Max frequency to give attenuation 96dB below PB?

Discussion in 'Homework Help' started by chebyshev, Jan 2, 2013.

  1. chebyshev

    Thread Starter New Member

    Apr 27, 2010
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    0
    Hi there, I have a question that has left me rather confused and one which I can't find a way to figure out from the lecture notes either.

    A 4th order hihg-pass filter with a cut-off frequency of 1000 Hz is required. What is the maximum frequency which gives an attenuation of at least 96dB (ie. -96dB) below the pass band?

    I've found the inverse of the 96dB, which comes to be a very large number (3,981,071,706), so this gives me the gain again..but I don't know what to do next?

    Any pointers would be great, thanks.
     
    Last edited: Jan 2, 2013
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I thought an ideal Butterworth filter gives an attenuation of 20 dB/decade for each "order" (?), so that would let you calculate the rolloff of the gain for a fourth order filter going down in frequency below the cutoff frequency.
     
  3. chebyshev

    Thread Starter New Member

    Apr 27, 2010
    2
    0
    Ah yes I forgot about that. I know what to do now thanks :)
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    Remember to account for the attenuation at the corner frequency being 3 dB below that of the passband.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I suspect the question means "96dB below maximum passband gain", which makes the 3dB attenuation at the corner frequency irrelevant.
     
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