StopbandAttenuation

Stopband attenuation is a specification of how strongly a filter suppresses frequencies outside its desired passband. It refers to the minimum attenuation, typically expressed in decibels (dB), that the filter provides for frequencies in the stopband. A higher stopband attenuation means greater suppression of unwanted spectral components, reducing leakage and interference from adjacent channels or noise.

In filter design, stopband attenuation is defined alongside other parameters such as passband ripple and the transition band. The stopband begins at the stopband edge frequency, and the filter should ensure that frequencies beyond this edge are attenuated by at least the specified As dB. The requirement often drives the necessary filter order: larger As generally necessitates more aggressive filtering, which increases computational or implementation complexity for FIR and IIR designs.

Common design approaches include finite impulse response (FIR) methods using windowing, frequency sampling, or optimization techniques (such as equiripple designs). For infinite impulse response (IIR) filters, classical families (Butterworth, Chebyshev, Elliptic) offer different trade-offs between passband flatness and stopband attenuation. The transition band width, determined by the gap between passband and stopband edges, is a key design lever alongside As.

Applications span communications, audio processing, and instrumentation, where adequate stopband attenuation suppresses adjacent-channel interference, out-of-band noise, or spectral aliases. Practically, As is specified so that the filter meets system-level performance targets at the frequencies of interest, guaranteeing acceptable attenuation of unwanted signals.