The best way to build a low pass filter for this?

Discussion in 'The Projects Forum' started by gte, Jun 30, 2010.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
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    If my calculations are correct, this would be ~58hz signal? I'd like it to filter out anything above 260hz, so would I need to do this with a microcontroller or can I use a resistor/capacitor combo?


    fcutoff = 1/(2πRC)

    fcutoff = 1/(2πC) * R
    fcutoff * (2πC) = 1/R
    fcutoff * 2 * 22/7 * 10uf = 1/R
    260 * 2 * 22/7 * .00001 = 1/R
    0.016342857 = 1/R
    61.1888 = R


    If I use 61Ω even, it puts me at 260.804hz
    If I use 60Ω even, it puts me at 265.151hz


    If the voltage were a square wave as pictured below (which I have not scoped yet) how would the voltage look at 260hz?



    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That's the -3db point, or half power point.

    I have a feeling you want more like -30dB down.
     
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  3. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I have a feeling you are correct sir, what do you suggest?

    Micro controller that passes the signal through until it reaches that threshold, and then it just mimics it?


     
  4. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    So is this grand fathered in?

    If so, any ideas?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Low-pass filters are not expressly prohibited.

    Have a look at the attached simulation. You really need to get familiar with SPICE simulations, as they can answer a lot of questions before you take them to a breadboard.

    Remember though, the simulations are only as good as the model that was built. With low-frequency, low-power stuff, you are generally OK. However, once you start getting into higher frequency circuits, seemingly inexplicable things start happening.

    Increasing the value of R and C of the attached schematic will decrease the ripple on the output waveform, but it will take considerably more time to settle.
     
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  6. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    Thanks Sgt!

    I have shied away from spice because of the lack of a GUI combined with all of the other stuff I've been trying to cram into my brain. I will take your advice though and try to start learning it and simulate it there first.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Google "LTSpice download" and you'll find it - it is freeware, and runs under Win2k, WinXP, Vista, Win7, and in Unix under wine.

    It DOES have a GUI interface. For most of the parameters, you just enter a few numbers in dialogs. LTSpice comes with all of their products in the library, as well as an assortment of other manufacturer's products like diodes, resistors, capacitors, MOSFETs, transistors, etc - and a goodly number of example circuits already made for you to experiment with and learn from.
     
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