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Advanced Direct Wideband Analog to Digital Conversion for Radar

Description:

TECHNOLOGY AREA(S): Electronics 

OBJECTIVE: Develop a direct wideband (WB) analog-to-digital conversion (ADC) capability for all types of radar systems to improve performance capability and reduce costs. 

DESCRIPTION: The U.S. Army is seeking research and development in Wideband (WB) direct sampled digital downconversion technologies that can be implemented for use with all types of radar systems - past, present, and future. Many radar systems currently utilize two or three analog frequency downconversions and other signal processing operations prior to the conversion to digital inphase signal (I) and quadrature phase signal (Q) data. With multiple mixers, filters, and local oscillators (LO), these systems have variable performance over time requiring manual adjustments to maintain adequate performance. As these systems age, they require technical refreshing (tech refresh). There exists the opportunity to simplify the system and reduce the cost of the tech refresh. Operational availability for these systems can be below military benchmarks, while maintenance costs are increasing due to parts obsolescence. Digital technology has matured rapidly over the past two decades with the state of the art now defined by, among other applications, software defined radio. However, radar requirements present unique challenges in the art of signal design, signal transmission, reception, and signal processing. As far as the radar receiver is concerned, the development of an advanced, robust, truly direct WB ADC could replace the entire radar receiver subsystem. It could reduce the receive to a simple filter, low noise amplifier, and the direct WB ADC, with no stages of mixing. "Bandpass sampling can be a powerful tool that allows a relatively high frequency signal to be sampled by a relatively low-performance digitizer, which can result in considerable cost savings” (Ref. 1). If the bandpass sampling downconversion process is successfully demonstrated, then significant cost and SWAP savings could be realized by a large reduction in required parts. New radar systems will also be able to make use of direct WB ADC. Future trends will call for pluralities of smaller networked radar systems that are inexpensive yet achieve desired performance. The direct WB ADC will enable this by providing much of the radar receiver processing, requiring no mixers or associated LOs, with an corresponding reduction in analog hardware. The cost and performance is a major factor of a direct WB sampling approach that can be achieved by developing a direct WB ADC for the L band and beyond. It is anticipated that the new system will have fewer parts therefore reducing maintenance costs and more cost-effective tech refreshes. This effort requires an assessment of the feasibility of a receiver that can take in radio frequency (RF) radar return signals and output baseband I and Q in real-time with a relatively low noise figure. High Frequency and high dynamic range WB ADCs are inhibited by analog circuitry (principally down-conversion stages) in the receive chain. For example, mixers in down-conversion stages can introduce harmonics and nonlinearities. Conversion from the radio frequency (RF) domain directly to the digital domain eliminates most of these problems. Fortunately, advances in high-speed ADCs make this possible. Consequently, high-speed direct ADC presents an attractive means for high frequency and high bandwidth receivers. The WB ADC should improve on the performance of currently available ADCs. Cost, performance, and reliability are the major factors driving development of the direct digital sampler. Evidence of design optimization of these parameters, as well as a comparison between model predictions and measured performance are expected. The direct Wideband (WB) Analog-to Digital Converter (ADC) should include filtering, as required, to eliminate spurious noise. Proposed technologies should highlight innovation in the areas of frequency bandwidth, downconversion methods, SWaP, cost, reliability, and sustainability. A successful implementation of Wideband ADC for radar should reduce the cost and complexity of radar systems. 

PHASE I: The company will define and develop a concept for a Direct Wideband Analog/Digital Converter (ADC) Digital Down converter that meets the requirements as stated in the topic description. The company will demonstrate the feasibility of the concept in meeting Army needs and will establish that the concept can be developed into a useful product. Material testing and analytical modeling will establish feasibility. The concept development effort should assess the importance of several factors, such as instantaneous bandwidth, dynamic range, and sampling rates. Evidence of design optimization of these parameters, as well as a comparison between model predictions and measured performance are required. Plans for implementing the Direct WB ADC will be included as an output of Phase I, along with estimated performance. The Direct WB ADC will initially be designed to operate at L band frequencies, but demonstration at higher bands will also be desired. Bandwidths on the order of 500 MHz or greater will also be demonstrated. Dynamic Range of the ADC should also be greater than 16 bit. 

PHASE II: Based on the results of Phase I, the company will develop a prototype L-band Direct WB ADC, with a bandwidth of at least 500 MHz, for evaluation. The prototype will be evaluated to determine the capability in meeting performance goals and Army requirements. System performance will be demonstrated through prototype evaluation and modeling or analytical methods over the required range of parameters. Evaluation results will be used to refine the prototype into a design that will meet Army requirements. The system should include filtering as required to reduce potential alias input. Documentation should include analysis comparing sampling rates, bandwidths, analog downconversion, noise figure, calculation of data throughput and recommendations for data handling/reduction. The company will prepare a Phase III development plan to transition the technology to Army field use. 

PHASE III: The company will support the Army in transitioning the technology for Army field use. The company will develop a Direct WB ADC/Digital Downconverter system according to the Phase III development plan for evaluation to determine its effectiveness in an operationally relevant environment. The company will support the Army for test and validation to certify and qualify the system for Army use and transition the Direct WB ADC to its intended platform. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Direct digital downconversion has application to the commercial radar market, as well as additional military applications. The proliferation of small solid-state radars for remote sensing and navigation benefits from cost-saving digital technologies that drive affordability and consequently expand the market even further. The commercial market is typically quick to adopt technology that enhances performance while controlling cost. The technology developed under this effort will facilitate a shift from expensive RF analog receiver circuitry to receivers based on commercial microprocessor technology. Even complex commercial radars such as weather radar can benefit from this technology, as digital processing is inherently scalable, allowing radars of various size and complexity to achieve improved performance at reduced cost. 

REFERENCES: 

1: Skolnik, M. RADAR Handbook. New York: McGraw-Hill 2008.

2:  Tseng, Ching-Hsiang, Chou, Sun-Chung. "Direct Downconversion of Multiple RF signals Using Bandpass Sampling". IEEE Paper, 0-7803-7802, April, 2003.

KEYWORDS: Direct Digital To Analog, Radar 

CONTACT(S): 

David Ligon 

(301) 394-1799 

david.a.ligon.civ@mail.mil 

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