butterworth digital low pass filter

Discussion in 'General Electronics Chat' started by vead, Jan 13, 2014.

  1. vead

    Thread Starter Active Member

    Nov 24, 2011
    621
    8
    butterworth digital low pass filter has following specification

    passband-0-500 hz
    stopband-2-4 khz
    passband ripple-3 db
    stopband attenuation-20 db
    sampling frequency-8 khz

    determine the passband and stopband edge frequency


    please tell me the hint or formula to calculate passband and stopband frequency
     
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    550
    75
    Surely whatever class or book this question came from also gave you the tools to answer it. Else what is the point of the class?

    However, it doesn't make sense. A Butterworth filter shouldn't have passband ripple.

    If you can assume it's a Butterworth filter (no passband ripple) or that it's a Chebyshev filter with 3dB passband ripple, then the beginning of your answer is right there. You have the minimum frequency your passband must extend to, you have the maximum frequency your stopband must start at, and you have the attenuation that must be seen at the start of the stopband. The class or book or whatever should have explained how to take those numbers and determine the filter order needed and cutoff calculation.
     
  3. Fritz Kreisl

    New Member

    Jan 13, 2014
    1
    0
    butter designs lowpass, bandpass, highpass, and bandstop digital and analog Butterworth filters. Butterworth filters are characterized by a magnitude response that is maximally flat in the passband and monotonic overall.
    <snip>
     
    Last edited by a moderator: Jan 13, 2014
  4. vead

    Thread Starter Active Member

    Nov 24, 2011
    621
    8
    this is unsolved question I don't have anything to solve it i have googled and spend lot of time to find out but I didn't found anything
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,383
    495
    Ok. You have a transfer function. You plug a frequency into the transfer function, do the math, you come up with a number between 0 and 1.

    When you plug a cutoff frequency of the filter into the transfer function, the result is 0.707, so 0.707=transfer function. Now solve it for the frequency.
     
  6. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Hi vead,
    Some days ago you have opened another thread dealing with the same subject :
    http://forum.allaboutcircuits.com/showthread.php?t=93040

    In my answer (post#2) I told you that there is no simple formula for calculating such a filter. First, you have to choose one of different design strategies available.
     
  7. vead

    Thread Starter Active Member

    Nov 24, 2011
    621
    8
    that was different post but topic is same I thought may be someone can help in this way but now I think no one can help me I have to do myself
     
  8. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Rather, I think it is not possible to help you based on some rough information in this forum. The task is rather complex and requires more work than can be done by simply ask questions and give answers.
     
  9. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,645
    759
    Not a bad idea, I think.
     
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