>Your ultimate attenuation should be infinite, barring the numerical
>issues mentioned.
I am using MATLAB to see how much attenuation I am getting at 60Hz. Here
is what I see:
for a 2nd order FIR filter with zero at 60Hz(on the unit circle): 85db
attenuation
for a 2nd order IIR filter with zero at 60Hz(on the unit circle) : 65dB
attenuation
for a 4th order IIR butterworth filter with two zeros at 60Hz(on the unit
circle) : 130db attenuation
I agree when you say if I have a zero on the unit circle at 60Hz, it
should have infinite attenuation, barring the numerical issues. And I
think
that the finite value of attenuation MATLAB is showing, is because of
this.
But I don't understand why a 4th order filter shows a (much)higher
attenuation at 60Hz than a 2nd order filter? Is this by chance or is there
some reason to it, like having 2 zeros at the same location instead of 1?
If this is the reason, why 2 zeros at the same point is better than 1
zero?
What consequences I will face if I switch to a 4th order IIR notch from a
2nd order? The implementation is on a PC with sufficient computing power.
Thanks.
>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
>
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