We present a 64-channel 1-bit/2-level cross-correlation system for a passive millimeter wave imager used for indoor human body security screening. Sixty-four commercial comparators are used to perform 1-bit analog-to-digital conversion, and a Field Programmable Gate Array (FPGA) is used to perform the cross-correlation processing. This system can handle 2016 cross-correlations at the sample frequency of 1GHz, and its power consumption is 48.75 W. The data readout interface makes it possible to read earlier data while simultaneously performing the next correlation when imaging at video rate. The longest integration time is up to 68.7 s, which can satisfy the requirements of video rate imaging and system calibration. The measured crosstalk between neighboring channels is less than 0.068%, and the stability is longer than 10 s. A correlation efficiency greater than 96% is achieved for input signal levels greater than −25 dBm.

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A Ka-band 1024-channel passive millimeter wave (PMMW) imager with 1 GHz bandwidth called BHU-1024 has been developed by Beihang University for security screening. BHU-1024 uses linear phased array to obtain resolution in the horizontal direction and uses aperture synthesis to obtain resolution in the vertical direction. The non-ideal characteristics of the hardware cause the decrease of system sensitivity and always blur the reconstructed image, hampering applications where high resolution and accurate recognition are emphasized. Hence, effective calibration is a prerequisite to ensure the quality of the millimeter wave image. In this paper, the overall calibration approach is presented, focusing on the main errors relevant to the BHU-1024 instrument design: The basic correlator counts are preprocessed to correct for comparators offset and quadrature error. The residual offset is estimated by continuously sweeping the local oscillator (LO) phase. And for the baseline error, a novel and effective calibration approach, which makes use of an external noise source mounted on the metal frame, is proposed to tackle this problem. The results of simulation and measurement demonstrate the validity of the overall calibration approach. Finally, the actual calibrated data is used to reconstruct the millimeter wave image, which further validates the calibration process. INDEX TERMS Calibration of visibility samples, hardware imperfections, passive millimeter wave imaging, security screeningThis work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.

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Interferometric aperture synthesis is a proven technique in radio astronomy and earth remote sensing, which also shows great potentials in security screening. An aperture synthesis passive millimeterwave (PMMW) imager is under development at Beihang University, which is designed for concealed contraband detection on the human body in an indoor environment. This imager uses 256 antenna-receiver channels with 1 GHz bandwidth and can obtain a radiometric sensitivity less than 1 K at a video imaging rate (∼25 frame/s). One of the greatest challenges in this system is the development of a digital correlation subsystem capable of analog-to-digital (A/D) conversion and subsequent signal processing among the system's 256 channels. In this paper, a comparator-based 1-bit/2-level (1B/2L) A/D conversion architecture is presented. The main error sources during sampling are identified as the timing error of sampling clocks and threshold offset of comparators and analyzed in detail. The sampled data are captured by field programmable gate arrays (FPGAs) to perform further signal processing, and a data capture module performing the serialto-parallel conversion and per-bit deskew is designed in the FPGA to transfer sampled data from the sampling clock domain to the internal processing clock domain. A 64-channel test system is built to verify the design, and a correlation efficiency of 92.5% to 99.6% is observed at 1 GHz sampling frequency. It is found that the correlation efficiency degradation to less than 98% is caused by the threshold offsets of comparators which can be compensated using a digital-to-analog converter (DAC) or programmable potentiometer.INDEX TERMS Interferometry, aperture synthesis, 1-bit, analog-digital conversion, comparator, FPGA.