Passbands

Passbands are the ranges of frequencies that a system transmits with acceptable attenuation. In filter design and signal processing, the passband is the portion of the spectrum where the system's gain remains above a defined criterion, while frequencies outside this range are suppressed or attenuated.

For a bandpass filter, the passband is bounded by a lower cutoff frequency f1 and an upper cutoff frequency f2. The bandwidth is B = f2 − f1, and the center frequency is f0, often approximated as (f1 + f2)/2. The edges at f1 and f2 mark where the response drops to the specified level, commonly -3 dB for many analog filters; real filters may exhibit passband ripple or a transition region beyond the nominal edges.

In digital signal processing and communications, the passband defines the frequencies that carry the desired information. The quality of a passband is described by parameters such as the passband ripple, the transition band, and the order of the filter. The Q factor, defined as f0/B, is a measure of selectivity.

In optics, light filters also define passbands by wavelengths rather than frequencies, transmitting only certain wavelengths and blocking others. Passbands are critical in spectroscopy, imaging, and fluorescence applications.

Practical design involves trade-offs among bandwidth, attenuation, ripple, and manufacturing or environmental tolerances. Temperature changes and component variability can shift the actual passband from its nominal specification, requiring calibration or compensation.

Related topics include bandpass filters, stopbands, bandwidth, and filter design.