Signal Conversion Takes the Fast Track

January 2002
By John Lillington

Real-time processing enables rapid detection of fleeting signals.

A new digital signal processing technology originally developed for the commercial world now is being incorporated in military systems where it offers significant improvements over current techniques. Known as pipelined frequency transform, the architecture is a licensable intellectual property of cores, or engines, that can be included in programmable logic devices such as semiconductors or system-on-chip designs. Major defense application areas include advanced radar, signals intelligence, secure wireless communications and electronic warfare.

Most modern electronics, whether in a new refrigerator or the latest airborne radar, have a common characteristic—digital signal processing. The technique that was once reserved for the more sophisticated electronic systems is now an integral part of everyday life, from placing a simple telephone call to a child interacting with a toy that contains a voice recognition chip. Making this technology even smaller, faster and more complex can change the many systems it influences.

The digital world starts with the conversion of analog signals into the binary system. The speed of this analog-to-digital conversion determines the amount of information that can be gathered. Analog-to-digital converters (ADCs) operating at extremely high speeds are readily available, and both speed and performance are still increasing rapidly. Real-time processing of this information at the very highest speed, however, is both difficult and expensive.

Pipelined frequency transform (PFT) is a new processing method that overcomes current limitations and can take the digitized data at the full ADC speed and process it in real time with no loss of data. The design of the architecture readily accommodates further speed increases in the analog-to-digital conversion rate. All the digitized information is presented in a channelized form and with better definition of each channel than was possible previously, allowing the entire spectrum of interest to be seen all of the time.

PFT is an example of a technology designed for a commercial application, in this case for satellite communications monitoring, that is now being applied to military and government needs—a reversal of traditional trends. As a commercial product, it has been developed to be flexible and consequently can be adapted to these different applications. In practice, the design can be tailored for each specialized project. For example, it can be adapted for an eight-channel communications system or even a 32,000-channel signal intelligence system. PFT can zero in on a signal of interest while continuing to monitor in real time the complete spectrum of interest.

The technology provides a solution for signal processing designs that were not practical until now because of the cost and size of their semiconductor chips. To some extent, PFT has been regarded as a disruptive technology because it challenges the previously accepted design approaches that are incorporated in advanced digital systems on the market today. These systems are the product of design work done within the constraints of a seriously limited ability to process high-speed digital outputs.

However, the military has been open to exploring the new approach because this technology will impact a key element of modern electronic warfare systems: the ability to acquire and identify enemy signals quickly, which allows time for the operator to choose the appropriate action. Speed of operation and the ability to detect and acquire signals that are only fleetingly apparent, are tracking across the spectrum very quickly or can barely be seen against the background electronic noise—particularly across a wide frequency range—are the key requirements of any signals intelligence system. The design of the radar-warning receiver is critical; however, constraints on space, cost, environment and power must be traded to optimize performance.

Similarly, modern radar systems such as multibeam or synthetic aperture radar require very sophisticated real-time signal processing. This ranges from channelization and matched filtering to pulse compression and wideband frequency analysis to extract echoes of small and fast-moving targets from a background of noise and clutter, including echoes from ground and sea and false targets, for example.

With PFT, it is now possible to provide real-time processing of up to 100 megahertz bandwidth in a single chip, compared to current systems that may use a number of circuit boards to perform the same function. Specialist functions that could be carried out only on large aircraft or fixed installations can now be carried out by smaller systems that are capable of equal or superior performance and that deliver significant advantages in the battlefield and airborne spaces.

PFT receives wide bandwidth signals that have been digitized. It then splits the signal into many sharply filtered channels so that each channel can be analyzed for signal content. It is a real-time process, acting on all of the data all of the time.

Signal detection systems must be able to detect the enemy from as far away as possible. This means being able to detect signals at low levels with a high degree of sensitivity. PFT has the ability to detect a broad range of signal levels, known as dynamic range, by using a unique filter architecture.

Current techniques normally use either a fast Fourier transform (FFT) approach or a bank of digital down-converters. The FFT approach is very efficient in computational terms but suffers from poor filter performance. Improvements in FFT filter performance can be achieved by overlapping the analysis; however, this entails a significant amount of additional chip area and therefore an increase in cost. Adopting the digital down-converter filter bank approach provides the necessary dynamic range, but again requires a vast amount of silicon to carry out the processing. Consequently, neither of these approaches achieve the goal of a small, highly efficient design.

In contrast, PFT provides a unique combination of dynamic range and efficient filter design, which reduces the silicon and power requirements, enabling a compact design and deployment in many more applications. Moreover, a speed-increase factor of more than 20 times is achieved over conventional programmable digital signal processing techniques.

The architecture is scalable and offers the option of intermediate stage outputs if required. It is configurable so that trade-offs between dynamic range, selectivity, throughput rates and silicon gate requirements can be carried out under the designer’s control to provide the optimal solution for each application. In addition, it is completely cascadable. Adding PFT stages increases the number of points and therefore provides higher resolution. Conversely, finer resolutions are achieved when a smaller input bandwidth is used.

PFT also can be tuned to allow flexible dynamic channelization of broadband spectrum. Consequently, the system designer can identify the performance level that will be necessary to meet design objectives for applying the technology to specific projects or tasks. In the field of operations, this built-in flexibility can save power resources when the system is in background monitoring mode.

The technology is being designed into the next generation of U.S. and European radar and electronic surveillance systems following rigorous evaluations. Initial product deliveries are due in late 2002.

John Lillington is the chief executive officer and chief technology officer of RF Engines Limited.

Additional information on pipelined frequency transform is available on the World Wide Web at http:/

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