You may have heard the term synthetic instruments used to describe a new type of test system architecture that promises to revolutionize the way we test products. What is synthetic instrumentation, and where did it come from? This article will discuss the genesis of synthetic instrumentation, its unique modular architecture and its application to testing microwave systems for the U.S. armed services.
The term synthetic instrumentation was coined by the U.S. Department of Defense’s Next Generation Automatic Test Systems (NxTest) Integrated Product Team to describe a new test architecture that would support the charter of its group. In April 2002, the DoD Automatic Test Systems (ATS) Executive Agent Office formally chartered the NxTest IPT – made up representatives from the Air Force, Army, Marine Corps and Navy – with two main goals: to reduce the total acquisition and support costs of DoD ATS and to improve the inter- and intraoperability of the armed services ATS functions.
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NxTest IPT has undertaken a number of important initiatives. Among those initiatives is the conceptualization of an architectural approach to test system design and implementation that is now referred to as synthetic instrumentation. This architecture is envisioned as open, one that can support new test needs and permit flexible insertion of updates and new technology with minimal impact on existing ATS components. The initiative targets broad commercial applications as well as mil-aero to garner test industry support for COTS solution components development.
Modern test systems
The design, manufacture, programming, and operation of a test system implemented for testing a modern microwave system or subsystem encompasses a comprehensive list of tasks, as shown in Table 1.
Tasks required for design, manufacture, programming, and operation of testing a modern microwave system
Specifying the microwave test requirements
Specifying the microwave instrument components that meet or exceed the test requirement specifications
Defining the microwave plumbing necessary to route various stimulus and measurement signals to and from the various instruments required
Defining and implementing the calibration scheme
Selecting the COTS instruments to be utilized from an array of instruments offered
Defining the test software requirements and developing the software
Debugging, bringing up the test system, and creating documentation
Table 1
Consider the task of mapping this entire process across the entire high-frequency test system requirements of the Joint Services of the United States. Also consider the further challenges of replacing obsolete equipment while preserving Test Program Set (TPS) software, even as new requirements must be accommodated. At this point, there can be little wonder that the U.S. Department of Defense (DoD) created an Executive Office and chartered a team of experienced test people to seek ways to reduce the total acquisition and support costs of DoD Automated Test Systems (ATS).
Given the scale of complexity, cost, implementation, and maintenance challenges, it is also not surprising that an emphasis has been placed on microwave systems by the enfranchised team. The goals of the working group are to:
Reduce the total cost of ownership
Reduce time to develop and field new or upgraded automatic test systems
Provide greater flexibility to the warfighter through U.S. and coalition partner interoperable automatic test systems
Reduce the test system logistics footprint
Reduce the physical footprint
Improve test quality
Synthetic instruments
The technology of synthetic instrumentation is not new. National Instruments has taken the concept of software-defined instruments and systems to levels unimagined by conventional purveyors of boxed, specific-function instruments. And, of course, the impact that virtual instrumentation has made in the past decade did not go unnoticed by the NxTest group. The IPT created a focused group known as the Synthetic Instrument Working Group (SIWG) to pursue the breadth and depth of synthetic instruments. The specific charter of the group is to:
“Define synthetic instrumentation and its attributes. Develop a framework that balances user and supplier objectives, facilitates rapid technology advancements and adaptation while reducing test costs throughout the test life cycle, and complements/supports other relevant test and measurement industry activities.”1
The SIWG has defined synthetic test systems as follows:
“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.”2
A generic block diagram of a synthetic microwave test system defined by the SIWG is shown in Figure 1. The system consists of hardware and software blocks, Radio Frequency (RF) conversion, and the device under test.
Synthetic instruments as modular test solutions
Synthetic instrumentation tends, by nature, to be modular. That modularity enables system integrators to tailor solutions that are well matched to an individual application. That is fundamental to achieving the NxTest goal for test cost reduction. What makes an instrumentation component or system synthetic? Is it modularity? The answer is no. In fact, synthetic test solutions need not be modular. However, synthetic instruments, which are modular, will be superior in meeting the broad goals outlined above.
The majority of legacy mil-aero microwave test systems have been largely configured from specific-purpose instruments, which are designed to accommodate as many diverse applications as possible at a given price/performance point. When it comes to specifying new test solutions, the starting point is often the compilation of the specifications of the instruments that make up the legacy system. Such a compilation probably will not accurately reflect the specifications at the system level; that is, of course, what sets error margins and uncertainty of test, which ultimately dictate the quality of test. Furthermore, the best approach for supplying equipment that meets all of the specifications of a given instrument is probably use of that very instrument – which ultimately costs the same as before and won’t result in a cost reduction.
This line of reasoning suggests we look again at the definition of synthetic test and consider what it really means to replace a collection of instruments with “elemental hardware and software components…to generate signals or make measurements using numeric processing techniques.”3 Limiting ourselves to the RF signal channels represented in Figure 1, we find that we require the ability to configure stimulus and response measurement channels. Stimulus channels are shown to consist of the elemental hardware components of a D to A converter, upconverter, and RF signal conditioner. Response measurement channels may be shown to consist of the elemental hardware components of an RF signal conditioner, downconverter, and an A to D converter.
If we continue to dissect those elemental components in a microwave synthetic instrumentation channel, we find additional possibilities for defining the most elemental modules. And configuration flexibility can apparently be optimized, if modularity is implemented with the right boundaries between modules. For example, Figure 2 shows that an upconverter can be broken down into an Intermediate Frequency (IF) module, a Local Oscillator (LO) module, and a frequency translation module. A downconverter can be broken down into a frequency translation module, an LO module, and an IF module. The frequency translation modules may, in turn, consist of banded submodules.
