Chebyshev filter

Discussion in 'Homework Help' started by bwd111, Aug 23, 2013.

  1. bwd111

    Thread Starter Member

    Jul 24, 2013
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    Distinguishing features of a Chebyshev filter? Im thinking It allows ripple in the passband just because it doesnt have a maximally flat response over its passband. Pretty sure im correct thou
     
    Last edited: Aug 23, 2013
  2. Papabravo

    Expert

    Feb 24, 2006
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    Ripple in the passband
    Ripple in the stopband
    Steeper rolloff than Butterworth
     
  3. bwd111

    Thread Starter Member

    Jul 24, 2013
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    Ripple in the passband I s what Im going with
     
  4. LvW

    Active Member

    Jun 13, 2013
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    Sorry - not 100% correct.
    No ripple in the stop band;
    Steeper roll-off near the pass band edge only - in general, the roll-off is determined by the order only.
    In addition, pass band edge defined (mostly) NOT by -3 dB but by the ripple.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    That is for a Chebyshev Type I
    The rolloff for a Chebyshev type I is steeper than for a Butterworth of the same order.
    The Chebyshev type II has no ripple in the passband and does have ripple in the stopband and it's rolloff is not as steep as the Type I.

    http://en.wikipedia.org/wiki/Chebyshev_filter
     
  6. LvW

    Active Member

    Jun 13, 2013
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    Yes - if somebody asks for Chebyshev response (without any additional information) I always assume Chebyshev I - and not the inverse Chebyshev function.

    Sorry - not correct. As mentioned before, the roll-off is determined by the filter order only (starting approx. one cade above the pass band edge).
    Butterworth and Chebyshev (2nd order): 40 dB/dec.

    Hint: Do not blindly trust wikipedia.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    You have to trust them if you're too cheap to buy a book.
    You could also build one and measure it, but even that has become too difficult for most.
     
  8. LvW

    Active Member

    Jun 13, 2013
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    And what about using your own brain?
    A short look on the transfer function reveals that the denominator is the same for both functions - thus, the roll-off is the same, of course.
    (It was the only purpose of my last sentence to remind you and other readers that it is not a good approach to pick-up any claims/statements found in books or the Internet without a critical review).
     
  9. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Are you saying that the denominator of the transfer function for a Butterworth filter is the same (identical) as the transfer function for a Chebychev filter of the same order?

    Perhaps you meant to say "A short look on the transfer function reveals that the denominator is the same order for both functions"
     
  10. LvW

    Active Member

    Jun 13, 2013
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    The_Electrician, thanks for the attempt to correct me.
    However, what I had in mind is the 2nd order denominator in its general form:

    D(s)=1+s/(wpQp) + (s/wp)^2

    Of course, the pole Q (Qp) is different for both functions (and the same wp).
     
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