Ten Essential Skills for Electrical Engineers

7

HOW TO DESIGN ANALOG FILTERS

Filters are used to pass or reject signals based on their frequency content. Filter applications vary widely; a radio receiver uses filtering to isolate a single desired station from all of the signals arriving at its antenna, a radio transmitter uses filtering so that it emits signals only in the frequency band for which it is licensed, a measurement system uses filters to remove noise from sensor inputs, and printed circuit boards use bypass capacitors to filter the power connections to integrated circuits. Analog filters can be implemented with active topologies using op-amps, or passive topologies using inductors and capacitors.

Once a filter is fully specified, the components can easily be determined using excellent textbooks and online resources. The skill of specifying the filter is more valuable than the skill of selecting the components because it requires knowledge of the system design goals, knowledge of the signals to be filtered, knowledge of the components and systems surrounding the filter, and knowledge of the different filter types. The examples and problems in this chapter are all designed to give you experience converting system specifications into filter specifications and ultimately into practical filters.

This chapter begins with a discussion of passive versus active filters. This is followed by a detailed discussion of the commonly encountered lowpass RC filter. Next we describe characteristics of three common filter types: ...