Filtering practice - Linear filtering: analog filtering

This work was born out of the need to make available to engineers and technicians a wide range of current filtering methods. It also aims to demystify aspects considered abstruse by providing keys to good practice in filtering operations.

In 1988, the late Jacques Max, scientific assistant at the Commissariat à l'Énergie Atomique in Grenoble, wrote a very interesting contribution to "Techniques de l'Ingénieur" on the practice of analog filtering: our article is intended to complement and extend the practice of filtering to recent developments in both "analog" and digital, one- and two-dimensional techniques.

For example, synthesizing an analog filter to meet constraints or specifications no longer requires the use of abacuses or tables, transformations of a prototype low-pass filter into a high-pass or band-pass filter, and even less normalization/denormalization. The filter is created directly by a MATLAB code that provides the parameters for the desired template, and all that's left for the experimenter to do is install the electronic components and obtain the desired analog filter.

But what is filtering?

Filtering is an operation that consists in transforming the information (contained in a signal) at the input of a hardware or software system into output information that is different from the original information, but more useful to the experimenter.

In the case of a one- or two-dimensional signal, this transformation can take the form, for example, of either the selection or elimination of certain frequencies, or the reduction or even suppression of unwanted information. Examples include white light being transformed into blue light, or an e-mail or website being blocked or "filtered" by an electronic device or computer code acting according to certain criteria. The extraction or estimation of relevant information and useful characteristics can also be considered as filtering.

The classification of filtering devices (hardware and software), called filters, can be based on several descriptors. We have opted for two main filter families: linear filters and non-linear filters. These two families can be further subdivided into continuous-time (or variable-time) filters, also known as "analog" filters, and digital filters.

This article reviews the basic concepts and methods for synthesizing and building analog linear filters from a desired template.

Throughout this article, the reader is provided with numerous examples and application exercises to illustrate the results obtained: the examples always have a pedagogical purpose or an approach to the concrete with a view to creating a "tailor-made" filter. MATLAB codes...