1-Bit/2-Level Analog-to-Digital Conversion Based on Comparator and FPGA for Aperture Synthesis Passive Millimeter-Wave Imager

Abstract

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.

Odd/Even Order Sampling Soft-Core Architecture Towards Mixed Signals Fourth Industrial Revolution (4IR) Applications

“…In related fields such as control systems, the analog-to-digital stage is very important in maintaining a high-quality control feedback signal [22]. In other related fields such as interferometric aperture, synthesized passive millimeter wave high resolution images can be produced using a 1-Bit/2-level comparator quantizer [23].…”

A Novel Near Field Image Reconstruction Method Based on Beamforming Technique for Real-Time Passive Millimeter Wave Imaging

“…last few years [10]- [12], which are used to verify the ability to detect concealed objects and validate the advantages of high imaging rate and large FOV. At present, an improved PMMW imager using 1024 antenna-receiver channels with 1GHz bandwidth has been developed by Beihang University [13]- [15]. As shown in Figure 1, the 1024-channel PMMW imager (BHU-1024) uses linear phased array to obtain resolution in the horizontal direction and uses aperture synthesis to obtain resolution in the vertical direction.…”

Calibration of Visibility Samples for Real-Time Passive Millimeter Wave Imaging

“…In correlation processing, 64 baseband IQ signals are digitized and power detected, then transmitted to a digital correlator array which is controlled by a host computer to get visibility function samples. In view of the high cost and power consumption of the multi-bit analog-to-digital convertor (ADC), the 1-Bit/2-Level ADC is adopted, which can be implemented using comparators [12]. Finally, the host computer uses the visibility function samples to create images.…”