中文说明：大多数实用的滤波器都表现出稳定的衰减特性
English Description：
Most practical filters exhibit steady roll-off characteristic, in contrast to the sharp transition from the pass and stop bands. The important families of filters are: Butterworth, Chebyshev and Elliptic. As part of this lab, we study and design Butterworth filters. Butterworth filters are simple to design and have a maximally flat pass band response. In contrast the Chebyshev and Elliptic filters are difficult to design and have ripples in pass/stop band but have sharper roll off characteristics than Butterworth filters. In the following we shall study the important characteristics of a Butterworth low pass filter that we