The demand for fast evolution and increased flexibility of electronics systems in many different industries and applications has driven the trend for increasing software content of electronics systems. In measurement and automation, the prevailing trend over the past 20 years has been towards measurement instruments that define their capability through software. Virtual instrumentation, which emerged in the mid-1980s, has been at the forefront of this trend. Recently, the U.S. Department of Defense (DoD) articulated their desire for more flexble, software-based test systems through an initiative called Synthetic Instrumentation.

Virtual Instrumentation

Virtual Instrumentation (VI) is defined as a software-defined system, where software based on user requirements defines the functionality of generic measurement hardware. A virtual instrument shares many of the same functional blocks as its traditional counterpart, the standalone box instrument, but allows the end user to define the core functionality of the instrument through software. Where a traditional instrument has vendor-defined embedded firmware, a virtual instrument has open software defined by the user. In this way, the virtual instrument can be reconfigured for a variety of different tasks or completely redefined when an application's needs change. A synthetic instrument is a type of virtual instrument; currently synthetic instruments are being defined specifically for RF stimulus and measurement within military test systems.

The benefits of software-defined virtual instruments include:

  • Increased system flexibility through reconfiguring software.
  • Increased system longevity by adapting to future needs.
  • Lower system size by creating multiple software personalities on shared measurement hardware.
  • Lower system cost through hardware reuse.
  • Ability to solve unique system requirements not addressed by existing traditional instruments.

Software is critical to a virtual instrumentation system. It is, after all, the user configurability through software that differentiates a virtual instrument from its traditional counterpart. Because the needs of measurement and automation systems are so diverse, no single bus or I/O standard can meet every need. For example, USB is well suited for applications requiring easy desktop connectivity, while internal PC buses like PCI and PCI Express provide the highest performance in latency and throughput.

Virtual instrumentation software should also be able to integrate traditional instruments into a hybrid system. This is valuable for two reasons: First, many systems must take advantage of existing measurement equipment to save costs, and second, there may be highly specialized requirements that are met by a particular traditional instrument. A comprehensive set of I/O drivers and a wellarchitected measurement and control services layer enable the user to create an integrated hybrid system.

Virtual and traditional instruments share many of the same functional subsystems, but differ in the way in which software is applied.
While virtual instrumentation software should be able to integrate hybrid systems comprised of both generic virtual instrumentation hardware as well as traditional instrumentation, there are several hardware attributes that make a compelling virtual or synthetic instrumentation hardware platform. These include: 1) A general-purpose hardware architecture to address the broadest set of applications; 2) A high-speed connection between the hardware and the VI processing element(s); and 3) Modularity so that parts of the system can be upgraded as needs evolve.

A primary benefit of virtual instrumentation is the flexibility that comes through reconfiguring a measurement and automation system in software. In order to maximize the degree of software reconfigurability in a system, the hardware should be designed to be as generic as possible. For analog measurement, virtual instrumentation hardware is responsible for digitizing the signal; all other processing for creating a measurement from the digitized signal is accomplished in software.

Once a signal is digitized in a VI system, it must be transferred over a data bus to a processing element running the appropriate software routine. Because buses vary in their strengths, certain buses offer better performance for particular applications than others. When evaluating bus performance, two important factors to consider are latency and bandwidth. Latency measures the delay of transmission of data, while bandwidth measures the rate at which data is sent across the bus, typically in MB/s. Lower latency improves the performance of applications that require a large number of small commands or data sets to be transferred. Higher bandwidth is important in applications such as waveform generation and acquisition.

DoD Synthetic Instrumentation Initiative

The U.S. Department of Defense, as the largest single purchaser of test equipment in the world, is a key adopter of next-generation instrumentation technology. Maintaining their vast array of disparate, application-specific test equipment has proved to be a significant and costly challenge. Recently, the DoD has begun articulating the need for a more flexible, software-centric approach to building test equipment. A report to Congress from the DoD Office of Technology Transition in February 2002 stated, "Recent commercial technology allows for the development of synthetic instruments that can be configured in real time to perform various test functions... A single 'synthetic' instrument can replace numerous single-function instruments, thereby reducing the logistics footprint and solving obsolescence problems."

The current capabilities of VI measurement hardware, expressed as the digitization resolution versus the sampling frequency. In many areas, the capabilities of generic VI hardware exceed traditional instrumentation.
The DoD has created a standards body called the Synthetic Instrument Working Group (SIWG) whose role is to define standards for interoperability of synthetic instrument systems. The SIWG defines a synthetic instrument (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.

The focus of the SIWG has been primarily on the SI concepts as applied to RF stimulus and measurement systems. The group has created a standard block diagram for an RF synthetic instrument, as shown in the diagram to the left. The functional blocks in this diagram are:

  • The frequency translation devices (RF up and down converters),
  • The IF (Intermediate Frequency) input and output and,
  • The processing engine where the application-specific software is hosted.

To meet the performance of many RF applications, there must be a high-bandwidth connection between the IF devices and the processing engine, where real-time analysis is performed. For example, to digitize a 50-MHz wide RF signal requires at least 200 Mbytes/s of bandwidth (100-MS/s sampling rate at 2 bytes of resolution per sample). For both an input and output channel, this grows to 400 MB/s. And for increasingly common multi-channel, or MIMO (Multi Input, Multi Output) applications, the bandwidth required can quickly scale to multiple gigabytes per second.

An RF Synthetic Instrument

The SIWG model for an RF synthetic instrument.
Commercial technologies currently are available for building systems using the synthetic instrument model. A platform that is often used to build these systems is PXI, a multi-vendor industry standard supported by over 70 companies with over 1,200 available products, including modules from several vendors for building RF systems. PXI includes a shared high-speed backplane combined with shared timing and synchronization resources. The combination of a modular form factor, a highspeed bus, and integrated timing features makes PXI ideal for creating modular, software-based systems.

Let's take a look at an example application: a system for generating and measuring signals up to 2.7 GHz. In this example, we'll stream the data back to the host for performing software- defined measurements. This is useful because it will provide the flexibility to change the software to generate entirely different types of modulated stimulus signals or different classes of measurements. We could also use this system as a software-defined radio to prototype a real-world communication system.

In this system, the RF block downconverter translates the signal, with a real-time bandwidth of up to 20 MHz, down to the input range of the IF digitizer. The IF digitizer uses an on-board digital downconverter, implemented in an FPGA, to filter and decimate the data. The data is then streamed over the high-speed PXI backplane to a host controller running a user-defined LabVIEW program. [LabVIEW is a graphical development environment that uses a block diagram syntax for programming the system.] The LabVIEW program can thus be reconfigured to change the personality of the instrument. For example, using built-in spectral analysis functions, the system can operate as a real-time spectrum analyzer. By adding demodulation functions, measurements such as modulation error ratio and even bit-error rate can be performed. And by changing the type of modulation performed, the system can test any type of standards-based communication signal that is within its frequency and bandwidth capability. The same basic components and capabilities are also available for generation using the IF generator and block upconverter.

This PXI-based synthetic instrument can be combined with other types of instruments to create a hybrid system to extend its capabilities. For example, when paired with VXI or standalone up and down converters, the frequency can be extended to 26.5 GHz and beyond. And because the IF generation and digitization is still done in the PXI modules, the system still can stream and process data at the high data rates needed by avionics applications.

This article was written by Eric Starkloff, Director of Test Marketing at National Instruments, Austin, TX. For more information, Click Here .