Active Low/High Pass Filters graphing

Discussion in 'Homework Help' started by kee23, Feb 5, 2012.

  1. kee23

    Thread Starter New Member

    Feb 1, 2012
    hello, this is my first thread here :)

    anyways i need some help on graphing Active Low/High Pass filters
    and I was wondering that when you're finding the roll off rate, is it must be in 24dB/OCT?(our circuits were 4th order and it was -80dB/decade so it should be -24dB/OCT.).

    Because when i was graphing from my lab results, I was only able to get 16dB/OCT. Also they said they don't need to be in exact numbers like 24dB/OCT, but mine was so low, so i got confused.

    Umm so is it ok to have less roll off rate like this? maybe i made wrong measurements? :(. was it when you found critical frequency, its doubled of that in low pass and half of it in high pass to find roll off rate in OCT, right? and then difference of these two?

    our circuits were 4th order and it was -80dB/decade so it should be -24dB/OCT.

  2. Audioguru


    Dec 20, 2007
    One order is 6dB per octave so arithmatic calculates 4 orders to be 24dB per octave.

    Lowpass: Maybe you used an old opamp that has poor high frequency response so the higher frequencies went through it without being attenuated.
    Highpass: Maybe your wiring picks up mains hum.

    Maybe you made a Bessel filter that has a gradual corner instead of a Butterworth filter that has a sharp corner.

    Post your schematic for us to see.
  3. crutschow


    Mar 14, 2008
    Between what two points are you measuring the rolloff rate?