IIR notch filter
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From: itsh11 on 26 Jun 2008 13:30 I am trying to design a second order digital IIR band stop (notch) filter with the following specs: 3dB cut off frequencies: 55Hz and 65Hz I want the notch at 60Hz with atleast 90dB attenuation at the 60Hz. Sampling frequency: 200hz I tried various filter configurations like a Butterworth or Chebyshev but could not get the attenuation higher than 70 dB.I want a second order filter to accomplish this i.e I do not want to go to a higher order filter. Can someone suggest any possible solutions to this problem. Many thanks
From: Oli Charlesworth on 26 Jun 2008 20:05 On Jun 26, 6:30 pm, "itsh11" <its...(a)yahoo.com> wrote: > I am trying to design a second order digital IIR band stop (notch) filter > with the following specs: > > 3dB cut off frequencies: 55Hz and 65Hz > I want the notch at 60Hz with atleast 90dB attenuation at the 60Hz. > Sampling frequency: 200hz > > I tried various filter configurations like a Butterworth or Chebyshev but > could not get the attenuation higher than 70 dB.I want a second order > filter to accomplish this i.e I do not want to go to a higher order > filter. > > Can someone suggest any possible solutions to this problem. Many thanks With a 2nd-order bandstop filter, I'm not sure there's any difference between Chebyshev and Butterworth. You're pretty much limited to sticking your zeros on the unit circle at 60 Hz, and your poles close enough to meet the Q requirements. However, this kind of filter will be extremely sensitive to numerical effects (quantisation, noise shaping, etc.). -- Oli
From: Tim Wescott on 26 Jun 2008 21:28 On Thu, 26 Jun 2008 12:30:23 -0500, itsh11 wrote: > I am trying to design a second order digital IIR band stop (notch) > filter with the following specs: > > 3dB cut off frequencies: 55Hz and 65Hz I want the notch at 60Hz with > atleast 90dB attenuation at the 60Hz. Sampling frequency: 200hz > > I tried various filter configurations like a Butterworth or Chebyshev > but could not get the attenuation higher than 70 dB.I want a second > order filter to accomplish this i.e I do not want to go to a higher > order filter. > > Can someone suggest any possible solutions to this problem. Many thanks Your ultimate attenuation should be infinite, barring the numerical issues mentioned. I don't design these from a Butterworth, Chebychev, etc., point of view; I just make a plain ol' notch. For a transfer function given a notch frequency and bandwidth, look here: http://www.dsprelated.com/ showmessage/81610/1.php. A good book on DSP should show you how to calculate your numerical effects. -- 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
From: Vladimir Vassilevsky on 26 Jun 2008 21:43 "itsh11" <itsh11(a)yahoo.com> wrote in message news:brKdnUByb8CyT_7VnZ2dnUVZ_uLinZ2d(a)giganews.com... > I am trying to design a second order digital IIR band stop (notch) filter > with the following specs: Trying is good. > 3dB cut off frequencies: 55Hz and 65Hz > I want the notch at 60Hz with atleast 90dB attenuation at the 60Hz. > Sampling frequency: 200hz This is not unreasonable. Will require a configuration with the non-recursive part after recursive; probably with 24 bit coefficients in numerator. For denominator, 16 bits should do. > I tried various filter configurations like a Butterworth or Chebyshev but > could not get the attenuation higher than 70 dB.I want a second order > filter to accomplish this i.e I do not want to go to a higher order > filter. Keep trying. > > Can someone suggest any possible solutions to this problem. Sure. Hire a consultant. > Many thanks How many thanks? Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com
From: Jerry Avins on 26 Jun 2008 23:22
Tim Wescott wrote: > On Thu, 26 Jun 2008 12:30:23 -0500, itsh11 wrote: > >> I am trying to design a second order digital IIR band stop (notch) >> filter with the following specs: >> >> 3dB cut off frequencies: 55Hz and 65Hz I want the notch at 60Hz with >> atleast 90dB attenuation at the 60Hz. Sampling frequency: 200hz >> >> I tried various filter configurations like a Butterworth or Chebyshev >> but could not get the attenuation higher than 70 dB.I want a second >> order filter to accomplish this i.e I do not want to go to a higher >> order filter. >> >> Can someone suggest any possible solutions to this problem. Many thanks > > Your ultimate attenuation should be infinite, barring the numerical > issues mentioned. > > I don't design these from a Butterworth, Chebychev, etc., point of view; > I just make a plain ol' notch. For a transfer function given a notch > frequency and bandwidth, look here: http://www.dsprelated.com/ > showmessage/81610/1.php. > > A good book on DSP should show you how to calculate your numerical > effects. Tim, you need to figure out how to stop your newsreader from breaking URLs. http://www.dsprelated.com/showmessage/81610/1.php. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ |