A Butterworth filter does not have passband ripple, which is why it is called a maximally-flat topology. The two parameters usually used to specify one are the corner frequency (frequency at which response id down 3 dBv), and either the attenuation slope (in dB/octave or dB/decade) or the attenuation at a particular frequency below the cutoff. For example, to keep AC power line hum out of a long-wire intercom you might want a cutoff of 300 Hz and a minimum attenuation of 30 dB at 60 Hz. When starting a filter design, these parameters come from the application requirements. They are used to determine the kind of filter to be used and its general parameters. Then, once the filter is designed, you can go back and calculate the specific performance at the same parameters to see if the filter matches or exceeds the requirements.
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