Synthetic instruments (SI) promise a flexible, cost-effective future for military systems testing. These modular function blocks can be combined under the command of different software engines to perform all of the functions currently offered by traditional "rack-and-stack" test systems, but at a fraction of the cost and without the built-in obsolescence that plagues application-specific measurement solutions. Several new

SI modules from Agilent Technologies (Santa Rosa, CA), including high-speed data converters and several frequency converters, hint at the road that lies ahead for the military's use of SI solutions.

According to a study performed by the United States' General Accountability Office (GAO) in March 2003, traditional methods of testing and maintaining military systems cost about $500 million per year. These traditional methods include racks of application-specific systems with conventional dedicated-function test equipment, such as power meters, spectrum analyzers, signal generators, voltmeters, and power supplies. Because of the high annual costs of maintenance, the US military sought a different approach to testing, and the concept of the SI-based system emerged.

In an April 2003 study, the GAO found that the DoD spent $50 billion on acquisition and support of ATE from 1980 to 1992, or about $250 billion in total ownership costs over the lifetime of the test equipment. The DoD uses more than 460 different test systems with about 85-to-95-percent redundancy in form and capability. Just by eliminating the redundancy (which includes display screens, keypads, frequency converters, microprocessors, and digitizers), the GAO projects potential savings of $81 billion.

The US Department of Defense (DoD) chartered the US Navy to develop that new strategy for testing, and the Navy responded with the NxTest next-generation automatic-test-equipment (ATE) systems program. The concept of NxTest is simple: to provide a seamless common system architecture for the testing, maintenance, support, diagnostic, and prognostic capability of combat systems across the four branches of the military. The NxTest SI systems should employ common but scalable modular hardware, flexible test software, and data-base management software that can be networked across all branches of the military and across all service levels of support.

The goals of the NxTest program include a reduction in the cost of ownership; reduction in long-term support by 30 to 50 percent; establishment of a common system architecture that can be deployed across all services, with 80-to-100-percent system interoperability; a reduction in the size and weight of test systems by 30 to 50 percent; a reduction in the time needed to develop and field new or upgraded test systems; scalability in those systems; and the ability to reduce repair times for electronic systems by 20 to 50 percent.

Commercial-off-the-shelf (COTS) approaches to building ATE systems were aimed at achieving many of these goals, but COTS equipment has its shortcomings. COTS equipment typically suffers from short operating lifetimes and quick obsolescence. When a commercial tester becomes obsolete, it must be replaced with the next-generation equivalent instrument, which usually requires changing the software and requalifying the entire test system and test procedures.

Systems built from SI modules include basic measurement functions that can be reconfigured through software, somewhat like the concept of a software-defined radio (SDR) for commercial cellular communications. By providing enough basic, building-block functions, an SI system can be reconfigured simply by changing the software. An SI module itself may not perform complex measurements, but when combined with other SI modules, can execute all the functions of a traditional ATE system. By working with the Test Program Set, which defines the type of measurements and sequence of those measurements required for a particular military application, SI software can be written to meet the needs of many different military electronic systems with a common SI test platform.

The SI concept can already be found in instruments designed into VXI, PXI, and LXI formats. Such modules can work together and coexist in an SI measurement system. LXI, which stands for LAN eXtension for Instrumentation, differs from the other modular formats in using an external computer and local-area network (LAN), rather than embedded computers, for control. The LXI standard is under development by the LXI Consortium (www.lxistandard.org), a nonprofit organization of test and measurement companies that included Agilent Technologies and VXI Technology. The first draft of the LXI standard is expected in the fall of this year.

The LXI standard supports the IEEE 1588 time synchronization and protocol standard, which allows synchronous triggering of different instruments, even with different-length LAN cables. The IEEE 1588 precision time protocol (PTP) enables a common sense of time over a distributed system. For a 100-Mb/s LAN, for example, LXI trigger resolution is about 10 ns.

The N8200 series of measurement modules from Agilent Technologies (Fig. 1) provides a glimpse into the measurement capabilities of the SI approach. The series includes analog frequency converters, a vector frequency converter, an intermediate-frequency (IF) digitizer, and an arbitrary waveform generator. The N8241A arbitrary waveform generator (Fig. 2) is a dual-channel SI module based on the company's model N6030A modular arbitrary waveform generator. The N8241A features 15-b and 1.25 GSamples/s performance levels that allow operators to generate single-ended or differential output signals with 500-MHz bandwidth per channel or 1–GHz bandwidth in an in-phase (I) and quadrature (Q) arrangement. As many as eight of the waveform generators can be synchronized for phase-coherent, multiple-signal creation.

The spectral purity of the N8241A matches that of the N6030A module, with phase noise of –95 dBc/Hz offset 1 kHz from the carrier, –115 dBc/Hz offset 10 kHz from the carrier, and dropping to the noise floor of –150 dBc/Hz at 1 MHz offset from the carrier. Harmonic distortion is –65 dBc or less while spurious (nonharmonic) distortion is –75 dBc or less.

Agilent's SI frequency-converter modules include the analog model N8201A 26.5-GHz downconverter, the analog model N8211A 20- or 40-GHz upconverter, and the vector model N8212A 20-GHz upconverter. The N8201A (Fig. 3) downconverter is a simple module with single RF input port (rated to +30 dBm) that handles signals from 3 Hz to 26.5 GHz and provides IF outputs at 7.5, 21.4, and 321.4 MHz. With dynamic range similar to that in the company's E4440A PSA spectrum analyzer, the downconverter features a modulation bandwidth of 100 MHz for carriers to 3 GHz and a modulation bandwidth of 200 MHz from 3 to 26.5 GHz. It also allows operators to bypass the built-in preselection filtering. Its input range can be further extended to 110 GHz by connecting the appropriate frequency mixer to a front-panel local-oscillator (LO) connection.

The model N8211A analog upconverter is designed to translate low-frequency input signals to an output range reaching 20 or 40 GHz. It provides adequate signal bandwidth to support amplitude modulation (AM), frequency modulation (FM), and pulse modulation without contributing additional noise to the original signal source (typically an analog signal generator). The model 8212A vector upconverter also provides output signals to 20 GHz with AM, FM, and pulse modulation, but supports an I and Q modulation bandwidth of better than 1 GHz for true wideband signal generation.

The model N8221A digitizer module (Fig. 4), which is leveraged from the digitizer in the company's model E4440A PSA spectrum analyzer, provides an 80-dB dynamic range by merit of its 14-b resolution. It samples 21.4-MHz IF signals at a rate of 30 MSamples/s and captures a modulation bandwidth of 8 MHz. The digitizer module, which includes external triggering capabilities, has a built-in 10-MHz reference oscillator.

In all SI system cases, it is important to note that software provides the "intelligence" for a particular measurement and is as essential as the hardware. The software configures a measurement, takes and processes the data, and formats the data for display, presentation, or storage. For example, to create an SI signal generator, an arbitrary waveform generator and upconverter module would be coordinated under software control. To create a spectrum analyzer, a downconverter module would be combined with an IF digitizer and vector-signal-analyzer software for display and analysis.

The SI measurement evolution is in its early stages and these are but examples of the many functions to come in the LXI-compatible format. The firm is already at work on a DC-to-26.5-GHz switch-matrix module (with versions available to 40 GHz) as well as single- and dual-channel peak power meters based on the company's EPM P-series power meters. Agilent Technologies, 1400 Fountaingrove Pkwy., Santa Rosa, CA 95403; (707) 577-1400, Internet: www.agilent.com.