right05 wrote:> Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working > out a formula for the co-efficients for the fourth order low pass > filter. > But if I have 2 second order low pass filters in cascade, can I use > the second order co-efficients for both the filters..I mean using the > second order formula.No. To achieve a Butterworth response, each section has its own Q. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
IIR Filter Co-efficients Formula
Started by ●October 16, 2007
Reply by ●October 16, 20072007-10-16
Reply by ●October 16, 20072007-10-16
On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote:> right05 wrote: > > Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working > > out a formula for the co-efficients for the fourth order low pass > > filter. > > But if I have 2 second order low pass filters in cascade, can I use > > the second order co-efficients for both the filters..I mean using the > > second order formula. > > No. To achieve a Butterworth response, each section has its own Q. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF==AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF= =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF But for a low pass filter ,all it needs is a cut off frequency. ???
Reply by ●October 16, 20072007-10-16
right05 wrote:> Thats a complex math for a fourth order LPF.Wish there > is a generic formula for the coefficients.I can give you the formula. The cost is $500. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●October 16, 20072007-10-16
On Oct 16, 2:24 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote:> right05 wrote: > > Thats a complex math for a fourth order LPF.Wish there > > is a generic formula for the coefficients. > > I can give you the formula. The cost is $500. > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.comThanks.With the help of the friends online ,I should be able to work out the co-efficients myself and then I will posit it for free!
Reply by ●October 16, 20072007-10-16
right05 wrote:>>>Thats a complex math for a fourth order LPF.Wish there >>>is a generic formula for the coefficients. >> >>I can give you the formula. The cost is $500. >> > Thanks.With the help of the friends online ,I should be able to work > out the co-efficients myself and then I will posit it for free!No you won't. It is too tough for a seeker of a free canned solutions. VLV
Reply by ●October 16, 20072007-10-16
On Tue, 16 Oct 2007 19:24:53 +0000, right05 wrote:> On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote: >> right05 wrote: >> > Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working >> > out a formula for the co-efficients for the fourth order low pass >> > filter. >> > But if I have 2 second order low pass filters in cascade, can I use >> > the second order co-efficients for both the filters..I mean using the >> > second order formula. >> >> No. To achieve a Butterworth response, each section has its own Q. >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> > > But for a low pass filter ,all it needs is a cut off frequency. ???If that statement were correct, you wouldn't specify a Butterworth or a Chebychev or an Elliptical filter -- you'd just specify a "low pass". So, no. Actually if you look at the page you quoted you'll see that the denominator polynomial for the 4th order system is already split into two dissimilar polynomials. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Reply by ●October 16, 20072007-10-16
right05 wrote:> On Oct 16, 2:21 pm, Jerry Avins <j...@ieee.org> wrote: >> right05 wrote: >>> Thanks,Tim. I got hold of a Oppenheim & Schafer textbook and working >>> out a formula for the co-efficients for the fourth order low pass >>> filter. >>> But if I have 2 second order low pass filters in cascade, can I use >>> the second order co-efficients for both the filters..I mean using the >>> second order formula. >> No. To achieve a Butterworth response, each section has its own Q. >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> ����������������������������������������������������������������������� > > But for a low pass filter ,all it needs is a cut off frequency. ???Not so. "Butterworth" specifies a maximally flat shape in the passband, There are many 4th order low-pass filters with a particular cutoff frequency. A Butterworth filter is a specific one of those. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●October 16, 20072007-10-16
On Oct 16, 2:17 pm, right05 <vidu...@gmail.com> wrote:> Thanks Robert. Thats a complex math for a fourth order LPF. Wish there > is a generic formula for the coefficients.This website does > http://www.apicsllc.com/apics/Sr_3/Sr_3.htm but it's not right.take a look at: http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt see how it spells out the coefficient values for a 2nd-order LPF section of known resonant frequency and known Q. for an Nth-order Butterworth (where N is even) the general formula for the Q of the nth section is: 1/Q = 2*cos(pi/N*(n+1/2)) where 0 <= n < N/2 and for all sections, the resonant frequency, f0 (or w0), is the same for a Butterworth (this is not the case for Tchebyshev or Elliptical or anything other than Butterworth, as far as i know). it's all spelled out, at least for even-order Butterworth implemented as cascaded 2nd-order sections (which is the manner you should implement it), and will not get less complex. r b-j
