6.1 Introduction

Filters are general signal processing blocks used in virtually every modern electronic system. Whenever a filter's parameters must track poorly controlled or time-varying conditions, adaptive filters are an attractive option. Currently, the vast majority of adaptive filters are implemented digitally and a wealth of literature has been published on the topic [1]. This chapter provides an overview of analogue adaptive filters which represent an important niche in adaptive filter theory and practice. ^[1]

At low speeds, adaptive filtering is easily and efficiently performed using digital circuits. On the other hand, analogue filters are preferable at high speeds when low power consumption, small integrated area and moderate linearity are required. As digital logic continues to shrink and increase in speed, the minimum speed at which analogue signal processing becomes beneficial increases. Therefore, our focus will be on analogue adaptive filters suitable for high-speed applications, where they will continue to be an important part of systems for years to come. At the same time, considerable attention is paid to historical developments. Although the developments themselves may no longer be of practical use, they provided a foundation for the development of modern filters in use today.

The designer of a modern analogue adaptive filter is required to simultaneously consider both system- and circuit-level issues. However, to provide this chapter with some logical organisation, a top-down approach is used. Specifically, the reader is first introduced to the algorithms (Section 6.2) and filter structures (Section 6.3) used...