Designing a lowpass Butterworth filter

[rusty009]

Hi all,

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

Passband edge f = 10 Khz (Ω_p)
passband stop f = 30 Khz (Ω_s)
Minimum stopband attenuation = 50db (1/A)
Maximum passband attenuation = 0.4db( 1/\sqrt{1+ε²} )

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/\sqrt{(A²-1)})/log(Ω[/B]_p/Ω_s)

can anyone help me out? Thanks

 

[Enthalpy]

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.