Figure 2. Modularity Configuration Diagram
Synthetic learning curve
Initially, the system integrator response to such modularity may not be completely positive. At some point, elemental modularity presents major challenges and may be a cost driver – not a cost reducer – in the business of system integration. A system integrator may not want to take on replacing a state-of-the-art synthesized signal generator with a synthetic stimulus channel. That may mean that not only must the integrator specify the I/O signals typical for a signal generator, they must also specify the individual elemental components of the synthetic stimulus channel including the bandwidth of the D to A converter, characteristics of the antialiasing filter of the D to A, signal conditioning characteristics at the input of the upconverter IF module, mixer gain, post mixer filtering, and amplification. It is a daunting task.
However, by integrating the appropriate test modules, the synthetic test system architecture supports many diverse applications such as:
Multichannel stimulus or measurement path configurations
Mix-and-match performance, size, and price levels for each channel, including routing individual channels through multiple signal conditioning blocks to provide test-specific functionality
Easily upgrading the system to add or remove functionality at any time, or completely reconfigure it for an entirely new application
Adding processing power or new test algorithms as required
Providing sophisticated system-level calibration and diagnostics capabilities
However, there is the dilemma outlined above for the system integrator. To achieve these benefits, the integrator needs interchangeable elemental hardware components, with elemental driver software, that enable configuration of the stimulus and response measurement channels. Interchangeable components require standardization at the boundaries – common IF frequencies, common LO frequencies, common impedances, common control signaling with Interchangeable Virtual Instruments (IVI)-type standardized software drivers, and Application Programming Interfaces (APIs).
An individual supplier of synthetic systems or elemental components at the stimulus or response measurement channel level is motivated to provide interchangeable components within its own architectural constructs, to enable configurability of a given channel. For example, the ability to alternate between a broadband Arbitrary Waveform Generator (AWG) and a narrowband AWG at the input of the upconverter allows a system to provide a high-resolution stimulus on the one hand, suitable for network analysis measurements. On the other hand, it provides a broader banded digitally modulated, frequency-hopped signal for functional testing. The synthetic test channel vendor who supplies a system option to multiplex between the two choices programmatically will enjoy an advantage in certain applications.
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Agile reconfigurable global combat support program
The NxTest team is taking on these types of challenges. To date, it has spun off groups to focus on Automated Test Markup Language (ATML), Common Test Interface (CTI), and synthetic instruments. The ATML Working Group was chartered with developing a common XML schema for all critical software interfaces in the test environment. This group has now transitioned to the formal IEEE SCC20 committee, and they will be releasing the first Automated Test Markup Language schema standards in the coming months. The CTI Working Group was chartered with developing a common, general purpose, and scalable test interface that could provide for interoperability between test systems used by the various DoD services. The SIWG is looking at standardization of elemental component interfaces, among other topics related to SI hardware and software architecture.
The DoD has begun major tester programs specified in terms of synthetic implementation as driven by NxTest. Possibly the most significant of these is the Agile Reconfigurable Global Combat Support (ARGCS) program, which was awarded in the fall of 2004. The creation of this challenging Advanced Concept Technology Demonstration (ACTD) program was sponsored by the NxTest IPT and authorized by the Office of the Secretary of Defense (OSD). The ARGCS test platform will demonstrate the most scalable and reconfigurable test system architecture ever fielded. The platform will support both simple and complex system configurations. Industry-leading flexibility to be demonstrated with the ARGCS platform will ultimately allow the U.S. Air Force, Army, Marine Corps, and Navy – as well as NATO and coalition partners – to individually procure systems configured for their specific operational scenarios. Sharing core hardware and software modules across diverse application systems provides desired cross-service interoperability. Use of common elemental hardware components will reduce costs.
The RF subsystem elements of the winning ARGCS proposal were based upon synthetic instrument technology that has been fielded for customers since the mid-1990s. That technology was provided in the form of large-scale, very broadband RF systems for satellite bent-pipe payload tests and transmit/receive module tests for electronically steered phased array radar systems. Up/downconverter subsystems were configured for 50 MHz to 26.5 or 32 GHz operating ranges. These components were not designed to be scalable for subsets of those frequency ranges. The hardware from those earlier generation testers has been repackaged so that it is scalable in frequency and more modular in design for the ARGCS program. Figure 3 shows an Aeroflex LXI-based NxTest system for microwave testing up to 26.5 GHz in a two-man transportable rack. The RF elements of the ARGCS program are lower risk because they build upon the installed base experience in hardware and software of predecessor synthetic test solutions.
Synthetic instrument software, as is to be utilized in ARGCS, enables the insertion of a translation layer designed to enable utilization of legacy TPS without modification. This software can translate “traditional instrument programming statements” 4,5 to equivalent test and measurement functions of the synthetic system. Demonstrating the ability to preserve TPS software investment more efficiently than previous generations of equipment is one of the major payoffs expected of synthetic test solutions. In virtually all industries that rely on ATS for their viability, test software costs tend to overshadow the hardware costs by a significant ratio. In terms of cost reduction, it is test system capital acquisition cost that seems to get the most management attention. In the long-term, it is likely to be the software contributions and advantages of synthetic test that emphasis on test.
Rick Humphrey is the director of technical marketing for the Aeroflex Test Systems (ATS) division. Rick has more than 30 years of experience in the electronics industry and is responsible for the technical marketing of Aeroflex microwave synthetic test systems. His work in test engineering spans positions with Texas Instruments, Hewlett Packard, two start-up IC industry-focused companies, and currently, Aeroflex.
For more information, contact Rick at:
Aeroflex, Inc.
Aeroflex Test Solutions - Powell Division
383 N. Liberty
Powell, OH 43065
Tel: 614-888-2700
E-mail: radar@aeroflex.com
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