The diversified needs of the US military for high-frequency measurement solutions is paving the way for future generations of multifunction, multipurpose synthetic measurement instruments.
Synthetic Instruments (SI) became an important issue for test-equipment suppliers as soon as the United States Department of Defense (DoD) announced the award of the ARGCS (Agile Rapid Global Combat Support) contract in September 2004. Although the ARGCS represents only a technology demonstration, it may well foretell the future of high-performance analog and microwave test equipment
used in automatic test systems (ATS). What follows is a brief description of the SI concept, how the SI approach differs from general-purpose bench-top instrumentation, and how advances in electronic component technologies might make SI successful in both the defense electronics and commercial industries.
According to the Synthetic Instrument Working Group,1 a synthetic instrument is 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. In other words, it's a concatenation of hardware and software modules used in combination to emulate a traditional piece of electronic instrumentation. Synthetic instruments contains up to four major components including: signal conditioners, frequency converters, data converters and numeric processors as shown in the figure.
This simplified architectural block diagram can describe most microwave instruments, like signal generators, spectrum analyzers, frequency counters, network analyzers, etc. However, the implementation using SI modules may require multiple similar modules to emulate the function of its original instrument.
At first glance, the SI architecture may appear more complicated than its traditional instrument counterpart largely because the general-purpose instrument has been assembled and optimized to achieve the desired performance and throughput. This would be correct if the only goal was to reproduce a single instrument. However, most ATSs have a wide range of different signal stimulus and measurement instruments so a new level of efficiency might be realized through the reduction of redundant modules. There are six goals the DoD expects to achieve with the implementation of synthetic instruments as shown in the table.
While these are lofty goals, the DoD and many instrument manufacturers think it is possible to achieve them over time. Probably the most challenging goal is achieving the reduction in the total cost of ownership. To many this means driving down the price of a test assets. While price is a major component in the total cost, long-term support is at least as large or larger given that these test systems must remain operational for greater than 20 years. The reason this is such a challenging goal is that the DoD expects these SI modules to come from a variety of vendors and to be mixed and matched as products from one vendor become obsolete and a substitute is required. This must be achieved with little to no software changes to the core test software program (some hardware driver would be required to support the new vendor's hardware). This can only be achieved if some standardized hardware/software interface can be defined much like what was done by the computer industry.
The idea of a synthetic instrument has been around for many years, so the concept is not new. Agilent Technologies (formally Hewlett-Packard Co., Palo Alto, CA) introduced the Modular Measurement System (MMS) more than 20 years ago, which achieved many of the goals stated in the table. Agilent's MMS components were integrated in many of the DoD's ATSs including CASS (Consolidated Air Support System), the US Navy's most successful test platform. However, only a few commercial test equipment manufacturers developed modules for this platform. As such, when modules became obsolete there were no available substitutes. So what has changed that might make synthetic instruments more widely adopted by a variety of test-equipment companies?
When Agilent developed the MMS platform, many of the key electronic components in the modules were developed and produced in Agilent's foundries. This achieved a higher level of performance than was commercially available, making it difficult for Agilent's competitors to make a viable substitute.
Over the last ten years there has been a large year-over-year growth in the wireless (cellular phone) and high-speed telecommunication industries attracting the attention of the large component manufacturers like Texas Instruments (Dallas, TX) and Analog Devices, Inc. (Norwood, MA), to name a few. This newfound attention has changed the cadence of performance advances in data converters and digital signal processors (DSPs). These are two of the core components that make up the data converter and numeric processor blocks in the SI architecture making it possible for instrument manufacturers to purchase high-performance components off-the-shelf. Unfortunately, there is no "high-volume" application greater than approximately 6 GHz to speed advances in microwave and millimeter-wave components.
Since some of these new components move at Moore's Law rate (performance doubles every 18 months) one could expect to achieve faster analysis and computation of the desired signals. Also, the data converters are getting faster at about 1.5 times Moore's Law (doubling every 24 to 36 months) achieving improved accuracies once only available through customized components. If an industry-standard interface emerges from the work of the Synthetic Instrument Working Group, a greater degree of competition will emerge. Test system integrators must be careful not to implement a system architecture where test modules become obsolete and unsupported, as in card cage modular instrumentation. Agilent is developing its synthetic instruments in the new LXI (LAN eXtension for Instruments) modular format to avoid the pitfalls of the card cage.
If one examines the architecture of a current microwave instrument, such as a vector spectrum analyzer or a vector signal generator, it would not be unlike the one shown in the figure. The main difference is that all the components including the keyboard, display, and numeric processor are housed in the same sheet metal versus individual modules. The key benefits to the customer are that the instrument manufacturer takes care of the interconnections between the modules including signal conditioning and switching. Unlike older microwave instruments where the performance and accuracy was based on the summation of the amplitude and phase errors of the components used, new data converters, DSPs and on-board calibration allow a modern microwave instrument to hold tighter specifications for the same measurement.
However, if one is using the highest performance microwave instruments available, like the DoD requires, then there is no easy way to add the latest technology when it becomes available, as it will be only offered from one source, the original equipment manufacturer (OEM).
If the frequency converter module were removed from the block diagram in the figure, what remains is the ideal synthetic instrument or software-definable instrument (SDI). While there are analog-to-digital converters (ADCs) that operate to 40 GSamples/s, the highest-performance ADC only provide 8-b resolution, not enough for many microwave applications.2
For most applications today and in the future, dynamic range (effective bits or spurious-free dynamic range) will be the pacing parameter for the data converters. When a digitizer or arbitrary waveform generator (AWG) with sufficient bandwidth is not available, a frequency converter to translate the signal to within the bandwidth of the data converter must be added. One example where data converters have achieved all the bandwidth and dynamic range required is the audio field where practically all equipment produced is direct conversion. Commercial ADCs are available with greater than 115 dB dynamic range and greater than 20 kHz bandwidth; all the human ear can resolve. What differentiates one manufacturer's product from the next is the signal conditioning and DSP algorithms to produce the best representation of the original voice or music produced.
In conclusion, The ARGCS contract may well be a turning point in the way automatic test systems are developed. There are many challenges ahead before the DoD's goals can be realized, promoting a true multi-vendor implementation. Driving down the total cost of ownership of ATSs will require careful consideration of what role the functional elements should take. Ideally, one would utilize as many commercial industry standards as possible without locking oneself into any standard that is likely to change through technology advancement. If successful, how long will it take to drive down the acquisition cost of a test system bringing sufficient volumes for commercial instrument manufacturers to support? With this in mind, Agilent's new LXI-based synthetic instruments offer the best compromise between cost, performance, size, and—most important—long life.
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