Image frequency suppression in frequency-scanned direction-of-arrival estimation systems

Abstract

A novel signal processing approach to the problem of image frequency suppression in wide-band frequency-scanned radio frequency (RF) receiving arrays that employ digital signal processing (DSP) techniques to estimate the direction of arrival (DOA) of incoming radiation has been presented. The conventional approach to image rejection in RF receivers is to employ a front-end RF tuner before downconversion to the intermediate frequency. As DOA processing employs a separate receiver channel for each array element, an RF tuner would be needed behind each element. Clearly for scanned systems at microwave frequencies, this is not a viable option where the tuners would generally have to be implemented in RF hardware. The need for RF tuners can be eliminated by downconversion down to baseband and employing I&Q mixers, which provide an intrinsic image rejection capability. Unfortunately, such a solution requires two analog-to-digital (A/D) converters per array element. An alternative approach is to use image rejection mixers in which case only one A/D converter per array element is needed. The approach also requires only one A/D per array element but achieves the image rejection through a DSP implementation. As a result, use of relatively expensive image rejection mixers is avoided without sacrificing performance. Experimental results are presented that validate the theoretical predictions.