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Designing a lowpass Butterworth filter

  1. Nov 3, 2011 #1
    Hi all,

    I am trying to design a lowpass filter using the following conditions

    Passband edge f = 10 Khz (Ωp)
    passband stop f = 30 Khz (Ωs)
    Minimum stopband attenuation = 50db (1/A)
    Maximum passband attenuation = 0.4db( 1/[itex]\sqrt{1+ε2}[/itex] )

    and am having quite a lot of trouble with it. I have tried to determine the order but it comes out as a complex number, the formula I am using is,

    n= log( ε/[itex]\sqrt{(A2-1)}[/itex])/log(Ω[/B]ps)

    can anyone help me out? Thanks
    Last edited: Nov 3, 2011
  2. jcsd
  3. Nov 27, 2011 #2
    There, download FilterCad:

    By the way, Butterworth is just the worst of all filters except the straight wire. Huge number of poles and components for a given selectivity, AND by far worst pulse response for a given selectivity. Chebychev is better, elliptic even better, and inverse (or type II) Chebychev better.

    So unless some kind of teacher imposes you the Chebychev, take a more sensible option.
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