A bandpass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Optical bandpass filters are of common usage.
An example of an analogue electronic bandpass filter is an RLC circuit (a resistor–inductor–capacitor circuit). These filters can also be created by combining a lowpass filter with a highpass filter.[1]
Bandpass is an adjective that describes a type of filter or filtering process; it is to be distinguished from passband, which refers to the actual portion of affected spectrum. Hence, one might say "A dual bandpass filter has two passbands." A bandpass signal is a signal containing a band of frequencies away from zero frequency, such as a signal that comes out of a bandpass filter.[2]
An ideal bandpass 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 passband where frequencies are attenuated, but not rejected. This is known as the filter rolloff, and it is usually expressed in dB of attenuation per octave or decade of frequency. Generally, the design of a filter seeks to make the rolloff as narrow as possible, thus allowing the filter to perform as close as possible to its intended design. Often, this is achieved at the expense of passband or stopband ripple.
The bandwidth of the filter is simply the difference between the upper and lower cutoff frequencies. The shape factor is the ratio of bandwidths measured using two different attenuation values to determine the cutoff frequency, e.g., a shape factor of 2:1 at 30/3 dB means the bandwidth measured between frequencies at 30 dB attenuation is twice that measured between frequencies at 3 dB attenuation.
Bandpass filter controls
Lower cutoff: Defines the lower cutoff frequency fL for the filter
Upper cutoff: Defines the upper cutoff frequency fH for the filter
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