Intermittently nonlinear analog filters for mitigation of technogenic interference in the process of analog-to-digital conversion

摘要

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 (e.g. impulsive) 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, and the broadband non-Gaussian noise becomes more Gaussian-like. This 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 nonlinear analog filters preceding an analog-to-digital converter (ADC). Such ADCs thus combine analog-to-digital conversion with analog rank filtering, enabling mitigation of various types of in-band non-Gaussian noise and interference, especially that of technogenic origin, including broadband impulsive interference. This can considerably increase quality of the acquired signal over that achievable by linear filtering in the presence of such interference. An important property of the presented approach is that, while being nonlinear in general, the proposed filters largely behave linearly. They exhibit nonlinear behaviour only intermittently, in response to noise outliers, thus avoiding the detrimental effects, such as instabilities and intermodulation distortions, often associated with nonlinear filtering.