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.

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.