18 dB per octave high/lowpass filter

Calculating the correct RC values for high- and lowpass filters can be something of a chore and is often regarded by amateur constructors as a subject that is best left well alone. This is especially true the more complicated the filter and the steeper the slope of the filter becomes. That being the case, the following circuit for a third order (i.e. with a slope of 18 dB per octave) high/lowpass Butterworth filter, together with the accompanying nomogram which supplies the correct RC values for any given turnover point, should prove extremely useful.