A team from the Georgia Tech Research Institute (GTRI) led by Mike Willis, principal research engineer, has been awarded U.S. patent 7,113,229 jointly with the U.S. government for a DSP technique that can enhance the robustness of radar warning receivers (RWRs). The patent covers a new device, the digital crystal video receiver (DCVR), which is a significant improvement on the original crystal video receiver, an analog subsystem in RWRs.

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Willis stated that conventional crystal video receivers use banks of direct-conversion receivers, with each circuit covering a specific portion of the total radar spectrum monitored by the warning system. The specific frequency range of each receiver is determined by its front-end bandwidth; each front end is connected through a detector diode to the input of a logarithmic amplifier. The log-amp performs analog signal processing on the captured RF signals for use as radar target video data by the subsequent stages of the RWR.

The fundamental advance realized in the prototype of the DCVR is to implement the logarithmic transfer function digitally, thereby eliminating the need for a bank of redundant receiver circuits. The digital system replaces all the associated log-amps in the original crystal video receiver with detector diodes feeding their corresponding digitizers, which in turn, collectively feed a single DSP.

According to Willis, analog components such as log-amps are particularly vulnerable to fluctuations in temperature. Because the new receiver architecture shifts the logarithmic transfer function from the analog domain to the digital domain, the characteristics of that transfer function cannot be altered by temperature or other factors that usually affect analog circuits. While the analog transfer function of the detector diode remains a consideration in the new DCVR's architecture, any transfer function shifts are easily managed with a compensation algorithm controlled by the temperature of the corresponding detector diode. The diode's temperature can be measured directly from a simple sensor.

The ADC sampling rate for the DCVR prototype was approximately 40 MHz to support a video-processing bandwidth of 20 MHz. The DSP was implemented in a conventional FPGA, which is generally faster than a microcontroller. Because the prototype DCVR was intended for operation as a subsystem in a conventional analog RWR, the processed digital video data was converted back to analog using a DAC. However, Willis stated that keeping the data in digital form can enable advanced capabilities in future RWRs, such as those provided by radar signature analysis algorithms.

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