band Combiner refers to a filter that can pass through frequency components in a certain frequency range, but attenuates frequency components in other ranges to very low levels, contrary to the concept of band stop filters. An example of an analog band Combiner is a resistance-inductor-capacitor circuit (RLC circuit). These filters can also be produced by combining a low-pass filter and a high-pass filter. An ideal band Combiner would have a completely flat passband with no amplification or attenuation within the passband, all frequencies outside the passband are completely attenuated, and in addition, conversion outside the passband is done in a small frequency range. In fact, there is no ideal band Combiner. The filter cannot fully attenuate all frequencies outside the desired frequency range, especially the attenuated but unisolated range outside the desired passband. This is often referred to as the phenomenon of the filter's rolldown, expressed in dB for every tenfold attenuation.

In general, the design of the filter should ensure that the roll-down range is as narrow as possible, so that the performance of the filter is closer to the design. However, as the roll down area becomes smaller, the passband becomes less flat and "ripples" begin to appear. This phenomenon is particularly pronounced at the edge of the passband. This effect is known as the Gibbs phenomenon as the difference between f1 and the resonant frequency between the lower cutoff frequency f1 and the upper cutoff frequency f2.

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