Simplified 1/f noise calculations

A new tool, called the equivalent number of decades ND_e, is introduced to simplify the analysis of noise in circuits containing 1/f noise sources. ND_e is similar to the familiar concept of equivalent noise bandwidth for white sources. A simple expression for ND_e is derived for a single-pole lowpass filter and is shown to be approximately correct for Butterworth and Chebyshev filters as well.