Stopbandets

Stopbandets, in English usually called the stopbands, is a term used in filter design and signal processing to denote the frequency ranges where a filter must suppress a signal. In a typical specification, the spectrum is divided into passbands, transition bands, and stopbands. The stopband is the region where the filter's magnitude response must be below a specified attenuation level, measured in decibels (dB). The exact edges of the stopband depend on the chosen filter type (low-pass, high-pass, band-pass, or band-stop) and the desired balance between selectivity and implementation complexity.

Stopband attenuation is denoted as As, while allowable passband ripple is denoted as Ap. Designers select a transition band—the frequencies over which the response changes from passband to stopband—to balance sharpness against filter order. Common families, such as Butterworth, Chebyshev, and elliptic, differ in how rapidly they reach the stopband and how much ripple they allow in the passband.

In practice, stopbands are used to reject unwanted frequencies: anti-aliasing and anti-imaging filters before digital sampling, channel filters in communication systems, and noise suppression in audio or instrumentation. The effectiveness of Stopbandets is assessed from the magnitude response or the frequency-domain impulse response to ensure adequate attenuation over the designated stopband.

The term also appears in Swedish-language texts about filters, where Stopbandets refers to one or more stopbands of a given filter design.