"Kai7918" <kai_the_ruler@n_o_s_p_a_m.yahoo.co.in> wrote in message
news:v82dnS21gdCDP3fWnZ2dnUVZ_uSdnZ2d@giganews.com...
> Hey guys,
>
> I have created a butter worth filter using SPTOOL in matlab.
>
> the specifications are
> �?� Sampling frequency: 1000Hz
> �?� Passband ripple: 3dB
> �?� Stop band edge frequency: 250Hz
> �?� Stopband attenuation: 40dB
> Pas band edge frequency: 225Hz
> Minimum order design
> Single section
>
> The filter is stable.
>
> Here is a screen shot of the Magnitude response:
> http://img43.imageshack.us/i/checkvt.jpg/
>
> see how i have the thing @ about 350-400Hz. What is that. Is that unwanted
> frequencies. If yes, then y do i get such thing.
>
> Also 1 more question. What would the transition width be for this filter?
>
> Thx
>
What do you mean "What would the transition width be"? May be I do not
understand, but is'nt this just 25 Hz? You said so your specifications?
(250-225)? btw, I do not understand those ripples you show in the stopband,
there should not be such when using butterworth filter.
I run your specs through a program I wrote for a class which designed an IIR
low pass filter using butter, and I get order N=30 for this. The program
implements impulse invariance or bilinear transformation. I used Bilinear
method, and this is the result I copied from my screen: (I do not know what
you mean by "single section" ) (I list the H(z) poles, the list shows each
pole's real and imaginary value:
if you are interested in reading my report on this program, here is the link
http://12000.org/my_notes/IIR_digital_filter_design/index.htm
note that my Mathematica program and my report is not yet completely
polished and still in beta :)
***** S T A R T I N G F I L T E R D E S I G N *********
passband corner frequency (hz)=225
stop corner frequency (hz)=250
sampling frequency (hz)=1000
attenuation at passband corner (db) =-3
attenuation at stopband corner (db) =-40
Using Bilinear method
Filter Order N=30
H(z) poles=
{{0.14481876621354606, 0.9384373651677788},
{0.1319143750846099, 0.8454503212687019},
{0.12128495319828964, 0.7601980391715206},
{0.11247308374564964, 0.6813596047822589},
{0.10513579564319177, 0.607865551076532},
{0.09901260308329161, 0.5388376367669333},
{0.09390359551696477, 0.47354456664470596},
{0.08965414987894381, 0.4113688863686502},
{0.08614410199270338, 0.35178182199533364},
{0.08327998296589807, 0.29432383541544355},
{0.08098940518977824, 0.2385893292591055},
{0.07921698766919846, 0.18421437951083425},
{0.07792140930180781, 0.13086667592050333},
{0.07707331151574555, 0.07823705647504915},
{0.07665386283631981, 0.02603216276033525},
{0.07665386283631981, -0.02603216276033525},
{0.07707331151574555, -0.07823705647504915},
{0.07792140930180781, -0.13086667592050333},
{0.07921698766919846, -0.18421437951083425},
{0.08098940518977824, -0.2385893292591055},
{0.08327998296589807, -0.29432383541544355},
{0.08614410199270338, -0.35178182199533364},
{0.08965414987894381, -0.4113688863686502},
{0.09390359551696477, -0.47354456664470596},
{0.09901260308329161, -0.5388376367669333},
{0.10513579564319177, -0.607865551076532},
{0.11247308374564964, -0.6813596047822589},
{0.12128495319828964, -0.7601980391715206},
{0.1319143750846099, -0.8454503212687019},
{0.14481876621354606, -0.9384373651677788}},