Filter Specifications

Let us first consider the design of a lowpass filter. The first task is to specify our design criteria. Referring to Fig.B.33, we define the following design parameters:

• : stopband ripple ( dB typical)
• : passband ripple ( dB typical)
• : stopband edge frequency
• : passband edge frequency
• TW: transition width
• SBA: stop-band attenuation

The passband ripple is larger than the stopband ripple for a couple of reasons: First, it is a deviation about 1 instead of 0. A passband ripple of dB, for example, translates to on a linear scale. A stopband ripple of dB, on the other hand, equals on a linear scale. Thus, a typical passband ripple specification may be 10 times larger than a typical stopband ripple specification, on a linear scale. For a stop-band gain down around dB, keeping the passband ripple at dB, the passband ripple becomes around 100 times larger than the stopband ripple, on a linear scale.

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.