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The Chebyshev II filter is the “mirror image” of the Chebyshev I filter. The passband is perfectly flat (no ripples at all in the frequencies to keep). The stopband (blocked region) has ripples — meaning the rejection of unwanted frequencies is not perfectly smooth. This makes it useful when preserving the exact amplitude of the signal in the passband is more important than how evenly the stopband is shaped.
Chebyshev II filter may be used for several reasons including:
•Flat passband: Ideal when even small amplitude distortions are not acceptable in the useful frequency range.
•Sharper cutoff than Butterworth: Like Type I, it cuts faster than Butterworth, but with the trade-off shifted to the stopband.
•Alternative to Chebyshev I: Used when passband accuracy is more critical than stopband smoothness.
When applying a Chebyshev II filter, the following parameters should be defined:
•Band type (low-pass, high-pass, band-pass, or band-stop).
•Order (higher order = sharper cutoff).
•Critical frequencies (Hz): Cutoff point(s) where filtering begins.
•Minimum attenuation (dB): How much the filter must reduce unwanted frequencies in the stopband. Example: “40 dB attenuation” means unwanted frequencies are reduced to 1/100th of their original amplitude.
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Figure: Chebyshev II filter dialog
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Figure: FFT Spectrum of B-ICC record before (red) and after (blue) applying Chebyshev II filter
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