Tunable bandpass filter

Discussion in 'General Electronics Chat' started by MikeD_72, Dec 30, 2008.

  1. MikeD_72

    Thread Starter Active Member

    Nov 11, 2008
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    I'd like to design a (quick and dirty) tunable bandpass filter for some experiments. I haven't found any schematics or tips for designing such a beast. Maybe someone can help?

    The circuit must be tuned with a pot/variable resistor. I'd like to have constant gain, but it's not necessary (I can amplify the signal with an inverting amp afterwards). Having a constant Q (~5 to 10) or bandwidth (~100 to 200Hz) would also be a nice plus.

    Thanks!
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Look up active filters on Wikipedia.

    There are some good books on the subject too, my old bible is "Designing Active Filters" I think it was called. Can't put my hands on it at the moment.

    Also think about simple RC tuned circuits, such as Wien Bridges, also on Wikipedia.
     
    Last edited: Dec 30, 2008
  3. MikeD_72

    Thread Starter Active Member

    Nov 11, 2008
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    Is there a filter that that you could recommend that will provide large variation in the center frequency with minimal change to the gain and Q?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Almost all of them will do it, the problem variable is Q. Try building a Wien Bridge Oscillator with insufficient gain, I think it will do what you want.

    Or you could build a Wien Bridge followed by a simple op amp amp with whatever gain you need to do what you want.

    Almost any RC design needs dual pots, if you can get an actual dual potentiometer that would work best.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here is a page from the EDUCYPEDIA on filters:
    http://www.educypedia.be/electronics/analogfil.htm
    There is also an application note from National Semiconductors.
    There is a part on switching filters in that note starting at page 17.

    Greetings,
    Bertus
     
  6. mik3

    Senior Member

    Feb 4, 2008
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  7. flat5

    Active Member

    Nov 13, 2008
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    Hello mik3,

    That calculator is interesting but it always gives a negative value for R3.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Your Fc isn't high enough for your bandwidth.

    For starters, try a bandwidth of 150 and a Fc of 150, gain of 1, and 10nF for the caps.
     
  9. flat5

    Active Member

    Nov 13, 2008
    403
    17
    SgtWookie, thanks for your help.

    I was using 300-2700 Hertz. Figured center would be 1660 Hz.
    Bandwidth is 2400
    First problem is the gain must be 1.
    Then it gets weird.
    If center freq. is 1698, R3 is 5,960,859
    If center freq. is 1697, R3 is -99,992,634
    If center freq. is 1700 things start to get real.
     
  10. lovlyman3030

    Member

    Apr 28, 2008
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    0
    can anyone help me to design a tunable band pass filter with constant band width
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    I have the perfect solution: it's called a Vox Crybaby. You might check with your friendly local garage band to see if one's available.

    Eric
     
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