Nonlinear rank-based analog loop filters in delta-sigma analog-to-digital converters for mitigation of technogenic interference

Abstract

Since at any given frequency a linear filter affects both the noise and the signal of interest proportionally, when a linear filter is used to suppress the interference outside of the passband of interest the resulting signal quality is affected only by the total power and spectral composition, but not by the type of the amplitude distribution of the interfering signal. Thus a linear filter cannot improve the passband signal-to-noise ratio, regardless of the type of noise. On the other hand, a nonlinear filter has the ability to disproportionately affect signals with different temporal and/or amplitude structures, and it may reduce the spectral density of non-Gaussian interferences in the signal passband without significantly affecting the signal of interest. As a result, the signal quality can be improved in excess of that achievable by a linear filter. Such non-Gaussian (and, in particular, impulsive) noise can originate from a multitude of natural and technogenic (man-made) phenomena. The technogenic noise specifically is a ubiquitous and growing source of harmful interference affecting communication and data acquisition systems, and such noise may dominate over the thermal noise. While the non-Gaussian nature of technogenic noise provides an opportunity for its effective mitigation by nonlinear filtering, current state-of-the-art approaches employ such filtering in the digital domain, after analog-to-digital conversion. In the process of such conversion, the signal bandwidth is reduced, which substantially diminishes the effectiveness of the subsequent noise removal techniques. In this paper, we focus on impulsive noise mitigation, and propose to incorporate impulsive noise filtering of the analog input signal into loop filters of ΔΣ analog-to-digital converters (ADCs). Such ADCs thus combine analog-to-digital conversion with analog nonlinear rank filtering, enabling mitigation of various types of in-band non-Gaussian noise and interference, including broadband impulsive interference. An important property of the presented approach is that, while being nonlinear in general, the proposed ADCs largely behave linearly. They exhibit nonlinear behavior only intermittently, in response to noise outliers, thus avoiding the detrimental effects, such as instabilities and intermodulation distortions, often associated with nonlinear signal processing.