Band-pass filter
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A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit (a resistor-inductor-capacitor circuit). These filters can also be created by combining a low-pass filter and a high-pass filter.
An ideal filter would have a completely flat passband (e.g. with no gain/attenuation throughout), and would completely attenuate all frequencies outside the passband. Additionally, the transition out of the passband would be instantaneous in frequency. In practice, no bandpass filter is ideal. The filter does not attenuate all frequencies outside the desired frequency range completely; in particular there is a region just outside the intended pass band where frequencies are attenuated, but not rejected. This is known as the filter roll-off, and is usually expressed in dB of attenutation per octave of frequency. Generally, the design of a filter seeks to make the roll-off as narrow as possible and thus for the filter to perform as close as possible to its design. However, as the roll-off is made narrower, the passband is no longer flat — it begins to "ripple." This effect is particularly pronounced at the edge of the passband, an effect known as Gibbs phenomenon.
Outside of electronics and signal processing, one example of the use of band-pass filters is in the atmospheric sciences. It is common to band-pass filter recent meteorological data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.
Between the lower cutoff frequency f1 and the upper cutoff frequency f2 of a frequency band is the resonant frequency, at which the gain of the filter is maximum. The bandwidth of the filter is simply the difference between f2 and f1.
See also
es:Filtro paso banda fr:Filtre passe-bande pl:Filtr środkowoprzepustowy pt:Filtro passa-faixa ru:Полосовой фильтр sv:Bandpassfilter