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Digital RF memories.

The DRFM was conceived in the early seventies as a method for storing and replicating RF signals.|1~ The generic DRFM consists of five main subassemblies. The DRFM process typically involves the following steps:

1. Tuning a local oscillator (LO) to the approximate frequency of the intercepted signal. A variety of emissions can be captured -- the pulse train of a search radar, the lock-on signal of an active missile, an IFF interrogation and so forth.

2. Down-converting the intercepted waveform to baseband, producing in-phase and quadrature (I and Q) components of the signal waveform. Down-conversion is accomplished in the mixer by combining the input waveform with the LO output. 3. Sampling and digitizing (in the A/D converter) the I and Q signal components. 4. Digitally storing the I and Q waveforms in memory. Once in memory, the signal can be studied, corrected or modified.

5. Reconstituting the analog I and Q video by digital-to-analog (D/A) conversion at playback time.

6. Reconstructing an RF version of the original signal, at an appropriate time determined by the control sub-assembly. The RF output is generated by means of single-sideband modulation with the I and Q video signals producing the modulating waveforms. The LO of step 1 supplies the carrier for this upconversion process.

For faithful reproduction of a signal of bandwidth B, in hertz, and duration T, in seconds, at least 2BT samples are required (BT in-phase samples and BT quadrature samples). Modern radars employ coded waveforms with signal bandwidths up to several hundred megahertz, requiring sampling rates of comparable magnitude for the DRFM. The accuracy of sampling degrades as the sampling rate goes up and the coarseness of quantizing grows as the time available for each A/D conversion decreases.

DRFM APPLICATIONS

Early radar deception approaches involved the reception of interrogating waveforms, followed by a relatively short time delay and reradiation to mislead the enemy trackers. As radar operators and electronic counter-countermeasures grew in sophistication, this relatively simple approach lost its effectiveness. The advent of coherent radars, especially those found in pulse Doppler fire control and active missile guidance systems, placed greater demands upon the ECM community.

The DRFM allowed greater manipulation of the captured signal and opened a new arena of ECM. An array of apparent targets can be simulated which impart range, angle or velocity information to the victim radar. These targets can be false, confusing or nonexistent. Since the DRFM preserves the phase of the incoming signal, retrodirective jamming signals can be generated with no need to measure or control the phase of the output signal.

As the potential of DRFMs became better understood, additional applications have evolved. Present-day applications include:

* radar signal reception, storage and analysis for ELINT applications

* deception of covert communications systems, especially spread spectrum frequency hoppers

* disruption of |C.sup.3~I and IFF systems

* receivers for unique RF IFF systems

* testing and simulation of sophisticated radar, radio and communication systems

* antiradiation missile decoys, by storing the main-, side- and backlobes of friendly fire control radars and calling them up for reradiation from false locations

* radar matched filter technology, etc.

WHERE ARE DRFMs GOING?

Speed appears to be a significant factor driving DRFM technology. Important operational benefits are expected with the introduction of gallium arsenide (GaAs) digital microcircuits. Under Defense Advanced Research Projects Agency sponsorship, technology insertion efforts were awarded to demonstrate the potential of high-speed GaAs circuits for upgrading existing programs.|2~

ITT Avionics (Nutley, NJ) explored the suitability of a GaAs-based DRFM for use in the ALQ-136 airborne jammer used in Army helicopters and special electronic mission aircraft. The ITT program resulted in the development of a 15-W, 20-cu-in. DRFM (downsized from its 70-W, 70-cu-in. predecessor) with improved operational characteristics. KOR Electronics (Garden Grove, CA) investigated this technology for the pod-mounted ALQ-167 jammer simulator, and Lockheed Sanders (Nashua, NH) incorporated GaAs-based DRFMs into the Navy's ALQ-126 jammer.

Another promising area of research is the joint project between the Air Force's Wright Laboratory and the Institute of Technology to build DRFMs on a chip.|3~ The goal of this work is to achieve digital modulation over both the amplitude and phase of the captured signal on a sample-by-sample basis. Ultimately, all the required converters, memory and control functions will be placed on a single chip.

ABOUT THIS SURVEY

Information for this survey was gathered in three phases. Initially, an attempt was made to identify the DRFM fabrication community. Some manufacturers produced DRFMs for the commercial market; others, such as Northrop, ITT and Sanders, produced very specialized devices for use only in their own systems and subsystems. We solicited a sampling of specifications of some of the commercially available DRFMs.

This information was sifted and digested and a preliminary questionnaire encompassing a selection of properties was generated. This questionnaire was submitted to several vendors for "beta" testing. We sought their expertise in polishing and fine tuning the inquiry. Our sincerest appreciation is extended to those who shared their time and knowledge with us at this point.

Finally, a questionnaire incorporating many of the suggestions and corrections was generated and submitted to the involved community. Perhaps the most difficult part of the process was identifying the active DRFM manufacturers. At best we can only say that the survey presented on these pages reflects the products of a portion of the DRFM community. To those companies whose products were overlooked, we express apologies. Please contact us to ensure your being included in future revisits to DRFM technology.

REFERENCES

1. "Digital Storage System for High Frequency Signals," US Patent #3,947,827, Dautremont, Jr. et al., dated March 30, 1976, Whittaker Corp., Los Angeles, CA. 2. Klass, Philip J., "Defense Dept Contracts to Spur Use of Digital GaAs Microcircuits," Aviation Week & Space Technology, June 12, 1989, p. 283.

3. Gehly, Darryl T., "Business as Usual," Journal of Electronic Defense, September 1991, p. 66.

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Title Annotation:EW Reference & Source Guide: Survey Section; radio frequency
Publication:Journal of Electronic Defense
Date:Jan 1, 1994
Words:983
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