitizer and down-converter are easily identified as separate functions. These are two of the elemental components you typically would find in an RF SI. Because they are reconfigurable, the functions could be part of several different instruments such as a modulation analyzer, a spectrum analyzer, or a distortion analyzer. A block diagram of a generic SI test system is shown in Figure 1.

Figure 1. SIWG Block Diagram of a Generic RF Test SystemCourtesy of National Instruments

Some benchtop instruments come close to fitting the SIWG definition. For example, an oscilloscope, or at least part of it, may satisfy the requirements depending on the accessibility of data at different points within the acquisition and processing systems. In some scopes, signal conditioning and digitizing can be considered a separate module because raw digitized data is made available to the user. For example, at Autotestcon 2006, Data Translation presented an SI demonstration that used a scope as a digitizer to provide data for a software-based spectrum analyzer.

Tim Ludy, product marketing manager at Data Translation, suggested that a trend may be developing toward a browser-based mechanism for setting up and controlling instruments. The instruments then would be combined to perform various sequenced tests controlled at the system level by a test engineer.

The company is promoting its Measure Foundry drag-and-drop programming environment as a tool for integrating SI test and measurement systems. Test programs are developed by dragging and dropping selected functions onto a form. Configuring the relevant property pages completes the exercise.

Is an SI the same as a virtual instrument? Maybe. Virtual instruments use hardware to condition and digitize signals, but most processing is done in software or by a special signal-processing device controlled by software.

Many virtual-instrument hardware modules can be used within SI systems, although some modules with highly integrated functions may not be reconfigurable at a low enough level. Effectively, these kinds of modules replace benchtop instruments in an ATE system but display results on a computer screen.

Present Activity in SI System Development
In addition to the company�s responsibility for the ARGCS RF/microwave subsystem, Aeroflex also provides synthetic satellite-payload test instruments and transmitter/receiver modules, the STI-1000 and TRM-1000, respectively. SI systems aren�t always the size of a 6′ rack, nor do they serve only military and aerospace test needs. Mr. Lupinetti explained that both the company�s cPCI-based 6400 Series Phone Protocol Test System and the Model 3900 Digital Radio One-Box Test Systems appear to be conventional commercial test sets but internally are modular SI test systems.

As can be seen from even the few examples cited, the range of complexity addressed by SI systems is very large. Within that range, the size of the elemental hardware and software modules referred to in the SIWG definition is not constant.

For example, Bustec has developed a series of elemental function cards compatible with VXI carrier boards. These cards provide basic functionality such as A/D and D/A conversion, counters, signal conditioning, and voltage references. However, the cards are not without sophistication. For example, the Model 3424 8-Channel ADC offers 24-b resolution, a 216-kS/s sample rate, programmable gain, and multicard synchronization.

In contrast, an oscilloscope typically combines signal conditioning, one or more voltage references, digitizing, signal processing, and a display. However, in some cases, a scope�s high level of integration may not disqualify it as part of an SI test system.

Dr. Mike Lauterbach, LeCroy�s director of product management, described an application in which the overall test-system reconfigurability was provided by the elemental nature of the software: �LeCroy has developed a synthetic interface that allows engineers working on many types of high-speed serial data components, subsystems, or end products to run a comprehensive set of conformance tests. Several different serial data standards are handled by the software, using scopes and protocol-layer test gear to access the data.�

Keeping with the software theme, RF Engines (RFEL) develops FPGA-based frequency-domain signal analysis IP, which is about as elemental as it gets. John Summers, the company�s managing director, said, �The individual IP blocks and system-on-chip designs that we supply for test and measurement applications can be thought of as subsystems for an SI. From our perspective, we see a trend toward software/firmware-defined instruments, although the market remains dominated by the manufacturers of stand-alone benchtop instruments.�

SIs in the Commercial World
Mr. Summers added that one of the attractions of conventional instruments is the sheer convenience of using them. Customers generally are unwilling to invest a large amount of time to configure low-level hardware and software elemental components to perform a straightforward function. It�s far more economical just to use the appropriate traditional instrument.

Of course, the decision is not so clear-cut in situations such as production testing involving a number of product types. In this case, reconfigurability is necessary if a relatively small, inexpensive ATE system is to address many different requirements.

Bustec�s Dr. Fred Bloennigen elaborated: �Today, the applications that can take advantage of SIs are either very simple or complex. Typical commercial applications lie in-between because they require more than one or two instruments but not the speed and complexity of a typical military test system.

