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 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.

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, visit http://info.hotims.com/10966-520.