A Compact Analog Complex Cross-Correlator for Passive Millimeter-Wave Imager

The design, analysis, implementation, and measurement of a compact analog complex cross-correlator for passive millimeter-wave imaging applications are presented in this paper. The correlator uses the “add and square” and “subtract and square” detection scheme by adopting the Schottky diodes and RF distribution network. Meanwhile, in order to realize the correlator with a compact size and low cost, commercially available surface mounting devices and multilayer PCB technique are employed. The relationship between the phase error of the correlator and the phase unbalance of the RF distribution network is also analyzed. Single-frequency test is applied to the correlator, and the results of the measurement reveal that the correlator operates well within the frequency range of 1.5–2.5 GHz. Moreover, a measurement system based on an actual imaging scenario is also developed to characterize the performance of the correlator when injected with correlated broadband noise signals, and the measurement results show that the correlator is well suited for imaging applications. The bandwidth of the current version correlator is around 1 GHz, but it could easily be changed by replacing distributed components.

I. Introduction

Interferometric imaging techniques are widely used in a variety of applications, such as satellite remote sensing [1], radio astronomy [2], and security sensing [3], [4]. For the interferometric imagers, the image is reconstructed through the complex cross correlation of the signals received from each antenna pairs [4], [5]. Since the correlator is used to perform the cross correlation measurement (which is also termed as visibility function measurement) [6], it plays an important role in the interferometric imagers [7].