A 3.5-8GHz Analog Complex Cross-correlator for Interferometric Passive Millimeter-wave Security Imaging Systems

The detection of concealed-weapons and explosives on human body is important for protection against terrorist threats. Due to the low intrusiveness and the ability to pass through clothing and atmospheric occlusions, millimeter wave imaging technique has been gaining interest in security sensing applications. Interferometric imaging technique is well suited for passive millimeter wave security imaging systems since interferometric imaging has the benefits of high imaging rate and large field of view. For interferometric imaging systems, the cross-correlator is a vital component since it performs cross-correlation measurement of the outputs of the antenna pairs in the imaging system. In this paper, the design, analysis, and implementation of a 3.5-8 GHz analog complex cross-correlator for interferometric passive millimeter-wave security imaging applications are presented. The correlator achieves large bandwidth with low cost by using the "add and square" detection scheme which employs zero-bias Schottky diodes and simplified RF network. The bandwidth requirement for the security imaging system is also analyzed. In order to investigate the performance of the correlator, single-frequency test with different input power levels is applied to the correlator. The measurement results reveal the input power dependent property of the correlator. The frequency dependent property of the quadrature error presented in the correlator is also demonstrated. The proposed correlator is applied to a Ka-band passive millimeter-wave security imager which consists of 256 receivers, the image of a point source and an actual imaging scenario are produced in order to show the validity and performance of the correlator. From the single-frequency test and the imaging system test, the correlator is well suited for interferometric security imaging applications.