# Band-pass filters

## Frequency response

Bandwidth ΔΩ=f0 - fu within the stop band of the band-stop filter is defined by the cut-off frequencies f0 and fu. They are defined as frequencies where the amplitude dropped by –3dB (~0.707). When a band-stop filter is configured, the bandwidth is determined by specifying the quality factor (filter[i].guete) . The quality factor is the ratio of the characteristic filter frequency to bandwidth ΔΩ:

For this reason, the frequency range f0-fu reduces as the quality rises. The following relationship exists between the characteristic frequency of the band-stop filter and the cut-off frequencies f0 and fu (geometric mean):

With band-stop filters, the filter prototype has only a slight influence on filter response.

Amplitude frequency response of band-pass filters (4th order, quality = 1)