�With SIs, the integrator must have knowledge of how an instrument works and be able to build up functionality from all of the pieces. In the commercial world,� he continued, �engineers do not have the time to build their own instruments. They will continue to use dedicated instruments with easy-to-use software until the SI approach becomes more attractive. For example, this may occur as more drivers become available for modules such as the Bustec function cards.�

Phase Matrix supplies RF SI modules to ATE defense contractors such as BAE Systems and recently joined with National Instruments (NI) in a commercial proof-of-concept demonstration. Michael Granieri, the vice president for advanced programs/business development at Phase Matrix, commented, �The reduced test system footprint and mitigation of test-system obsolescence are the primary drivers for the adoption of SI-based systems in the commercial test and measurement marketplace. Established software environments such as NI�s LabVIEW aid development of spectrum analysis or network analysis measurement applications and are an added incentive for commercial users to employ SI.�

The block diagram of a typical VXI SI test system is shown in Figure 2. The stimulus hardware equipment (SHE) and measurement hardware equipment (MHE) consist of specific combinations of Phase Matrix products. Suitable DAC, ADC, and controller modules are available from multiple vendors.

Figure 2. Block Diagram of RF SI Test SystemCourtesy of Phase Matrix

Virtual instruments represent the largest group of devices that may meet the SIWG SI definition. In most cases, solutions to specific applications have been accomplished by developing special-purpose software to configure a number of general-purpose hardware modules.

The distinction here is that the resulting functionality tests brakes or monitors blood-sugar level or processes product images to find imperfections. In general, virtual instruments are not designed to perform a range of different types of measurements as an SI is. That�s not to say that they couldn�t be designed in this way, just that often they aren�t.

This point was made in a Data Translation datasheet that discussed Measure Foundry�s role in SI test systems, �The promise of SIs is the ability to change the capability of an instrument through software. Beyond virtual instruments, which represent a specific device, SIs allow a generic device to be used as the instrument [requires.]�

Data Translation�s view is clear, but the line between virtual and synthetic instruments can be blurred depending on whom you talk to in the industry. Distinguishing between the terms on the basis of potential vs. actual reconfigurability may seem arbitrary at first glance. However, if you consider the intention of mil/aero ATE architects, the difference between the approaches becomes clear.

To deliver the performance improvements required of a system such as ARGCS, most of the hardware and low-level software modules must serve multiple purposes controlled by specific test software. This is the distinction the system designers had in mind when designating ARGCS as a synthetic test system.

Owen Golden and Eric Starkloff, both with NI, made this point in a paper delivered at Autotestcon 2006: �A primary benefit of synthetic instrumentation is the flexibility that comes through reconfiguring a measurement and automation system in software. To maximize the degree of software reconfigurability in a system, the hardware should be designed to be as generic as possible. [As an example,] for analog measurement, synthetic instrumentation hardware is responsible for digitizing the signal; all other processing for creating a measurement from the digitized signal is accomplished in software.

��While the ultimate hardware for synthetic instrumentation is a truly universal measurement device that can do both high-resolution measurements as well as high-speed measurements, the authors continued, �in practice, there are trade-offs in cost and current semiconductor capability. It is possible, though, to architect systems based on a relatively small number of generic measurement devices and cover a large percentage of requirements through software routines.�¹

An example from NI illustrates a type of application in which virtual and synthetic instruments are equivalent. Darcy Dement, modular instruments senior product manager, described how the University of California at Berkeley used virtual instruments to address RF education.

NI�s LabVIEW and PXI instruments are used to streamline experimental radio development. Students design their own 900-MHz front-end software radio, validate their system with hardware, and characterize their radio. Each student validates his radio�s hardware by inserting it into the tool chain, replacing whatever COTS hardware was used at that step in the communications system.

The low cost and modularity of the platform opened the door for multiple experiments and areas of study. These included electromagnetic wave transmission and propagation, the effects of multipath fading and interference, and the comparison of digital and analog modulation and encoding on a communications system.

Evolution Through SIs
The benefits of an SI test system go beyond mitigating obsolescence and reducing equipment footprint and cost. As Bill Accolla, Southeast sales manager for Acqiris USA, put it, �Legacy instrumentation emulation can stifle true innovation in SI. The movement to SIs is not just aimed at replacing obsolete instruments, but also at embracing newer technologies that will provide higher performance, precision, and speed.

�If TPSs continue to be written using the legacy model, then we will be stuck with that instrument emulation, not being able to progress through evolving technologies,� he continued. �Ideally, SIs will eliminate instruments, not emulate them. The SI model enables the orderly insertion of new SI components at a pace set by demand for higher performance of each individual component.�

For developers of commercial semiconductor test systems as well as some communications ATE, this is a well-understood message. The modularity, flexibility, and extensibility provided by the SI approach have allowed these companies to respond quickly to rapidly changing test requirements. Lower cost of ownership and simpler upgrades also followed from adopting an SI-based system.

WiFi is a good example of a rapidly evolving communications standard. To address the needs of design, test, and manufacturing engineers, GaGe developed the NEXUS 802.11 family of WiFi testing systems. Based on the company�s CompuGen and CompuScope Arbitrary Waveform Generator and digitizer cards combined with application-specific software, the systems achieve accurate measurements at a relatively low cost.

