What is a narrowband filter?

Narrow bandpass filters are defined according to the amplitude-frequency characteristics of the filter's frequency characteristics. If a pole filter is set up at the frequency of the signal to be extracted by the digital filter, the pole is selected as d=re^j2πkfT. When the r value is close to 1, a very narrow passband and a relatively steep transition band can be obtained, so it is called is a narrow bandpass filter.

In practice, bandpass filters are not ideal and will not completely attenuate every frequency outside of the preferred frequency selection. In particular, where frequencies are attenuated, there is a portion outside the proposed passband, but which is not discarded, this is called filter roll-off, and typically, for each octave, attenuated The unit is dB, otherwise it is ten times the frequency.

Typically, filter designs look as thin as possible to establish roll-off, so let the filter do the proposed design. Typically, this is accomplished by consuming passband ripple and otherwise stopband ripple.

Filter bandwidth can be defined as the dissimilarity between higher and lower frequencies. The shape factor is the fraction of bandwidth calculated using two different attenuation values ??and is used to determine the cutoff frequency. For example, a shape factor of 2:1 at 20/2dB means that the bandwidth calculated over the frequency range with 20dB attenuation is double that calculated over the frequency range with 2dB attenuation.

Optical BPF is commonly used in photography and lighting work in theaters. These types of filters employ the outline of a transparent colored film that would otherwise be thin.

Extended information

The ratio of the upper cutoff frequency to the lower cutoff frequency of a narrowband bandpass filter is approximately 2 or less, so it cannot be divided into separate low-pass and high-pass filters device to achieve.

When the ratio of the upper cutoff frequency to the lower cutoff frequency decreases, the attenuation at the center frequency will increase; when the ratio of the upper cutoff frequency to the lower cutoff frequency is close to 1, the filter has greater suppression effect.

Baidu Encyclopedia-Narrowband Bandpass Filter