iir butterworth bandpass filter

[vani]

Hello everyone,I am trying to generate bandpass butterworth filter coefficients for one of my class projects.I am using "Butterworth CoefficientsVI" to generate the filter coefficients and using these coefficents I filter my input signal using "IIR filter VI".I can also do this filtering using "Butterworth Filter VI". But I get different results. "Butterworth CoefficientsVI" generates IIR 4th order filter structure even when i specify order lesser than 4. In my example attached, if i specify order above 3, the filter using "Butterworth CoefficientsVI" becomes unstable, whereas "Butterworth Filter VI" is still stable. In my application I need to get the filter coefficients, and I cant use the "Butterworth Filter VI". So can someone tell me how to get a stable filter coefficients?Message Edited by vani on 07-24-2008 01:37 PM

filt_question2.vi:
http://forums.ni.com/attachments/ni/300/4407/1/filt_question2.vi

[Ipshita_C]

Hi Vani,
With the Butterworth Coefficients VI you will only get only 4th order cascaded sections for the bandpass filters. The Digital Filter Design Toolkit offers more flexibility in the fact that you can design your own Butterworth Filter there and it will also let you do 2nd order cascaded sections which you can use to get to higher orders. For more information on the Digital Design toolkit and cascade form IIR filtering:
<a href="http://zone.ni.com/reference/en-XX/help/371361D-01/lvanlsconcepts/iir_filters/#Butterworth_Filters" target="_blank">Cascade Form IIR Filtering</a>
<a href="http://zone.ni.com/devzone/cda/tut/p/id/4240#toc2" target="_blank">Digital Filter Design Toolkit</a>
Hope this helps!
Ipshita C.