The company�s sales manager/business development, Dr. Andrew Dawson, added, �The communications revolution and continual evolution of standards reinforce the advantages of SIs. Modularity, scalability, and economy result from taking an SI approach, and users are provided with a testing solution that has a much longer lifespan than a dedicated fixed-box system.�

SIs figure prominently in Teradyne test systems. Peter Hansen, core system instrumentation marketing manager at the company, described some of the capabilities of the Bi-410 Synthetic Serial Bus Test Instrument. �The Bi-410 can be reconfigured on the fly to emulate several instruments previously required to test serial bus protocols. SI provides a smaller footprint, reduced cost of ownership, and greater flexibility.�

A smaller footprint is critical in test systems that must be transported to and from battlefields and air bases around the world. The Bi-410 is part of the RT-CASS system used by the Marines for V22 Osprey field testing. Lean logistics and mission readiness are improved through an SI approach to test systems.

Flexibility, the last of Mr. Hansen�s points, means that the Bi-410 can go far beyond the capabilities of earlier rigid, standards-driven instruments to handle the bus protocols of next-generation boards and boxes. In fact, the Bi-411, based on the Bi-410, is used in ARGCS, the highest profile example of a military next-generation flexible SI test system.

Tom Sarfi, business development manager for functional test at VXI Technology, provided an example of a modal testing upgrade. �For years, a 16-b multichannel digitizer was sufficient, but recently, researchers became interested in having a wider dynamic range. A 24-b converter was needed. Because the system was modular, an easy upgrade path was available once the 24-b digitizer had become available. Users simply removed the 16-b unit and replaced it with the 24-b digitizer without needing to change any other component in the system.�

Conclusion
SIs are becoming well-entrenched in mil/aero ATE systems. The benefits of the approach are undeniable. When SIs are considered for commercial test applications, however, this type of architecture may not be an obvious choice. Nevertheless, the appearance of different types of instrumentation system architectures is not the only change occurring today in the test arena.

Just as virtual and synthetic instrumentation are attractive alternatives to traditional benchtop equipment, FPGA-based designs have unique attributes beyond either hardware or software solutions. For example, RFEL�s SpectraChip� IP core is a digital replacement for analog IF filtering used in spectrum analyzers. The core is intended to be embedded within an FPGA device and provides a digital implementation of features such as resolution bandwidth filtering, video bandwidth filtering, and conversion to log power. A block diagram of the core functions is shown in Figure 3.

Reprogrammability of the FPGA itself has made possible capabilities difficult to implement otherwise. As an RFEL datasheet noted, �A number of filter banks can be supplied to meet different measurement requirements. Standard Gaussian shape filters can be used to produce the usual spectrum analyzer effect, or higher-specification, flat-top, low transition band filters can be used to achieve power measurement accuracy and good frequency isolation.�

Reprogrammability extends from the subdevice to the instrument-scale end of the spectrum and is one of the factors that influenced development of the LXI standard. Bob Rennard, president of the LXI Consortium, said that LXI supports SI systems by providing many instrumentation functions as small, half-rack, faceless blocks that consume little rack space. Breaking instruments into more elementary building blocks increases the chances that at least some of the components will be reused for a different application.

Because LXI is Ethernet-based, both modules and complete instruments can be combined to form instrumentation systems. In addition, Ethernet offers the stability of an established standard as well as the opportunity to create systems of physically distributed hardware.

Mr. Rennard made the point that system upgrade cost is yet another important aspect of SIs. The separate building blocks in any test system or instrument have different technology innovation cycles, and the SI approach allows a digitizer to be upgraded, for example, while keeping an RF down-converter intact.

Communications among SI building blocks are well-served by LXI. The standard offers high data rates, multiple triggering alternatives, and the flexibility of peer-to-peer communications and multicast messaging. Keithley Instruments has developed a further scripting capability that can co-exist with LXI interfaces.

The test script processor (TSP) capability embedded within the company�s Series 2600 SourceMeter� instruments allows test scripts to be run on a master or multiple instruments in a cluster. Such scripts may incorporate test algorithms including mathematical functions and machine interface control capabilities. The result is very flexible and fast communications within a TSP cluster that may be part of a larger SI-based ATE system.

Regardless of scale or implementation details, SIs really are traditional instruments broken into fundamental building blocks that can be used in different combinations to make various types of measurements. The approach is most attractive for test systems required to make several different but related measurements and where extensions to the present capabilities are likely.

For small, single-purpose test systems, the low level of functionality typically integrated in an SI module may lead to unwarranted expense. On the other hand, the opportunity to develop reconfigurable embedded FPGA-based systems should not be overlooked.

Reference
1. Golden, O. and Starkloff, E., �Developing Synthetic Instruments With COTS Technologies,� IEEE Autotestcon 2006 Proceedings, pp. 32-37.

December 2006