Designing a lowpass Butterworth filter

  • Thread starter rusty009
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Main Question or Discussion Point

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:

Answers and Replies

  • #2
667
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There, download FilterCad:
http://www.linear.com/designtools/software/

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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