Passbandedge

Passband edge refers to the boundary frequency that defines where a filter or system transitions from passing a signal with minimal attenuation to attenuating it more significantly. It delineates the end of the passband and the start of the transition region between passband and stopband.

In filter design, the passband is the range of frequencies that are allowed to pass with little loss, while the stopband contains frequencies that are attenuated to meet a specification. The passband edge frequency, often denoted as fp, marks the upper limit of the passband for low-pass filters or the lower limit for high-pass filters. For band-pass filters, there are typically two passband edges, fp1 and fp2, defining the lower and upper bounds of the passband. The exact position of the edge depends on the design specifications, including permissible passband ripple and the required amount of attenuation in the stopband.

Specifications commonly use Ap to describe allowable ripple within the passband and As to describe minimum attenuation in the stopband. The region between the passband edge(s) and the stopband edge(s) is the transition band, where the filter’s attenuation increases from the passband level toward the stopband level. In digital or RF systems, edge frequencies can be given in Hertz, normalized frequency, or radians per sample, depending on the context.

Practical filters never have an infinitely sharp edge; the edge is defined by performance criteria and chosen to meet system requirements while balancing factors such as complexity and latency.