passband

A passband is the range of frequencies that a filter or other system allows to pass with acceptable attenuation. It is defined by the portion of the frequency response where the magnitude meets a specified criterion for transmission, often within a small deviation from the designed gain. Outside the passband, frequencies are increasingly attenuated in the stopband, with a transition region bridging the two.

In filter design, passbands are characterized by their shape and extent. A low-pass filter has a passband from near zero frequency up to a cutoff frequency, a high-pass filter from a cutoff frequency upward, and a band-pass filter confines transmission to a finite band around a center frequency f0. For a band-pass, the passband is typically defined by lower and upper cutoff frequencies f1 and f2, with bandwidth B = f2 − f1. In practical devices, the passband is not perfectly flat and may exhibit ripple; the transition to the stopband occurs over a range of frequencies rather than at a sharp edge.

In communications and signal processing, the term passband is also used to describe the spectral region that a system or channel occupies. A carrier signal centered at f0 is modulated to create a passband around f0 with a bandwidth determined by the modulation scheme. The term contrasts with baseband, which refers to frequencies close to zero, and with sampling considerations, where the sampling rate must be sufficient to capture the highest frequency present in the passband to avoid aliasing. Optical and RF filters likewise specify passbands in terms of transmitted wavelength or frequency range.