stopbandedge

Stopband edge is a term used in filter design to describe the boundary of the stopband in the frequency spectrum. It designates the frequency, often labeled f_s, at which the filter is required to meet a specified minimum attenuation and beyond which the signal is suppressed to the stopband level. The stopband edge is typically defined together with the passband edge, f_p, and together they bound the transition region where the filter’s attenuation rises from the passband level to the stopband level.

In common terminology for low‑pass filters, frequencies above the stopband edge f_s must be attenuated to at least As, while frequencies up to the passband edge f_p remain in the passband with minimal ripple Ap. The transition bandwidth is the frequency range between f_p and f_s. For high‑pass filters, the stopband edge marks the upper boundary of the stopband (the region below f_s is attenuated). Band-pass and band-stop filters can have two stopband edges, creating a stopband between them.

The stopband edge is a design parameter, chosen to satisfy specification requirements on passband ripple (Ap) and stopband attenuation (As). In digital implementations, frequencies are often expressed as normalized frequencies or radians per sample, but the conceptual role remains the same: f_s defines where the attenuation meets the stopband specification and the stopband begins.