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Introduction to bandpass filter
Sep 06, 2018

An ideal bandpass filter should have a completely flat passband, no amplification or attenuation within the passband, and all frequencies outside the passband are completely attenuated, in addition, the conversion outside the passband is completed in a very small frequency range.

In fact, there is no ideal bandpass filter. The filter cannot attenuate all frequencies outside the desired frequency range completely, especially when there is a attenuated but not isolated range outside the desired passband. This is often referred to as a roll-down of the filter and is represented by the number of dB attenuations per ten-fold frequency. In general, the design of the filter should ensure that the rolling fall range is as narrow as possible, so that the performance of the filter is closer to the design. However, as the roll-down range becomes smaller, the passband becomes less flat and begins to "ripple." This phenomenon is particularly pronounced at the edge of the passband, an effect known as the gibbs phenomenon.

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