how to increased the roll off rate

Discussion in 'Homework Help' started by naspek, Mar 23, 2010.

  1. naspek

    Thread Starter Member

    Feb 17, 2010
    45
    0
    hey there...
    i wanted to modified my 1st order Butterworth low pass filter with K=1.
    i want to increased the roll off rate.. but, after changing the resistor value,
    the capacitor value and fo, i the roll of rate still the same..
    how can i increased my roll off rate?
     
  2. jlcstrat

    Active Member

    Jun 19, 2009
    58
    3
    I believe each reactive element will add another -20 Db/decade. You need to add more capacitors, but keep the total reactance the same to achieve the same cutoff frequency.
     
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    If it is a passive network, the rate is only 3 dB/octave.
     
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  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    A single RC rolls off at 6dB/octave which is 20db/decade. Opamps and ordinary audio tone controls use it.

    A fairly complicated 3db/octave filter is used to convert white noise into pink noise that needs a roll off of 3dB/octave.

    A Sallen and Key Butterworth 2nd-order filter rolls off at 12db/octave and a Sallen and Key 3rd-order Butterworth filter rolls off at 18dB/octave.

    Switched capacitor lowpass filter ICs roll off at 6db/octave per order. An 8th-order Butterworth filter rolls off at 48db/octave and will filter a square-wave into a pretty good sine-wave.
     
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  5. beenthere

    Retired Moderator

    Apr 20, 2004
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  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    If you want to increase to rolloff while keeping the flatness there are other types of filters. For example, an m-derived filter has a very sharp knee, but not as good rejection.

    http://en.wikipedia.org/wiki/M-derived_filter
     
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  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    A Bessel filter has good phase response but has a droopy frequency response.
    A Butterworth filter has the flattest frequency response and fairly sharp cutoff but rings a little at the cutoff frequency.
    A Chebychev filter has a very sharp cutoff but has ripple in the passband and ripple at higher frequencies.
     
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