Preface
This book should allow anyone with basic electronics skills to quickly design a passive Butterworth filter. All possible low pass and high pass filters up to tenth order, and all possible band pass and band stop filters up to eighth order are covered. This means schematics and component values for these filters are given along with formulas for scaling the values to the particular frequency the filter must operate at.
For each filter type and order there are four variations. Variation 1 is for a finite source resistance and infinite or very high termination resistance. Variation 2 is for an ideal zero resistance source and a finite termination resistance. Variations 3 and 4 are for the balanced case of equal source and termination resistance.
There is a design example for each of the filter types that shows how to scale the component values. A Spice simulation file for the design is given along with the frequency response. Running a simulation on a filter is useful for looking at the effect that non ideal components have on the frequency response. Some inductors for example may have significant series resistance which can be included in the simulation.
If you want band pass or band stop filters higher than eighth order the book explains how to use a low pass filter to construct them. So the information is there to construct filters up to twentieth order but we don't recommend trying to do that. The non ideal nature of the components makes it hard to get the expected extra performance.
The alternative to a passive Butterworth filter is of course an active Butterworth filter but active filters do not perform well at frequencies above $100$ kHz. The passive filters presented in this book are usually the only practical choice for frequencies from $1$ MHz up to about $1$ GHz. They are also the only choice in situations where the power required for active filters is not available such as in audio crossover networks inside speaker enclosures.
In some cases only a simple first or second order filter will do the trick so the book starts out discussing these filters. They require at most one inductor which makes them easy to design and build. Even if you are sure you need a higher order filter it is a good idea to read over this material for general insights into filter behavior we think it provides.
You can find more information on signal processing and other engineering related topics at our website: http://www.exstrom.com/pande/index.html
Information on digital Butterworth filters can be found in our book Recursive Digital Filters: A Concise Guide. The website for that book is http://www.abrazol.com/books/filter1/
Finally, the website for the book you are now reading is http://www.abrazol.com/books/filter2/ where we will post related resources.